Pennsylvania leads the Nation in the number of individuals that use groundwater for private domestic water supply; more than 3 million rural and suburban Pennsylvania residents rely on private domestic supplies for drinking water. These supplies are not regulated nor routinely monitored; thus relevant groundwater-quality information is not widely available. The U.S. Geological Survey (USGS), in cooperation with the Pennsylvania Department of Environmental Protection (PaDEP) Safe Drinking Water Bureau, established a statewide, fixed-station ambient groundwater quality network in 2015. The goals for the Pennsylvania Groundwater Monitoring Network (GWMN) include characterizing ambient groundwater quality conditions in rural areas of the State and documenting potential changes in conditions over time. Seventeen wells were selected for monitoring at 6-month intervals beginning in 2015. Since then, several wells have been added to the GWMN, bringing the total number of wells sampled in the fall of 2019 to 28. Routinely monitored constituents included physical characteristics and chemical concentrations in filtered and unfiltered samples (major and trace elements, nutrients, and organic compounds). Samples for volatile organic compounds (VOCs), radionuclides, and dissolved hydrocarbon gases were collected during the first sampling event at each well.
To offer insights on the quality of groundwater used for domestic supply in Pennsylvania, summary statistics for the 221 GWMN samples collected during 2015–19 are compared to U.S. Environmental Protection Agency (EPA) drinking-water standards, which are applicable to public water supplies. Results show that samples across the GWMN generally meet drinking-water standards for inorganic and organic constituents; however, a percentage of samples had concentrations that exceeded maximum contaminant level (MCL) thresholds for nitrate (3 percent) and secondary maximum contaminant level (SMCL) thresholds for iron (32 percent), manganese (36 percent), and aluminum (5 percent). Radon-222 activities, which were sampled only during the initial visit to a well, exceeded the lower proposed drinking water standard of 300 picocuries per liter (pCi/L) in 64 percent of wells in the GWMN; additionally, 7 percent of wells exceeded the higher proposed standard of 4,000 pCi/L. There were no exceedances for VOCs, but one well had a tribromomethane detection. Three wells had detectable concentrations of methane, with one sample exceeding the Pennsylvania action level of 7 milligrams per liter (mg/L).
The pH and dissolved oxygen concentrations varied widely across the GWMN and were correlated with dissolved metal concentrations and other chemical characteristics of groundwater samples. Considering all samples collected for the study, the pH ranged from 4.2 to 8.3; 42 percent of pH values were either above or below the SMCL range of 6.5–8.5. The highest pH values resulted from contamination of loose grout used in the construction of one well and decreased to levels consistent with other wells in the vicinity after repeated sampling rounds. Dissolved oxygen (DO), which ranged from 0 to 13.9 mg/L, influences the mobility and prevalence of constituents with variable oxidation state, including iron, manganese, and nitrogen species. Samples with acidic pH (less than 6.5) and (or) low DO had the highest concentrations of manganese and iron, whereas those with neutral to alkaline pH values had the highest concentrations of calcium, magnesium, sodium, and other major ions. Analysis of major ions indicates that calcium/bicarbonate water types are the most common, with a few characterized as calcium/chloride or sodium/chloride, and most others as mixed water types including calcium-magnesium/bicarbonate, sodium-magnesium/bicarbonate, and sodium/bicarbonate-chloride.
Nonparametric statistical methods were used to evaluate the data for spatial and temporal trends. A principal components analysis (PCA) model developed with ranked data values for the entire network resulted in three components, (1) dissolved solids, (2) redox, and (3) sodium-chloride, which explained 74.5 percent of variance in the dataset. On the basis of individual contributions to the PCA, certain wells were identified through hierarchical cluster analysis that shared relevant water-quality characteristics. The spatial distribution of sampling locations and the temporal trends of constituent concentrations indicate that hydrogeologic setting and topographic position as defined in the PCA model are important factors affecting the spatial and temporal patterns of groundwater quality in the GWMN.
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This study was funded by an ongoing cooperative effort between the U.S. Geological Survey (USGS) Pennsylvania Water Science Center and the Pennsylvania Department of Environmental Protection (PaDEP). The authors would like to thank Susan Weaver and Amy Williams of PaDEP for their assistance and guidance in operation of the network. Appreciation is extended to the local land managers, including the staff of the Pennsylvania Department of Conservation and Natural Resources and the Pennsylvania Game Commission for their cooperation in maintaining access to sampling locations. Thanks are extended to USGS colleagues Mitchell Weaver, Kyle Ohnstad, Dennis Low, and Dana Heston for groundwater sampling, and Brandon Fleming, Chuck Cravotta, and James Degnan for report reviews.
Multiply | By | To obtain |
Length | ||
---|---|---|
inch (in.) | 2.54 | centimeter (cm) |
foot (ft) | 0.3048 | meter (m) |
mile (mi) | 1.609 | kilometer (km) |
Area | ||
square mile (mi2) | 2.590 | square kilometer (km2) |
Volume | ||
gallon (gal) | 3.785 | liter (L) |
Flow rate | ||
gallon per minute (gal/min) | 0.06309 | liter per second (L/s) |
Pressure | ||
inch of mercury at 60 °F (in Hg) | 3.377 | kilopascal (kPa) |
Radioactivity | ||
picocurie per liter (pCi/L) | 0.037 | becquerel per liter (Bq/L) |
Specific capacity | ||
gallon per minute per foot ([gal/min]/ft) | 0.2070 | liter per second per meter ([L/s]/m) |
Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows: °F = (1.8 × °C) + 32.
Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows: °C = (°F – 32) / 1.8.
Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88).
Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).
Altitude, as used in this report, refers to distance above the vertical datum.
Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (µS/cm at 25 °C).
Concentrations of chemical constituents in water are given in either milligrams per liter (mg/L) or micrograms per liter (µg/L).
Activities for radioactive constituents in water are given in picocuries per liter (pCi/L).
Results for measurements of stable isotopes of an element (with symbol E) in water, solids, and dissolved constituents commonly are expressed as the relative difference in the ratio of the number of the less abundant isotope (ΔE) to the number of the more abundant isotope of a sample with respect to a measurement standard.
degrees Celsius
equal to
greater than
greater than or equal to
less than
less than or equal to
micrograms per liter
microsiemens per centimeter
Bureau of Laboratories (Pennsylvania Department of Environmental Protection)
digital elevation model
dissolved oxygen
dissolved organic carbon
U.S. Environmental Protection Agency
Groundwater Monitoring Network
health advisory level
maximum contaminant level
milligrams per liter
North American Datum of 1983
North American Vertical Datum of 1988
National Field Manual
National Geodetic Vertical Datum of 1929
nephelometric turbidity ratio units
Pennsylvania Department of Environmental Protection
principal component
principal components analysis
picocuries per liter
Palmer Drought Severity Index
per- and polyfluoroalkyl substances
residue on evaporation
specific conductance
secondary maximum contaminant level
total dissolved solids
U.S. Geological Survey
volatile organic compounds
Pennsylvania leads the Nation in the number of individuals that use groundwater for private domestic water supply, with approximately 3.47 million Pennsylvania residents (27 percent) in rural and suburban areas relying on private groundwater wells as a source of drinking water (
The PaDEP monitored long-term ambient groundwater quality from 1985 until the late 1990s statewide and continued monitoring in the southeastern part of the state until 2018 (Susan Weaver, Pennsylvania Department of Environmental Protection, written commun., 2020). Interest in an ambient, fixed-station network increased in the 2000s with the commencement of Marcellus Shale gas drilling activity in the northern and western parts of the State; recognizing the need to re-establish a long-term monitoring network, PaDEP partnered with the U.S. Geological Survey (USGS) in 2014 to create a network with the goals of determining the background quality of groundwater resources within the State and to monitor for changes in groundwater quality over time that may be related to a variety of human-caused and natural changes (
Sampling in the GWMN began in 2015 using 17 existing USGS groundwater-level monitoring wells that were completed in a variety of geologic settings. Since then, the GWMN has grown each year to include additional USGS groundwater monitoring wells and other private wells that did not previously contain instrumentation. The current GWMN consists of 28 wells in 27 counties and represents varied geologic, hydrologic, and land use settings. Sixteen wells have been sampled biannually for 5 years and the other 12 wells added to the GWMN have been sampled at the same frequency, but for shorter lengths of time. All samples collected for GWMN sampling are analyzed by PaDEP’s Bureau of Laboratories (BOL). Results are checked and approved by USGS personnel and uploaded to the National Water Inventory System (NWIS) database, which is accessible to the public.
Beginning in 1985, the Pennsylvania Department of Environmental Resources (now PaDEP) collected groundwater-quality data for an ambient, fixed-station groundwater monitoring program. The program was designed to allow for the evaluation of groundwater resources in the State based upon a groundwater basin prioritization scheme using socioeconomic and environmental factors (Susan Weaver and Patrick Bowling, Pennsylvania Department of Environmental Protection, written commun., 2020). The highest priority basins were located primarily near urban areas in the southern parts of the State (
Recognizing the need for a statewide characterization of shallow groundwater resources, the PaDEP partnered with the USGS in 2004 to compile electronically available groundwater-quality data for a 28-year monitoring interval between 1979 and 2006 (
Groundwater-quality monitoring has increased since 2007, mainly in the western, northcentral, and northeastern regions of the State, which coincides with the large-scale development of Marcellus Shale gas resources in those areas. The Marcellus Shale gas field, which underlies much of the state of Pennsylvania, contains a range of wet (natural gas that contains methane as well as other larger hydrocarbons including ethane and propane) and dry (natural gas that consists almost entirely of methane) gas that has applications in energy and industrial production. In addition to proprietary predrilling reconnaissance by gas extraction companies and concerned homeowners, publicly funded countywide studies of the quality of groundwater from private domestic supplies in Lycoming (2014), Wayne (2014), Pike (2015), Bradford (2016), Potter (2017), and Clinton (2017) Counties were conducted by the USGS (
This report presents the results of analysis of 221 groundwater quality samples collected semiannually from 2015 through 2019 from 28 wells in the Pennsylvania GWMN. All samples were analyzed for major ions, trace elements, nutrients, and organic compounds. Additional samples for analysis of volatile organic compounds (VOCs), hydrocarbons, and radon-222 were collected once, the first time a well was sampled. The measured concentrations of constituents are compared to drinking-water quality standards set by the EPA. Summary statistics for groundwater-quality data are presented, followed by the characterization of statewide spatial and temporal variations in water quality for the sampling period. The relations observed between groundwater-quality characteristics, climate, seasonality, land use, geology, and other environmental variables are evaluated to explain the variability in groundwater quality. Considerations are presented for modifications to the GWMN for future sampling, expansion, instrumentation, and supplemental sample collection and analysis.
The State of Pennsylvania occupies approximately 46,055 square miles (29.5 million acres), with elevations ranging from 0 feet (ft) at Philadelphia in the southeastern part of the State to 3,213 ft above the North American Vertical Datum of 1988 (NAVD 88) at Mount Davis, located in the southwestern part of the State (
According to the 2020 U.S. Census, 13,002,700 people reside in the State (
Locations of the 28 sampled wells within the Pennsylvania Groundwater Monitoring Network and associated spatial features including major metropolitan areas, counties with previous baseline groundwater quality studies, and locations of unconventional gas wells in Marcellus Shale.
Figure 1. Map showing the locations of the 28 sampled wells within the Pennsylvania Groundwater Monitoring Network and associated spatial features including major metropolitan areas, counties with previous baseline groundwater quality studies, and locations of unconventional gas wells in Marcellus Shale.
Land use varies widely across the State, with development largely concentrated in the south. Forest lands in the State total 16.9 million acres (57 percent) and include State and Federally managed forests and parks in addition to private land holdings; the acreage of forested lands in the State have been relatively stable in the years following 1965 (
Pennsylvania has six geologically complex physiographic provinces (
Table 1. General information including station name, station identification number, number of samples collected, and physical characteristics of wells in the Pennsylvania Groundwater Monitoring Network.
[ID, identifier; Fm, formation; NA, not applicable; --, no data]
Well name | County | USGS station ID | Latitude | Longitude | Elevation, in feet above sea level | Year of first sample (number of samples collected through 2019) | Rock formation well is completed in | Major aquifer type | Principle aquifer or secondary hydrologic region | Primary lithology | Depth, in feet | Casing length, in feet | Casing diameter, in inches | Specific capacity, in gallons per minute per foot |
AD 146 | Adams | 395846077040601 | 39.97928 | 77.06917 | 532 | 2018 (4) | Heidlersburg Member | Siliciclastic | Early Mesozoic | Mudstone | 100 | 17 | 6 | 0.4 |
AG 700 | Allegheny | 403734080063001 | 40.62619 | 80.10839 | 1,028 | 2016 (8) | Glenshaw Fm | Siliciclastic | Pennsylvanian | Shale | 100 | 24 | 6 | 2 |
BR 202 | Bradford | 414815076391801 | 41.80486 | 76.65644 | 1,071 | 2016 (7) | Catskill Fm | Siliciclastic | Northern Appalachian Basin | Mudstone | 180 | 147 | 6 | -- |
BR 203 | Bradford | 414748076403901 | 41.79646 | 76.67693 | 1,221 | 2016 (7) | Lock Haven Fm | Siliciclastic | Northern Appalachian Basin | Sandstone | 80 | 50 | 6 | -- |
CB 104 | Carbon | 410123075425401 | 41.02314 | 75.71464 | 1,294 | 2016 (8) | Mauch Chunk Fm | Siliciclastic | Valley and Ridge | Shale | 125 | 20 | 6 | 17 |
CE 118 | Centre | 404518077575501 | 40.75558 | 77.96617 | 1,146 | 2014 (10) | Gatesburg Fm | Carbonate | Valley and Ridge Carbonate | Sandstone | 130 | 40 | 6 | 56 |
CF 321 | Clearfield | 410627078313601 | 41.10744 | 78.52647 | 2,153 | 2018 (3) | Burgoon Sandstone | Siliciclastic | Mississippian | Sandstone | 150 | 26 | 6 | 0.3 |
CH 10 | Chester | 395450075485401 | 39.91372 | 75.81419 | 293 | 2018 (4) | Cockeysville Marble | Crystalline | Piedmont and Blue Ridge Carbonate | Marble | 34 | 18 | 6 | 375 |
CN 1 | Clinton | 411424077462201 | 41.24022 | 77.77278 | 2,045 | 2015 (11) | Huntley Mountain Fm | Siliciclastic | Mississippian | Sandstone | 75 | 38 | 6 | 2.4 |
CU 2 | Cumberland | 400209077183301 | 40.03592 | 77.30887 | 964 | 2014 (11) | Metarhyolite | Crystalline | Piedmont and Blue Ridge Carbonate | Metarhyolite | 44 | 19 | 6 | 0.7 |
CW 2417 | Crawford | 413658079572601 | 41.61604 | 79.95735 | 1,426 | 2019 (1) | Cuyahoga Group | Siliciclastic | Mississippian | Siltstone | 53 | -- | 6 | 0.2 |
FO 11 | Forest | 412823079030601 | 41.47312 | 79.05143 | 1,779 | 2014 (12) | Clarion Fm | Siliciclastic | Pennsylvanian | Sandstone | 110 | 23 | 6 | 0.7 |
LA 1201 | Lawrence | 410538080280801 | 41.09395 | 80.46868 | 1,045 | 2014 (11) | Connoquenessing Fm | Siliciclastic | Pennsylvanian | Sandstone | 150 | 30 | 6 | 1.2 |
LB 372 | Lebanon | 402207076180801 | 40.36883 | 76.30181 | 443 | 2014 (11) | Ontelaunee Fm | Carbonate | Valley and Ridge Carbonate | Dolomite | 80 | 0 | 6 | 180 |
LY 796 | Lycoming | 411640077215802 | 41.2779 | 77.36616 | 1,751 | 2017 (5) | Huntley Mountain Fm | Siliciclastic | Mississippian | Sandstone | 505 | 127 | 2.5 | 2.6 |
NU 579 | Northumberland | 404218076351501 | 40.70504 | 76.58755 | 709 | 2018 (4) | Trimmers Rock Fm | Siliciclastic | Valley and Ridge | Siltstone | 56 | -- | 8 | 0.6 |
PI 522 | Pike | 411833075133601 | 41.30919 | 75.22686 | 1,758 | 2015 (10) | Catskill Fm | Siliciclastic | Northern Appalachian Basin | Sandstone | 150 | 28 | 6 | 0.6 |
PO 72 | Potter | 414640077493801 | 41.77784 | 77.82694 | 1,815 | 2014 (11) | Catskill Fm | Siliciclastic | Mississippian | Sandstone | 110 | 21 | 6 | 5.6 |
SO 854 | Somerset | 395920079021501 | 39.98883 | 79.0375 | 2,320 | 2018 (3) | Allegheny Fm | Siliciclastic | Pennsylvanian | Sandstone | 121 | 42 | 6 | 2.5 |
SQ 61 | Susquehanna | 415323077451301 | 41.88997 | 75.75325 | 1,276 | 2014 (12) | Catskill Fm | Siliciclastic | Northern Appalachian Basin | Sandstone | 175 | 80 | 6 | 0.1 |
SU 169 | Sullivan | 412403076234802 | 41.40092 | 76.39675 | 2,333 | 2017 (5) | Huntley Mountain Fm | Siliciclastic | Mississippian | Sandstone | 180 | 150 | 2 | 2 |
TI 470 | Tioga | 414634077235801 | 41.77608 | 77.39925 | 1,160 | 2014 (11) | NA | Surficial | Northern Appalachian Basin | Glacial outwash | 73 | 69 | 6 | 0.5 |
UN 51 | Union | 405928077115501 | 40.99014 | 77.19772 | 1,568 | 2014 (11) | Reedsville Fm | Siliciclastic | Valley and Ridge | Shale | 115 | 94 | 6 | 0.9 |
VE 57 | Venango | 411958079540202 | 41.33158 | 79.90022 | 1,521 | 2014 (10) | Connoquenessing Fm | Siliciclastic | Mississippian | Sandstone | 215 | 9 | 6 | 0.6 |
WE 300 | Westmoreland | 402138079031802 | 40.36047 | 79.05583 | 1,283 | 2018 (3) | Clarion Fm | Siliciclastic | Pennsylvanian | Sandstone | 110 | 22 | 6 | 0.3 |
WN 64 | Wayne | 414333075153201 | 41.72522 | 75.25858 | 1,338 | 2014 (12) | NA | Surficial | Northern Appalachian Basin | Glacial outwash | 52 | 52 | 6 | 3.9 |
WR 50 | Warren | 414159079213601 | 41.69953 | 79.35983 | 1,215 | 2014 (11) | Venango Fm | Siliciclastic | Northern Appalachian Basin | Siltstone | 105 | 46 | 6 | 0.9 |
WY 197 | Wyoming | 412708076150201 | 41.45228 | 76.25053 | 1,972 | 2017 (5) | Burgoon Sandstone | Siliciclastic | Mississippian | Sandstone | 119 | 115 | 2.5 | 0.2 |
Physiographic provinces and physiographic sections of Pennsylvania and the locations of 28 sampled wells within the Pennsylvania Groundwater Monitoring Network.
Figure 2. Map showing the physiographic provinces and physiographic sections of Pennsylvania and the locations of 28 sampled wells within the Pennsylvania Groundwater Monitoring Network.
The bedrock geology and associated topography of Pennsylvania (
Figure 3. Maps showing
Surface water resources are dominated by three major rivers (Susquehanna, Delaware, and Ohio), which collectively drain 95 percent of the 45,000 miles of waterways in Pennsylvania. All network wells are in one of these watersheds, with the majority (57 percent) located in the Susquehanna River watershed. The degree of surface water-groundwater interaction varies with geology and topography, with particularly high connectivity in the limestone valleys found in the Ridge and Valley and Piedmont physiographic provinces. This connectivity can be of concern where chemicals from human activities, such as the disposal of wastes and the use of fertilizers and manmade organic compounds, can enter the groundwater supply. Natural lake features are limited to the northeastern and northwestern glacially influenced parts of the State; the majority of surface water bodies in the State are artificially formed or expanded reservoirs used for flood-control, water supply, and recreational purposes.
To characterize groundwater chemistry in aquifers, 221 groundwater quality samples were collected between 2015 and 2019 from 28 wells. Samples were analyzed for physical and chemical properties, major ions, metals and trace elements, nutrients, volatile organic compounds, radionuclides, and dissolved hydrocarbon gases including methane. All data presented in this report are available in the USGS National Water Information System (NWIS;
GWMN wells are located at existing USGS continuous monitoring stations or in locations where agreements exist between the USGS and landowners for continued access to the well. The selection of wells for the network initially focused on the northern and western parts of the State, with many being located in areas underlain by the Marcellus Shale (
Prior to being added to the GWMN, a prospective well undergoes an exploratory site visit to determine if factors such as accessibility, well integrity, and water quality would preclude the well from accurately representing the groundwater quality of the local aquifer. The integrity of candidate wells is evaluated by checking the casing for rust and the immediate vicinity for surface drainage issues; well casings also need to be above grade and have the ability to be secured to protect submersible pumps, tubing, and instrumentation inside the well. Each candidate well is pumped to examine physical and chemical water-quality properties as well as to determine whether the well is able to produce an adequate supply of water for sample collection. Physical and chemical properties are monitored during pumping of three borehole volumes to determine if stabilization of field parameters is possible (U.S. Geological Survey, variously dated). If the well is considered to be a good candidate for the GWMN, a dedicated Grundfos RediFlo 2 pump and precleaned, inert Teflon tubing is installed for future sampling events.
A total of 221 groundwater quality samples were collected from 28 well sites between 2015 and 2019 following standard USGS methods and protocols outlined in the USGS National Field Manual (NFM; U.S. Geological Survey, variously dated). The number of samples at a specific site range from 1 to 12 depending on when the well was added to the GWMN (
Water level daily values, 2000–19, from PO 72 Potter County observation well, displaying daily average minimum, 25th percentile, median, 75th percentile, and maximum values.
Figure 4. Hydrograph of water level daily values, 2000–19, from PO 72 Potter County observation well, displaying daily average minimum, 25th percentile, median, 75th percentile, and maximum values.
Samples were processed in the following order and manner, as recommended by the USGS NFM (U.S. Geological Survey, variously dated). A 0.45µm (micrometer) pore filter is rinsed with 2 liters of deionized water and a plastic sampling chamber is set up to reduce atmospheric contamination of samples as recommended in the USGS NFM (U.S. Geological Survey, variously dated). Unpreserved dissolved samples were collected for physical properties and general chemistry (pH, alkalinity, specific conductance). Dissolved nutrients were collected and preserved with sulfuric acid, followed by dissolved metals, and dissolved organic compounds that are preserved with nitric acid. Alkalinity samples were collected and placed on ice for titration after the site visit. After replacing the sampling chamber, unfiltered samples were collected for general chemistry, metals (preserved with nitric acid), nutrients and total organic compounds (preserved with sulfuric acid), and cyanide (preserved with sodium hydroxide).
All samples were placed on ice following collection and preservation and transported to the PaDEP laboratory in Harrisburg, Pennsylvania, for analysis; laboratory methods for each parameter suite can be found in
Throughout the course of the project, the reporting limits have changed for several constituents, mostly trace elements. This has largely consisted of increasing reporting limits (less sensitive detection levels), leading to reported values of some chemical concentrations measured from earlier samples that are less than later reporting limits. To account for changing reporting limits, each constituent was censored to the highest-available reporting limit, with all values below that level being up-censored to that reporting limit. Filtered constituents containing the most up-censored values include lead (82 percent of samples censored to the highest reporting limit), mercury (73 percent), selenium (71 percent), and antimony (67 percent). Unfiltered constituents containing the most up-censored values include silver (94 percent), orthophosphate (77 percent), mercury (70 percent), and beryllium (65 percent). To ensure consistent statistical analysis, values below the highest lab-established reporting limit for a given constituent have been up-censored to the highest reporting limit; a list of parameters with up-censored values is presented in
Table 2. Counts and percentages of censored constituent values following up-censoring to account for changes to Pennsylvania Department of Environmental Protection Bureau of Laboratories reporting limits.
[µg/L, micrograms per liter; mg/L, milligrams per liter; P, phosphorus]
Constituent | Reporting limit | Units | Number (percent) of censored values |
Inorganic constituents | |||
---|---|---|---|
Aluminum, dissolved | 8.92 | µg/L | 34 (15) |
Aluminum, total, recovered | 8.92 | µg/L | 16 (7) |
Antimony, dissolved | 1.6372 | µg/L | 148 (67) |
Antimony, total | 1.64 | µg/L | 136 (62) |
Arsenic, dissolved | 0.66 | µg/L | 102 (46) |
Arsenic, total | 0.661 | µg/L | 126 (57) |
Beryllium, total, recovered | 0.123 | µg/L | 144 (65) |
Bromide, total | 25 | µg/L | 51 (23) |
Cadmium, dissolved | 0.03 | µg/L | 112 (51) |
Cadmium, total | 0.025 | µg/L | 82 (37) |
Chromium, dissolved | 0.483 | µg/L | 29 (13) |
Chromium, total, recovered | 0.483 | µg/L | 14 (6) |
Cobalt, total, recovered | 0.101 | µg/L | 64 (29) |
Copper, dissolved | 0.412 | µg/L | 104 (47) |
Copper, total, recovered | 0.2743 | µg/L | 20 (9) |
Fluoride, dissolved | 0.0231 | mg/L | 54 (24) |
Fluoride, total | 0.0262 | mg/L | 53 (24) |
Iron, dissolved | 18 | µg/L | 54 (24) |
Iron, total, recovered | 18 | µg/L | 16 (7) |
Lead, dissolved | 0.101 | µg/L | 181 (82) |
Lead, total, recovered | 0.101 | µg/L | 114 (52) |
Lithium, dissolved | 3 | µg/L | 21 (10) |
Lithium, total, recovered | 3 | µg/L | 22 (10) |
Manganese, dissolved | 0.67 | µg/L | 46 (21) |
Manganese, total, recovered | 0.67 | µg/L | 33 (15) |
Mercury, dissolved | 0.15 | µg/L | 162 (73) |
Mercury, total, recovered | 0.15 | µg/L | 154 (70) |
Molybdenum, dissolved | 0.625 | µg/L | 92 (42) |
Molybdenum, total, recovered | 0.625 | µg/L | 126 (57) |
Nickel, dissolved | 2.25 | µg/L | 131 (59) |
Nickel, total, recovered | 2.25 | µg/L | 131 (59) |
Selenium, dissolved | 0.8 | µg/L | 158 (71) |
Selenium, total | 0.763 | µg/L | 137 (62) |
Silver, dissolved | 0.095 | µg/L | 153 (69) |
Silver, total, recovered | 0.12 | µg/L | 208 (94) |
Thallium, dissolved | 0.204 | µg/L | 71 (32) |
Thallium, total | 0.204 | µg/L | 94 (43) |
Uranium, dissolved | 0.328 | µg/L | 140 (63) |
Uranium, total | 0.33 | µg/L | 65 (29) |
Zinc, dissolved | 4.98 | µg/L | 132 (60) |
Zinc, total, recovered | 4.98 | µg/L | 123 (56) |
Organic constituents | |||
Bromomethane, total | 0.5 | µg/L | 14 (50) |
Dibromomethane, total | 0.5 | µg/L | 14 (50) |
4-Isopropyltoluene, total | 0.5 | µg/L | 14 (50) |
CHBrCl2, total | 2 | µg/L | 21 (75) |
Tetrachloromethane, total | 0.5 | µg/L | 14 (50) |
1,2-Dichloroethane, total | 0.5 | µg/L | 14 (50) |
Tribromomethane, total | 1 | µg/L | 9 (32) |
Dibromochloromethane, total | 2 | µg/L | 21 (75) |
Trichloromethane, total | 2 | µg/L | 21 (75) |
Toluene, total | 0.5 | µg/L | 14 (50) |
Benzene, total | 0.5 | µg/L | 14 (50) |
Chlorobenzene, total | 0.5 | µg/L | 14 (50) |
Chloroethane, total | 0.5 | µg/L | 14 (50) |
Ethylbenzene, total | 0.5 | µg/L | 14 (50) |
Chloromethane, total | 0.5 | µg/L | 14 (50) |
Dichloromethane, total | 0.5 | µg/L | 14 (50) |
Tetrachloroethene, total | 0.5 | µg/L | 14 (50) |
CFC-11, total | 0.5 | µg/L | 14 (50) |
1,1-Dichloroethane, total | 0.5 | µg/L | 14 (50) |
1,1-Dichloroethene, total | 0.5 | µg/L | 14 (50) |
1,1,1-Trichloroethane, total | 0.5 | µg/L | 14 (50) |
1,1,2-Trichloroethane, total | 0.5 | µg/L | 14 (50) |
1,1,2,2-Tetrachloroethane, total | 0.5 | µg/L | 14 (50) |
1,2-Dichlorobenzene, total | 0.5 | µg/L | 14 (50) |
1,2-Dichloropropane, total | 0.5 | µg/L | 14 (50) |
trans-1,2-Dichloroethene, total | 0.5 | µg/L | 14 (50) |
1,2,4-Trichlorobenzene, total | 0.5 | µg/L | 14 (50) |
1,3-Dichlorobenzene, total | 0.5 | µg/L | 14 (50) |
1,4-Dichlorobenzene, total | 0.5 | µg/L | 14 (50) |
CFC-12, total | 0.5 | µg/L | 14 (50) |
Naphthalene, total | 0.5 | µg/L | 14 (50) |
trans-1,3-Dichloropropene, total | 0.5 | µg/L | 14 (50) |
cis-1,3-Dichloropropene, total | 0.5 | µg/L | 14 (50) |
Vinyl chloride, total | 0.5 | µg/L | 14 (50) |
Trichloroethene, total | 0.5 | µg/L | 14 (50) |
Methane, w, diss | 0.5 | mg/L | 8 (30) |
Orthophosphate, total | 0.02 | mg/L as P | 170 (77) |
tert-Butyl alcohol, total | 5 | µg/L | 14 (50) |
Carbon disulfide, total | 0.5 | µg/L | 14 (50) |
cis-1,2-Dichloroethene, total | 0.5 | µg/L | 14 (50) |
n-Butyl methyl ketone, total | 2.5 | µg/L | 14 (50) |
Styrene, total | 0.5 | µg/L | 14 (50) |
m-Xylene, total | 1 | µg/L | 14 (50) |
o-Xylene, total | 0.5 | µg/L | 14 (50) |
1,1-Dichloropropene, total | 0.5 | µg/L | 14 (50) |
2,2-Dichloropropane, total | 0.5 | µg/L | 14 (50) |
1,3-Dichloropropane, total | 0.5 | µg/L | 14 (50) |
1,2,4-Trimethylbenzene, total | 0.5 | µg/L | 14 (50) |
Isopropylbenzene, total | 0.5 | µg/L | 14 (50) |
n-Propylbenzene, total | 0.5 | µg/L | 14 (50) |
1,3,5-Trimethylbenzene, total | 0.5 | µg/L | 14 (50) |
2-Chlorotoluene, total | 0.5 | µg/L | 14 (50) |
4-Chlorotoluene, total | 0.5 | µg/L | 14 (50) |
Bromochloromethane, total | 0.5 | µg/L | 14 (50) |
n-Butylbenzene, total | 0.5 | µg/L | 14 (50) |
sec-Butylbenzene, total | 0.5 | µg/L | 14 (50) |
tert-Butylbenzene, total | 0.5 | µg/L | 14 (50) |
1,2,3-Trichloropropane, total | 0.5 | µg/L | 14 (50) |
1,2,3-Trichlorobenzene, total | 0.5 | µg/L | 14 (50) |
MTBE, total | 0.5 | µg/L | 14 (50) |
Isobutyl methyl ketone, total | 2.5 | µg/L | 14 (50) |
Acetone, total | 10 | µg/L | 20 (71) |
Bromobenzene, total | 0.5 | µg/L | 14 (50) |
Tetrahydrofuran, total | 1 | µg/L | 4 (14) |
For quality control (QC), an equipment blank was collected using a randomly selected submersible pump and tubing section prior to installation to evaluate the potential effects of the sampling apparatus (sampling pumps, tubes, and filters) on the water chemistry results. At each sampling site, pesticide-grade blank water with a deionized (DI)-water purged filter was processed as the equipment blank for dissolved organic compounds analysis. With few exceptions, these blanks registered constituent concentrations below reporting limits. Additionally, replicate samples were collected at several wells during the course of the project, including at SO 854 during the first sampling event at that well during the fall 2018 sampling season and FO 11 and SQ 61 during the spring 2019 sampling season. Duplicate pairs of filtered or unfiltered samples were collected sequentially (standard sample followed by replicate sample). Results from replicate samples indicate reproducibility was within 5 percent for most major ions and trace elements at concentrations that were greater than two times the reporting limit and within 20 percent for samples that were less than two times of the reporting limit. For the replicate samples collected from FO 11, there was a 14.3 percent difference between environmental (6,600 micrograms per liter [µg/L]) and replicate (7,700 µg/L) results for total iron and a 45.6 percent difference between environmental (2.65 µg/L) and replicate (1.44 µg/L) results for molybdenum. For the replicate samples collected from SO 854, there was a 44.5 percent difference between environmental (17.98 milligrams per liter [mg/L]) and replicate (9.97 mg/L) results for chloride and a 9.3 percent difference between environmental (2.83 mg/L) and replicate (2.59 mg/L) results for calcium.
For quality assurance (QA), intrasample characteristics of inorganic chemical analyses were evaluated using standard procedures described by
Additional QA/QC checks involved comparisons of (1) the computed cation and anion equivalents concentrations and the corresponding ionic charge balance, (2) the ratios of cation or anion equivalents to measured specific conductance (SC), (3) the measured residue on evaporation (ROE) at 180 degrees Celsius (°C) to the computed total dissolved solids (TDS) as the sum of major ion concentrations, and (4) the measured SC to the computed SC. The ionic charge balance and computed SC were estimated with PHREEQC (
Comparison of field, laboratory, and (or) computed values of pH and specific conductance (SC) and total dissolved solids (TDS) for 221 samples collected from 28 wells within the Pennsylvania Groundwater Monitoring Network, 2015–19.
Figure 5. Comparison of field, laboratory, and (or) computed values of pH and specific conductance and total dissolved solids for 221 samples collected from 28 wells within the Pennsylvania Groundwater Monitoring Network, 2015–19.
Data analysis of samples within the GWMN focuses on several categorical variables, including geology and physiography, land use, topographic position, and drought severity. These variables were used to subset the samples into various groups for targeted analysis that were used to determine if common traits were shared among wells within the GWMN. Spatial data was accessed through the Pennsylvania Bureau of Topographic and Geologic Survey and accessed from the Pennsylvania Spatial Data Access (PASDA) database (
Land use was calculated for a boundary radius of 1,000 meters (m) around each well location. Land use data were gathered from the 2011 National Land Cover Database (
Land use within 1,000 meters of each sampled well within the Pennsylvania Groundwater Monitoring Network. Land use data from the 2011 National Land Cover Database (
Figure 6. Land use within 1,000 meters of each sampled well within the Pennsylvania Groundwater Monitoring Network.
The topographic position for each well in the GWMN was determined using a digital elevation model (DEM) raster dataset to evaluate peaks, valleys, and slopes on the landscape (Jenness, 2006). The DEM was converted to a raster dataset with 10-m resolution that evaluated the slope of each point in the file. To access slope values for each well, a spatial join was performed to match each well with the closest cell in the raster dataset. Topographic position was categorized into three general categories (valley, slope, and ridge) based on examination of regional slope and elevation patterns.
The Palmer Drought Severity Index (PDSI) estimates the relative dryness of a location using temperature, soil moisture, and precipitation data (Palmer, 1965; Dai and others, 2019). PDSI is a tool that can be used to measure periods of long-term wetness or drought over a regional space; limitations of PDSI include the limited spatial areas that were used to define the index, the absence of a way to handle frozen ground surfaces, and the arbitrary designation of parameters such as depth of soil where moisture is available to be mobilized via evapotranspiration (
All analysis was completed using R, an open-source environment for statistical computing (
The summary statistics of sampled constituents were computed using a dataset that had censored values removed to capture the distribution of values with detectable concentrations at the highest reporting limit of a particular constituent; these results are presented alongside EPA drinking water standards including maximum contaminant levels (MCLs;
Principal components analysis (PCA) and hierarchical clustering were computed with the FactoMineR package (Lê S and Husson, 2008) and used to evaluate multivariate correlations among dissolved constituents and field parameters including pH, specific conductance, and dissolved oxygen. Results of the PCA were used to identify geochemical processes or variables that explain patterns of constituent distributions and associations (
An analysis of differences between total and dissolved constituents was performed to evaluate the sensitivity of the laboratory’s instrumentation with regard to whole and filtered samples. Each sample set consists of filtered and unfiltered samples for major ions, trace elements, and nutrients. The nonparametric Wilcoxon rank-sum test was performed using the exactRankTests R package (Hothorn and Hornik, 2019). Within each rank sum test, filtered and unfiltered samples were paired to ensure that evaluations were being performed on the two variables for each sample collected; values that were unable to be paired were dropped from analysis. The exact Wilcoxon rank-sum test was employed owing to its robustness when evaluating non-normal distributions of data and because it uses the Streitber-Röhmel shift algorithm to handle ties within the dataset; this is particularly useful when analyzing environmental data, when many of the values in a dataset are either at or just above the reporting limit (
Wilcoxon rank-sum tests were also used to evaluate differences between samples that were collected in times of greater than average precipitation (PDSI > 0) and times of less than average precipitation (PDSI < 0). Rank-sum tests were performed using the PDSI values from the month that a particular well was sampled, as well as values lagged from 1 to 9 months prior to the sample to capture differences that may occur in long-term groundwater movement. Constituents, including calcium, magnesium, sodium, chloride, and lithium were evaluated for differences between positive and negative PDSI phases. Differences that may be caused by the changes in sampling seasons (such as spring and fall) were evaluated on both a network and by individual well using Wilcoxon rank-sum tests. Only constituents with more than 50 percent of recorded values above the reporting limit were included in this analysis. For the analysis of individual wells, only wells that had at least four spring and four fall samples were included to account for anomalously high or low readings that may be present. Temporal trends in constituent values among these wells were also evaluated using the Mann-Kendall Trend test; this test was performed using the trend R package (
Results of the 221 groundwater quality samples are presented in the context of drinking-water standards and discussed below. Three standard criteria—MCLs, SMCLs, and HALs—are considered (
Table 3. Results of Wilcoxon rank-sum tests that compare the paired filtered and unfiltered samples for selected constituents for 221 samples collected from 28 wells within the Pennsylvania Groundwater Monitoring Network, 2015–19.
[µg/L, micrograms per liter; mg/L, milligrams per liter; N, nitrogen]
Constituent | Units | Number of comparisons | P-value | Minimum difference | Median difference | Maximum difference |
Aluminum | µg/L | 49 | 0 | –52 | 11.1 | 1,439 |
Arsenic | µg/L | 73 | 0 | –0.72 | 0.06 | 7.64 |
Barium | µg/L | 220 | 0 | –20 | 0.4 | 67.9 |
Cadmium | µg/L | 57 | 0 | –0.067 | 0.003 | 0.1 |
Iron | µg/L | 125 | 0 | –400 | 180 | 22,900 |
Manganese | µg/L | 164 | 0 | –70 | 1.04 | 213.3 |
Phosphorus | mg/L as phosphorus | 91 | 0 | –0.01 | 0.001 | 0.064 |
Silica | mg/L | 219 | 0 | –7.87 | –0.07 | 5.838 |
Ammonia | mg/L as N | 72 | 0.0001 | –0.03 | 0 | 0.108 |
Potassium | mg/L | 219 | 0.0004 | –0.52 | 0.01 | 0.385 |
Copper | µg/L | 111 | 0.0005 | –2.08 | 0 | 6.89 |
Zinc | µg/L | 38 | 0.0011 | –8.64 | 0.5 | 152.7 |
Calcium | mg/L | 219 | 0.0810 | –5.2 | 0.01 | 4.3 |
Nickel | µg/L | 31 | 0.0951 | –1.34 | 0.01 | 1.89 |
Lithium | µg/L | 96 | 0.1160 | –10 | 0 | 10 |
Sulfate | mg/L | 203 | 0.1510 | –2 | 0 | 2 |
Sodium | mg/L | 219 | 0.1658 | –12 | 0.001 | 9 |
Magnesium | mg/L | 219 | 0.1691 | –2 | 0 | 0.6 |
Strontium | µg/L | 181 | 0.3094 | –30 | 0 | 60 |
Chloride | mg/L | 202 | 0.4189 | –109 | 0 | 4 |
Nitrate | mg/L as N | 129 | 0.6016 | –0.4 | 0 | 0.18 |
Molybdenum | µg/L | 46 | 0.6100 | –2.04 | 0.001 | 14.271 |
Boron | µg/L | 66 | 0.8621 | –10 | 0 | 11 |
Selenium | µg/L | 13 | 0.8799 | –0.9 | –0.1 | 0.5 |
Fluoride | mg/L | 144 | 0.9678 | –0.88 | 0 | 0.06 |
Thallium | µg/L | 109 | 0.9998 | –0.02 | 0 | 0.7 |
Filtered and unfiltered samples were collected for analysis of major ions and trace elements to evaluate differences and determine if both filtered and unfiltered samples are necessary. Filtering samples removes suspended particles greater than 0.45 microns, along with any chemicals sorbed onto those particles. Conversely, suspended particles and associated chemicals sorbed onto them are not removed from unfiltered samples.
Filtering groundwater samples is a standardized procedure that reduces variability associated with the disturbance of a well or aquifer by sampling and is considered more representative of ambient conditions (U.S. Geological Survey, variously dated). However, the suspended particles may dissolve in the human digestive tract once consumed. Thus, unfiltered samples are important for characterizing drinking water because private domestic supplies may not have filtration and (or) disinfection systems.
The differences between filtered and unfiltered constituent concentrations in groundwater arise as a result of varying levels of solubility; some constituents readily dissolve in water, whereas others have more restricted parameters for dissolution, leading to expected higher concentration levels in unfiltered samples than in their filtered counterparts. To determine if there are significant differences between the total and filtered samples, a Wilcoxon rank-sum test was used to compare the values collected for each constituent. Results of Wilcoxon rank-sum tests are presented in
Concentrations of constituents with higher differences between filtered and total concentrations, which included arsenic, iron, and manganese, are shown in cumulative distribution frequency plots (
Cumulative distribution frequency plots of total and dissolved arsenic, iron, and manganese using paired samples from Pennsylvania Groundwater Monitoring Network samples, 2015–19.
Figure 7. Cumulative distribution frequency plots of total and dissolved arsenic, iron, and manganese using paired samples from Pennsylvania Groundwater Monitoring Network samples, 2015–19.
Physical and chemical properties that are measured during well sampling include water temperature, dissolved oxygen, pH, specific conductance, turbidity, and alkalinity. Summary statistics for these physical and chemical properties and associated measures such as the air temperature and barometric pressure are given in
Table 4. Minimum, median, and maximum values of physical and chemical properties for 221 water samples collected from 28 wells within the Pennsylvania Groundwater Monitoring Network, 2015–19.
[°C, degrees Celsius, µS/cm, microsiemens per centimeter; mg/L, milligrams per liter; mV, millivolts; NTRU, Nephelometric Turbidity Ratio Units; %, percent; CaCO3, calcium carbonate; USGS, U.S. Geological Survey; NFM, National Field Manual; SM, Standard Methods for the Examination of Water and Wastewater; EPA, U.S. Environmental Protection Agency; MCL, maximum contaminant level; SMCL, secondary maximum contaminant; level,wu, undissolved water sample; wu,recov, undissolved water sample, recovered; dissolved, dissolved water sample; fld, field measurement; --, no MCL or SMCL established]
Parameter | Units | Method | Number (percent) above reporting level | Minimum | Median | Maximum | Number (percent) exceeding standard | EPA MCL | EPA SMCL |
Temperature, water | °C | USGS NFM chapter A6 | 221 (100) | 7.0 | 10.1 | 13.3 | 0 (0) | -- | -- |
Temperature, air | °C | USGS NFM chapter A6 | 133 (60) | –23.9 | 11.8 | 27 | 0 (0) | -- | -- |
Specific conductivity at 25 °C | µS/cm at 25 °C | USGS NFM chapter A6 | 219 (99) | 18 | 203.3 | 1,160 | 0 (0) | -- | -- |
Dissolved oxygen | mg/L | USGS NFM chapter A6 | 215 (97) | 0 | 2.85 | 13.9 | 0 (0) | -- | -- |
Dissolved oxygen, percent saturation | % Saturation | USGS NFM chapter A6 | 209 (95) | 0 | 26.5 | 123 | 0 (0) | -- | -- |
pH | pH units | EPA 200.7 | 220 (100) | 4.2 | 6.85 | 8.3 | 92 (42) | -- | <6.5 or >8.5 |
pH, lab | pH units | EPA 200.7 | 203 (92) | 5 | 7.35 | 8.5 | 0 (0) | -- | -- |
Alkalinity, dissolved, lab | mg/L as CaCO3 | USGS NFM chapter A6 | 191 (86) | 0.5 | 60.8 | 252 | 0 (0) | -- | -- |
Alkalinity, dissolved, field | mg/L as CaCO3 | USGS NFM chapter A6 | 191 (86) | 0.3 | 63.1 | 262 | 0 (0) | -- | -- |
Hardness, water | mg/L as CaCO3 | SM 2340 B | 219 (99) | 5.57 | 65.8 | 326 | 0 (0) | -- | -- |
Turbidity | NTRU | USGS NFM chapter A6 | 164 (74) | 0.1 | 1.85 | 340 | 59 (36) | 5 | -- |
Water temperatures for wells sampled in this study ranged from 7.0 to 13.3 °C, with a median of 10.1 °C (
The concentrations of dissolved oxygen (DO) ranged from 0 to 13.9 mg/L, with a median value of 2.85 mg/L. Although some wells, including LB 372, PI 522, WE 300, and WN 64, exhibited a range of DO concentrations, most other wells in the network consistently display anoxic or oxic conditions (
Relation between pH and dissolved oxygen (DO) concentrations in Pennsylvania Groundwater Monitoring Network samples, 2015−19. Anoxic water contains <0.5 milligrams per liter (mg/L) of DO, mixed water contains ≥0.5 mg/L but <2 mg/L of DO, and oxic water contains ≥2 mg/L of DO.
Figure 8. Graph showing relation between pH and dissolved oxygen concentrations in Pennsylvania Groundwater Monitoring Network samples, 2015−19.
The pH of water is a measure related to acidity and the potential of water to be corrosive and leach metals from pipes and plumbing (
Alkalinity is a measure of water’s ability to neutralize acid and is commonly the result of carbonate and bicarbonate ions in solution (
Water hardness, which is computed as the sum of dissolved calcium multiplied by a factor of 2.5 plus dissolved magnesium multiplied by a factor of 4.1, ranged from 5.57 to 326 mg/L as calcium carbonate (CaCO3;
Relation between pH and total hardness in Pennsylvania Groundwater Monitoring Network samples, 2015−19. Soft water contains hardness ≤60 mg/L, moderately hard water contains hardness from >61 mg/L to ≤120 mg/L, hard water contains hardness from >121 mg/L to ≤180 mg/L, and very hard water contains hardness >180 mg/L.
Figure 9. Graph showing relation between pH and total hardness in Pennsylvania Groundwater Monitoring Network samples, 2015−19.
Specific conductance (SC), expressed in units of microsiemens per centimeter (µS/cm), is a gross measure of the ability of ions in water to conduct an electrical current (
Relation between field-measured specific conductance and total dissolved solids concentrations in Pennsylvania Groundwater Monitoring Network samples, 2015–19. Dashed line represents linear relation (R2 = 0.87) between measured specific conductance and laboratory measured dissolved solids.
Figure 10. Graph showing relation between field-measured specific conductance and total dissolved solids concentrations in Pennsylvania Groundwater Monitoring Network samples, 2015–19.
Turbidity is a measure of the number of solid particles that are suspended in water that block the transmission of light through a sample. Turbidity is expressed in nephelometric turbidity ratio units (NTRU), which quantifies the degree to which light is scattered by the suspended particles in the sample; higher NTRU readings indicate a more turbid sample. Turbidity concentrations in 164 samples range from 0.1 to 340 NTRU, with a median concentration of 1.85 NTRU. In general, higher NTRU concentrations are associated with samples that have elevated levels of constituents that are commonly suspended in water. Fifty-nine samples (36.0 percent) from 6 wells (21 percent of wells) had a turbidity value greater than the EPA MCL of 5 NTRU for public water systems; this includes all or most samples collected from wells CU2, UN 51, FO 11, WN 64, TI 470, and SQ 61 (
Major ions (
Table 5. Minimum, median, and maximum concentrations of major ions for 221 samples collected from 28 wells within the Pennsylvania groundwater Monitoring Network, 2015–19.
[mg/L, milligrams per liter; °C, degrees Celsius; EPA, U.S. Environmental Protection Agency; MCL, maximum contaminant level; SMCL, secondary maximum contaminant level; HAL, health advisory level; total, total water sample; total, recoverable, total water sample, recovered; dissolved, dissolved water sample; --, no MCL or SMCL established; * indicates a HAL, these are not included in counting samples that exceed a standard]
Parameter | Units | Method | Number (percent) above reporting level | Minimum | Median | Maximum | Number (percent) exceeding standard | EPA MCL | EPA SMCL/HAL |
Calcium, dissolved | mg/L | EPA 200.7 | 219 (99) | 1.42 | 19.7 | 91.6 | 0 (0) | -- | -- |
Calcium, total, recoverable | mg/L | EPA 200.7 | 220 (100) | 1.42 | 20.2 | 91.6 | 0 (0) | -- | -- |
Magnesium, dissolved | mg/L | EPA 200.7 | 219 (99) | 0.45 | 4.51 | 25.1 | 0 (0) | -- | -- |
Magnesium, total, recoverable | mg/L | EPA 200.7 | 220 (100) | 0.46 | 4.56 | 24.4 | 0 (0) | -- | -- |
Sodium, dissolved | mg/L | EPA 200.7 | 220 (100) | 0.38 | 6.24 | 239 | 0 (0) | -- | 20* |
Sodium, total, recoverable | mg/L | EPA 200.7 | 219 (99) | 0.37 | 6.21 | 223 | 0 (0) | -- | 20* |
Potassium, dissolved | mg/L | EPA 200.7 | 219 (99) | 0.34 | 1.04 | 4.52 | 0 (0) | -- | -- |
Potassium, total, recoverable | mg/L | EPA 200.7 | 220 (100) | 0.37 | 1.03 | 4.6 | 0 (0) | -- | -- |
Chloride, dissolved | mg/L | EPA 300.0 | 203 (92) | 0.28 | 2.5 | 200 | 0 (0) | -- | 250 |
Chloride, total | mg/L | EPA 300.0 | 202 (91) | 0.3 | 2.58 | 200 | 0 (0) | -- | 250 |
Sulfate, dissolved | mg/L | EPA 300.0 | 203 (92) | 0.17 | 7.33 | 39 | 0 (0) | -- | 250 |
Sulfate, total | mg/L | EPA 300.0 | 203 (92) | 0.18 | 7.45 | 39 | 0 (0) | -- | 250 |
Fluoride, dissolved | mg/L | EPA 300.0 | 149 (67) | 0.03 | 0.06 | 1.04 | 0 (0) | 4 | 2 |
Fluoride, total | mg/L | EPA 300.0 | 150 (68) | 0.03 | 0.06 | 0.57 | 0 (0) | 4 | 2 |
Silica, dissolved | mg/L | EPA 200.7 | 219 (99) | 3.67 | 7.82 | 24.8 | 0 (0) | -- | -- |
Silica, total | mg/L | EPA 200.7 | 220 (100) | 0.64 | 8.16 | 25.4 | 0 (0) | -- | -- |
Dissolved solids, dried at 180 °C | mg/L | USGS I-1750 | 220 (100) | 10 | 124.5 | 632 | 7 (3) | -- | 500 |
Major ions are divided into two balanced groups consisting of positively charged cations (sodium, potassium, calcium, and magnesium) and negatively charged anions (chloride, nitrate, sulfate, fluoride, and bicarbonate). Additionally, silica is a major ion that is commonly uncharged, contributing to dissolved solids but not to specific conductance. Both filtered and unfiltered samples were analyzed to represent dissolved and total concentrations of major ions, respectively (
Total dissolved solids (TDS) is a measurement of the total amount of remaining solids following the evaporation of a water sample. TDS concentrations ranged from 10 to 632 mg/L, with a median concentration of 124.5 mg/L. Of 220 samples analyzed for TDS, only 7 (3.2 percent) from 2 wells exceeded the EPA SMCL of 500 mg/L. Six of the seven samples that exceeded 500 mg/L came from well BR 202, which also had the highest levels of dissolved sodium and chloride within the GWMN (median concentrations of 207 mg/L and 200 mg/L, respectively;
Concentrations of dissolved sodium ranged from 0.38 to 239 mg/L, with a median concentration of 6.24 mg/L (
Predominant water types collected from 28 wells within the Pennsylvania Groundwater Monitoring Network during the fall 2019 sampling season. The subset of wells labeled as high sodium contain sodium in concentrations greater than two times the expected 1:1 molar ratio of sodium and chloride in pure sodium chloride salt (NaCl).
Figure 11. Trilinear diagram showing predominant water types collected from 28 wells within the Pennsylvania Groundwater Monitoring Network during the fall 2019 sampling season.
Cumulative ion contributions of samples collected from 28 wells within the Pennsylvania Groundwater Monitoring Network during the fall 2019 sampling season. Positively charged cations are represented in shades of blue and negatively charged anions are represented in shades of red.
Figure 12. Cumulative ion contributions of samples collected from 28 wells within the Pennsylvania Groundwater Monitoring Network during the fall 2019 sampling season.
Metals and other trace elements are typically found in concentrations <1 mg/L in natural waters (
Table 6. Minimum, median, and maximum concentrations of metals and trace elements for 221 samples collected from 28 wells within the Pennsylvania Groundwater Monitoring Network, 2015–19.
[µg/L, micrograms per liter; EPA, U.S. Environmental Protection Agency; MCL, maximum contaminant level; SMCL, secondary maximum contaminant level; total, total water sample; total, recoverable, total water sample, recovered; dissolved, dissolved water sample; NA, not applicable; --, no MCL or SMCL established]
Parameter | Units | Method | Number (percent) above reporting level | Minimum | Median | Maximum | Number (percent) exceeding standard | EPA MCL | EPA SMCL |
Arsenic, dissolved | µg/L | EPA 200.8 | 82 (37) | 0.67 | 1.09 | 5.36 | 0 (0) | 10 | -- |
Arsenic, total | µg/L | EPA 200.8 | 95 (43) | 0.67 | 0.96 | 8.59 | 0 (0) | 10 | -- |
Barium, dissolved | µg/L | EPA 200.8 | 220 (100) | 0.66 | 54.7 | 985 | 0 (0) | 2,000 | -- |
Barium, total, recoverable | µg/L | EPA 200.8 | 221 (100) | 1.97 | 57.6 | 1,100 | 0 (0) | 2,000 | -- |
Beryllium, total, recoverable | µg/L | EPA 200.8 | 62 (28) | 0.13 | 0.13 | 0.53 | 0 (0) | 4 | -- |
Boron, dissolved | µg/L | EPA 200.7 | 66 (30) | 22 | 38 | 267 | 0 (0) | -- | -- |
Boron, total, recoverable | µg/L | EPA 200.7 | 69 (31) | 22 | 36 | 260 | 0 (0) | -- | -- |
Bromide, dissolved | µg/L | EPA 300.1 B | 25 (11) | 26 | 31 | 1,700 | 0 (0) | -- | -- |
Bromide, total | µg/L | EPA 300.1 B | 20 (9) | 25.2 | 310.5 | 1,530 | 0 (0) | -- | -- |
Cadmium, dissolved | µg/L | EPA 200.8 | 68 (31) | 0.04 | 0.04 | 0.34 | 0 (0) | 5 | -- |
Cadmium, total | µg/L | EPA 200.8 | 66 (30) | 0.03 | 0.04 | 0.16 | 0 (0) | 5 | -- |
Chromium, dissolved | µg/L | EPA 200.8 | 81 (37) | 0.54 | 0.74 | 0.83 | 0 (0) | 100 | -- |
Chromium, total, recoverable | µg/L | EPA 200.8 | 122 (55) | 0.54 | 0.83 | 7 | 0 (0) | 100 | -- |
Cobalt, total, recoverable | µg/L | EPA 200.8 | 8 (4) | 1 | 8 | 17 | 0 (0) | -- | -- |
Copper, dissolved | µg/L | EPA 200.8 | 112 (51) | 0.42 | 0.66 | 11.7 | 0 (0) | 1,300 | 1,000 |
Copper, total, recoverable | µg/L | EPA 200.8 | 199 (90) | 0.28 | 0.64 | 11.3 | 0 (0) | 1,300 | 1,000 |
Iron, total, recoverable | µg/L | EPA 200.7 | 193 (87) | 19 | 410 | 27,000 | 111 (50) | -- | 300 |
Iron, dissolved | µg/L | EPA 200.7 | 126 (57) | 19 | 83.5 | 27,000 | 70 (32) | -- | 300 |
Lead, dissolved | µg/L | EPA 200.8 | 31 (14) | 0.11 | 0.21 | 1.56 | 0 (0) | 15 | -- |
Lead, total, recoverable | µg/L | EPA 200.8 | 104 (47) | 0.11 | 0.29 | 12.9 | 0 (0) | 15 | -- |
Manganese, total, recoverable | µg/L | EPA 200.8 | 185 (84) | 0.72 | 29.6 | 1,260 | 86 (39) | -- | 50 |
Manganese, dissolved | µg/L | EPA 200.8 | 165 (75) | 0.77 | 19.95 | 1,250 | 79 (36) | -- | 50 |
Thallium, dissolved | µg/L | EPA 200.8 | 110 (50) | 0.21 | 0.22 | 1 | 0 (0) | 2 | -- |
Thallium, total | µg/L | EPA 200.8 | 109 (49) | 0.21 | 0.22 | 1 | 0 (0) | 2 | -- |
Molybdenum, dissolved | µg/L | EPA 200.8 | 59 (27) | 0.63 | 0.90 | 2.94 | 0 (0) | -- | -- |
Molybdenum, total, recoverable | µg/L | EPA 200.8 | 74 (33) | 0.63 | 0.93 | 14.9 | 0 (0) | -- | -- |
Nickel, dissolved | µg/L | EPA 200.8 | 37 (17) | 2.28 | 2.75 | 20.3 | 0 (0) | -- | -- |
Nickel, total, recoverable | µg/L | EPA 200.8 | 37 (17) | 2.28 | 3.55 | 19.8 | 0 (0) | -- | -- |
Silver, dissolved | µg/L | EPA 200.8 | 3 (1) | 0.19 | 0.23 | 0.34 | 0 (0) | -- | 100 |
Silver, total, recoverable | µg/L | EPA 200.8 | 2 (1) | 0.24 | 0.31 | 0.39 | 0 (0) | -- | 100 |
Strontium, dissolved | µg/L | EPA 200.7 | 182 (82) | 10 | 70 | 840 | 0 (0) | -- | -- |
Strontium, total, recoverable | µg/L | EPA 200.7 | 183 (83) | 10 | 83.5 | 900 | 0 (0) | -- | -- |
Zinc, dissolved | µg/L | EPA 200.8 | 44 (20) | 5.36 | 12.2 | 26 | 0 (0) | -- | 5,000 |
Zinc, total, recoverable | µg/L | EPA 200.8 | 61 (28) | 5.01 | 9.58 | 163 | 0 (0) | -- | 5,000 |
Antimony, dissolved | µg/L | EPA 200.8 | 59 (27) | 1.64 | 1.64 | 1.64 | 0 (0) | 6 | -- |
Antimony, total | µg/L | EPA 200.8 | 4 (2) | 1.78 | 2.5 | 5 | 0 (0) | 6 | -- |
Aluminum, dissolved | µg/L | EPA 200.8 | 49 (22) | 10.4 | 15.7 | 358 | 11 (5) | -- | 50 |
Aluminum, total, recoverable | µg/L | EPA 200.8 | 105 (48) | 10 | 38.95 | 1,460 | 45 (20) | -- | 50 |
Lithium, dissolved | µg/L | EPA 200.7 | 100 (45) | 4 | 8 | 235 | 0 (0) | -- | -- |
Lithium, total, recoverable | µg/L | EPA 200.7 | 99 (45) | 4 | 9 | 240 | 0 (0) | -- | -- |
Selenium, dissolved | µg/L | EPA 200.8 | 16 (7) | 1 | 1.2 | 6.5 | 0 (0) | 50 | -- |
Selenium, total | µg/L | EPA 200.8 | 16 (7) | 1 | 1 | 7 | 0 (0) | 50 | -- |
Mercury, dissolved | µg/L | EPA 245.1 | 0 (NA) | -- | -- | -- | 0 (0) | 2 | -- |
Mercury, total, recoverable | µg/L | EPA 245.1 | 0 (NA) | -- | -- | -- | 0 (0) | 2 | -- |
Iron, manganese, aluminum, and copper can have undesirable effects on the odor, taste, and color of water. Iron can cause water to have a rusty color and leave a red or orange stain on laundry and plumbing materials (
Spatial distribution of iron and manganese secondary maximum contaminant level exceedances of samples collected from 28 wells within the Pennsylvania Groundwater Monitoring Network during the fall 2019 sampling season.
Figure 13. Map showing the spatial distribution of iron and manganese secondary maximum contaminant level exceedances of samples collected from 28 wells within the Pennsylvania Groundwater Monitoring Network during the fall 2019 sampling season.
Relation between iron and manganese and dissolved oxygen (DO). Samples are split by those with low pH (≤6.5) and those with high pH (>6.5). Anoxic water contains <0.5 milligrams per liter (mg/L) of DO (below red dashed line), mixed water contains ≥0.5 mg/L but <2 mg/L of DO (between red and blue dashed line), and oxic water contains ≥2 mg/L of DO (above blue dashed line).
Figure 14. Graph showing the relation between iron and manganese and dissolved oxygen.
Dissolved copper concentrations ranged from 0.42 to 11.7 µg/L, with a median concentration of 0.66 µg/L; none of the 112 samples above the reporting limit were above the SMCL of 1 mg/L (1,000 µg/L). All the samples with >8 µg/L copper were collected from well BR 203; plumbing components may be responsible for the addition of copper to the water from this well and others. Aluminum is one of the most common minerals in Earth’s crust but is not commonly found dissolved in water in high concentrations unless the water also has a low pH (
Dissolved arsenic concentrations in the 82 samples that were above the reporting limit of 0.66 µg/L ranged from 0.67 to 5.36 µg/L, with a median concentration of 1.09 µg/L. Arsenic is naturally present in bedrock and sediments and can be mobilized by changes in redox or pH; mobilization can occur under oxidizing conditions where dissolved oxygen or nitrate oxidizes sulfide minerals that contain traces of arsenic, or under reducing conditions with high pH where arsenic adsorbed by iron oxide is released by reductive dissolution (
Concentrations of dissolved lead ranged from 0.11 to 1.56 µg/L, with a median concentration of 0.21 µg/L, whereas total lead ranged from 0.11 to 12.9 µg/L, with a median concentration of 0.29 µg/L. The differences between total and dissolved lead concentrations occur because of the low solubility of lead and its tendency to sorb to suspended particles (
The highest bromide concentrations (1,700 µg/L) were recorded at well BR 202, a level more than five times the median concentrations found in GWMN samples. This well had the highest median field-measured pH of 8.3 as well as the highest median concentrations for sodium (239 mg/L) and chloride (200 mg/L). Correlation analysis performed using the results of the PCA (
The concentrations of several trace element constituents are plotted against pH in
Selected major ion and trace element constituents and pH, grouped by oxic and anoxic water conditions. Anoxic water conditions contain <0.5 milligrams per liter (mg/L) of dissolved oxygen and mixed/oxic water conditions contain ≥0.5 mg/L of dissolved oxygen.
Figure 15. Graphs of selected major ion and trace element constituents and pH, grouped by oxic and anoxic water conditions.
Relation between median chloride concentrations and median chloride: bromide mass ratios in 28 wells in the Pennsylvania Groundwater Monitoring Network. Binary mixing curves are notated to represent various sources of chloride. Blue rectangle represents range of values expected in dilute groundwater.
Figure 16. Graph showing the relation between median chloride concentrations and median chloride: bromide mass ratios in 28 wells in the Pennsylvania Groundwater Monitoring Network.
Nutrients analyzed in this study included nitrogen and phosphorus species. Nitrogen species in water typically include nitrate, nitrite, and ammonia. Phosphorus is mainly present as orthophosphate. Although nutrients are naturally present in soil and rock, excessive concentrations of nutrients usually indicate an anthropogenic source, such as fertilizer or animal wastes, effluent from sewer systems and septic tanks, atmospheric deposition, and stormwater drainage. Excessive nitrate in drinking water can be dangerous for human consumption, especially for infants below the age of 6 months, as it disrupts the transport of oxygen in blood. The majority of well locations in the GWMN are in forested areas, although some wells are located in areas of agricultural land use. Summary statistics of the nutrients analyzed are presented in
Table 7. Minimum, median, and maximum concentrations of nutrients for 221 samples collected from 28 wells within the Pennsylvania Groundwater Monitoring Network, 2015–19.
[mg/L as N, milligrams per liter as nitrogen; mg/L as P, milligrams per liter as phosphorus; EPA, U.S. Environmental Protection Agency; MCL, maximum contaminant level; SMCL, secondary maximum contaminant level; total, total water sample; total,recov, total water sample, recovered; dissolved, dissolved water sample; --, no MCL or SMCL established]
Parameter | Units | Method | Number (percent) above reporting level | Minimum | Median | Maximum | Number (percent) exceeding standard | EPA MCL | EPA SMCL |
Ammonia, dissolved | mg/L as N | EPA 350.1 | 77 (35) | 0.02 | 0.05 | 0.33 | 0 (0) | -- | -- |
Ammonia, total | mg/L as N | EPA 350.1 | 78 (35) | 0.03 | 0.05 | 0.34 | 0 (0) | -- | -- |
Nitrite, dissolved | mg/L as N | EPA 353.2 | 4 (2) | 0.02 | 0.03 | 0.04 | 0 (0) | 1 | -- |
Nitrite, total | mg/L as N | EPA 353.2 | 4 (2) | 0.02 | 0.03 | 0.04 | 0 (0) | 1 | -- |
Nitrate, dissolved | mg/L as N | EPA 353.2 | 129 (58) | 0.08 | 0.5 | 12.1 | 7 (3) | 10 | -- |
Nitrate, total | mg/L as N | EPA 353.2 | 129 (58) | 0.09 | 0.5 | 12 | 6 (3) | 10 | -- |
Kjeldahl Nitrogen, dissolved | mg/L as N | EPA 351.2 | 18 (8) | 1.2 | 1.7 | 3.7 | 0 (0) | -- | -- |
Kjeldahl Nitrogen, total | mg/L as N | EPA 351.2 | 23 (10) | 1.1 | 1.75 | 3.2 | 0 (0) | -- | -- |
Phosphorus, total | mg/L as P | EPA 365.1 | 122 (55) | 0.01 | 0.02 | 0.15 | 0 (0) | -- | -- |
Phosphorus, dissolved | mg/L as P | EPA 365.1 | 97 (44) | 0.01 | 0.02 | 0.12 | 0 (0) | -- | -- |
Orthophosphate, dissolved | mg/L as P | EPA 365.1 | 89 (40) | 0.01 | 0.02 | 0.06 | 0 (0) | -- | -- |
Orthophosphate, total | mg/L as P | EPA 365.1 | 37 (17) | 0.02 | 0.03 | 0.07 | 0 (0) | -- | -- |
Organic carbon, total | mg/L | SM 5310C | 35 (16) | 0.52 | 0.61 | 1.4 | 0 (0) | -- | -- |
Organic carbon, dissolved | mg/L | SM 5310C | 27 (12) | 0.52 | 1 | 2.8 | 0 (0) | -- | -- |
Dissolved nitrate concentrations ranged from 0.08 to 12.10 mg/L, with a median concentration of 0.50 mg/L. Seven of the 129 (5.4 percent) samples above the reporting limit of 0.04 mg/L that were collected exceeded the EPA MCL of 10 mg/L; all of these samples were collected at the Lebanon County observation well LB 372, which is located in an urban setting and within a carbonate aquifer that is surrounded by heavy agricultural land use. The majority of nitrite samples analyzed had concentrations that were below the reporting limit of 0.01 mg/L. Only four wells (LB 372, AG 700, LA 1201, and VE 57) had concentrations greater than 0.01 mg/L and none were above the EPA MCL of 1.0 mg/L as N. Nitrite concentrations ranged from 0.02 to 0.04 mg/L, with a median concentration of 0.03 mg/L. Dissolved phosphorus concentrations from the 97 samples (44 percent) with concentrations greater than the reporting limit ranged from 0.01 to 0.12 mg/L, and had a median of 0.02 mg/L (
Dissolved organic carbon (DOC) can alter groundwater quality by acting as a pH buffer, interacting with DO and other ions in oxidation/reduction reactions, and through involvement in mineral dissolution/precipitation reactions (
Volatile organic compounds (VOCs) consist of a range of natural and synthetic carbon-based compounds; their properties include relatively low solubility in water and a high vapor pressure. VOCs are used for a wide range of industrial, commercial, and domestic applications and can enter groundwater through spills or deposition from atmospheric transport. VOCs are commonly found wherever human activities are ongoing; compounds including solvents, fuels, fumigants, and disinfection byproducts can enter the environment through pesticide application, leaking storage tanks, or spills. VOCs have historically been used and disposed of indiscriminately, leading to the widespread presence of these chemicals in the environment. The reporting level of the majority of VOCs sampled was either 0.5 µg/L or 1.0 µg/L; of the 68 VOCs that were sampled during the first sampling event at each well, only one, tribromomethane, was detected.
Tribromomethane was measured at a level of 2 µg/L in well FO 11; this trihalomethane has historically been used as a solvent and is a byproduct of water chlorination processes but is also produced naturally by seaweeds and phytoplankton (
Radioactivity is the release of particles and energy from unstable elements as they decay to more stable forms. Radionuclides are naturally present in bedrock and soils and can be dissolved or leached into groundwater. There are three types of radioactive decay: alpha decay, in which positively charged helium particles are released; beta decay, in which positrons or electrons are released from an element; and gamma decay, in which electromagnetic waves are released from an element. Radioactivity in samples collected from the GWMN is measured in picocuries per liter, which is equal to the activity of one nanogram of radium and results are presented in
Table 8. Minimum, median, and maximum concentrations of radionuclides for 221 samples collected from 28 wells within the Pennsylvania Groundwater Monitoring Network, 2015–19.
[pCi/L, picocuries per liter; µg/L, micrograms per liter; EPA, U.S. Environmental Protection Agency; MCL, maximum contaminant level; SMCL, secondary maximum contaminant level; total, total water sample; dissolved, dissolved water sample; --, no MCL or SMCL established]
Parameter | Units | Method | Number (percent) above reporting level | Minimum | Median | Maximum | Number (percent) exceeding standard | EPA MCL | EPA SMCL |
Gross alpha activity, total | pCi/L | EPA 900.0 | 14 (6) | –4.02 | 0.32 | 6.89 | 0 (0) | 15 | -- |
Beta radioactivity, total | pCi/L | EPA 900.0 | 23 (10) | –0.26 | 1.83 | 7.57 | 0 (0) | -- | -- |
Radon-222, total | pCi/L | SM 7500-Ra B | 26 (12) | 40 | 500 | 7050 | 2 (1) | -- | 4,000 |
Radium-226, total | pCi/L | EPA 903.1 | 22 (10) | 0 | 0.18 | 2.09 | 0 (0) | -- | -- |
Radium-228, total | pCi/L | 18 (8) | –0.48 | 0.44 | 1.9 | 0 (0) | -- | -- | |
Uranium, dissolved | µg/L | EPA 200.8 | 36 (16) | 0.33 | 0.67 | 4.83 | 0 (0) | 30 | -- |
Uranium, total | µg/L | EPA 200.8 | 29 (13) | 0.34 | 0.93 | 4.94 | 0 (0) | 30 | -- |
The isotopes primarily responsible for naturally occurring radioactivity in groundwater are uranium-238 and thorium-232, which are present in minerals. These particles decay over time through a series of steps that emit alpha or beta radiation, producing mostly short-lived daughter products until a stable isotope of lead is produced; the majority of naturally occurring radioactivity is caused by the decay of uranium-238 (
Radon-222 is considered by the EPA to be the second-leading cause of lung cancer and the leading cause of lung cancer in nonsmokers, with an estimated 20,000 deaths in the United States attributed to it each year (
Spatial distribution of radon-222 proposed maximum contaminant level exceedances collected from 28 wells within the Pennsylvania Groundwater Monitoring Network, variously dated.
Figure 17. Map showing the spatial distribution of radon-222 proposed maximum contaminant level exceedances collected from 28 wells within the Pennsylvania Groundwater Monitoring Network, variously dated.
Gross alpha activity of 14 samples with activities above the reporting level ranged from –4.02 to 6.89 pCi/L, with a median concentration of 0.32 pCi/L, and the gross beta activity of 23 samples ranged from –0.26 to 7.57 pCi/L, with a median concentration of 1.83 pCi/L. Negative activities of gross alpha and beta radiation are the result of the sensitivity of laboratory instruments used to measure radiation; the random nature of radioactive decay makes it possible for background radiation to exceed the activity of the original sample (
Methane and other hydrocarbons can occur naturally in groundwater owing to anaerobic bacterial processes that break down organic materials in shallow aquifers (biogenic production) or through the breakup of organic matter at elevated temperatures and pressures in deep sedimentary strata (thermogenic production). Biogenic production typically results in hydrocarbons that consist almost entirely of methane, whereas thermogenic production can also contain higher-chain hydrocarbons in addition to methane. Results of hydrocarbon gas analysis are summarized in
Table 9. Minimum, median, and maximum concentrations of hydrocarbons collected from 28 wells within the Pennsylvania Groundwater Monitoring Network, 2015–19.
[µg/L, micrograms per liter; EPA, U.S. Environmental Protection Agency; MCL, maximum contaminant level; SMCL, secondary maximum contaminant level; total, total water sample; dissolved, dissolved water sample; PaDEP, Pennsylvania Department of Environmental Protection; BOL, Bureau of Laboratories; --, no MCL or SMCL established]
Parameter | Units | Method | Number (percent) above reporting level | Minimum | Median | Maximum | Number (percent) exceeding standard | EPA MCL | EPA SMCL |
Methane, dissolved | µg/L | PaDEP BOL6019 | 3 (1) | 22 | 4,210 | 8,230 | 0 (0) | -- | -- |
Ethane, dissolved | µg/L | PaDEP BOL6019 | 1 (0) | 1,890 | 1,890 | 1,890 | 0 (0) | ||
Ethene, total | µg/L | PaDEP BOL6019 | 0 (NA) | -- | -- | -- | 0 (0) | -- | -- |
Propane, total | µg/L | PaDEP BOL6019 | 0 (NA) | -- | -- | -- | 0 (0) | -- | -- |
Statistical analyses of groundwater quality data from the 221 samples collected from 2015 to 2019 were completed to characterize trends and patterns within the GWMN. A range of analyses were used to look for similarities and differences between the wells within the GWMN, changes occurring over time within the wells, and with the sampling design and methods. Results are evaluated in order to inform future decisions regarding changing or expanding the GWMN or sampling protocols.
Principal components analysis (PCA) is a statistical method used to identify major factors that can account for variability and associations among variables within a large dataset. The goal of PCA is to reduce the dimensionality by combining those variables on a smaller set of principal components while retaining most of the original variation (
Collectively, three principal components (PCs) explain 74.5 percent of variability contained in the groundwater quality data of the GWMN and consist of 20 commonly detected constituent loadings; they will be identified as PC1 (dissolved solids), PC2 (redox), and PC3 (sodium-chloride)(
Table 10. Principal components analysis of major factors controlling the chemistry of wells in the Pennsylvania Groundwater Monitoring Network.
[°C, degrees Celsius; PC, principal component; TDS, total dissolved solids; redox, reduction-oxidation; Na, sodium; Cl, chloride]
Parameter | PC1 (TDS) | PC2 (Redox) | PC3 (Na-Cl) |
Specific conductivity at 25 °C | 0.93 | 0.30 | 0.07 |
Alkalinity | 0.91 | 0.26 | –0.17 |
Strontium | 0.89 | 0.04 | –0.19 |
Dissolved solids dried at 180 °C | 0.88 | 0.30 | 0.09 |
Calcium | 0.82 | 0.39 | –0.13 |
Magnesium | 0.82 | 0.28 | –0.07 |
pH | 0.74 | 0.27 | –0.34 |
Fluoride | 0.74 | –0.32 | –0.30 |
Barium | 0.74 | –0.42 | 0.28 |
Sodium | 0.72 | 0.30 | 0.46 |
Potassium | 0.72 | –0.09 | 0.29 |
Lithium | 0.71 | –0.35 | –0.01 |
Silica | 0.66 | –0.02 | –0.05 |
Sulfate | 0.64 | 0.27 | –0.14 |
Phosphorus | 0.56 | –0.33 | –0.39 |
Chloride | 0.48 | 0.47 | 0.60 |
Iron | 0.41 | –0.75 | 0.19 |
Manganese | 0.39 | –0.85 | 0.20 |
Nitrate | –0.28 | 0.81 | –0.02 |
Dissolved oxygen | –0.69 | 0.38 | 0.04 |
Eigenvalue | 10.06 | 3.54 | 1.29 |
Variance explained | 50.32 | 17.70 | 6.44 |
Cumulative variance | 50.32 | 68.03 | 74.46 |
PC1 (dissolved solids), which explains 50.3 percent of the variability in the data, indicates the effects of mineral weathering on groundwater quality and contains positive loadings for SC; alkalinity; dissolved solids; major ions including calcium, magnesium, sodium, potassium, chloride, sulfate, and fluoride; and pH. PC1 contains negative loadings for dissolved oxygen (
Results of principal components analysis, principal components 1 and 2: parameter variables (
Figure 18. Results of principal components analysis, principal components 1 and 2: parameter variables and individual samples plots.
Moderately significant relations between pH and specific conductance (
Figure 19. Scatterplots of moderately significant relations between pH and specific conductance, and dissolved oxygen and specific conductance.
Wells that exhibited oxic conditions (DO >0.5 mg/L) generally contained lower alkalinity and major ion concentrations; the exception are wells located in carbonate aquifers, which contain concentrations above GWMN medians for all dissolved major ions. An inverse relation between DO and major ions suggests that (1) organic materials and soluble minerals, such as those found in carbonate rocks, are not present in the aquifer and (or) (2) recharge is not recent, leading to residence time that is sufficient for extensive interactions between water and bedrock that consume oxygen while releasing ions to solution. End-member wells with the highest individual contributions for PC1 include CN 1, BR 202, and AG 700; CN 1 is a highly oxic water with low ionic strength, whereas BR 202 and AG 700 exhibit some of the highest conductivities and alkalinities in the GWMN (
PC2 (redox) explains an additional 17.7 percent of variability in the dataset and has positive loadings by nitrate, DO, calcium, and chloride with negative loadings by manganese and iron (
PC3 (sodium chloride) accounts for an additional 6.4 percent of the variability in the GWMN; positive loadings include chloride, sodium, and potassium, and negative loadings include phosphorus, fluoride, and pH (
Relation between sodium and chloride concentrations in Pennsylvania Groundwater Monitoring network samples, 2015−19. Line of concentration equality represents a 1:1 equilibrium of sodium and chloride concentrations.
Figure 20. Relation between sodium and chloride concentrations in Pennsylvania Groundwater Monitoring network samples, 2015−19.
Results of the PCA model were subsequently used to create a hierarchical clustering classification to group wells that are most similar within the GWMN. Using the 200 samples from 28 wells that were used to establish PCs 1 through 3, four sampling clusters were visually identified (
Table 11. Results of hierarchical cluster analysis of principal components analysis results of the major factors controlling the chemistry of wells in the Pennsylvania Groundwater Monitoring Network.
[+, positive association with a well cluster; -, negative association with a well cluster; SC, specific conductance; TDS, total dissolved solids; DO, dissolved oxygen; Alk, alkalinity; NA, no association]
Cluster | Wells | SC | TDS | DO | pH | Alk | Ca | Mg | Na | K | Cl | SO4 | F | SiO2 | NO3 | P | Ba | Fe | Mn | Sr | Li |
1 | CB 104, CN 1, CU 2, PI 522, PO 72, SU 169, TI 470, UN 51, VE 57 | - | - | + | - | - | - | - | - | - | - | - | - | - | NA | - | - | - | NA | - | - |
2 | CF 321, FO 11, LY 796, SO 854, WY 197 | NA | NA | NA | NA | NA | NA | NA | NA | NA | NA | NA | NA | NA | - | NA | NA | + | + | NA | NA |
3 | AD 146, BR 203, CE 118, CH 10, LB 372, NU 579, WN 64 | + | + | NA | NA | + | + | + | NA | NA | + | NA | NA | NA | + | NA | NA | - | - | NA | NA |
4 | AG 700, BR 202, CW 2417, LA 1201, SQ 61, WE 300, WR 50 | + | NA | - | + | + | + | + | + | + | NA | NA | + | + | - | + | + | + | + | + | + |
Spatial distribution of clusters from hierarchical clustering of principal components analysis results of 28 wells within the Pennsylvania Groundwater Monitoring Network.
Figure 21. Map of the spatial distribution of clusters from hierarchical clustering of principal components analysis results of 28 wells within the Pennsylvania Groundwater Monitoring Network.
The Palmer Drought Severity Index (PDSI) offers a proxy to how wet or dry conditions are in a given location on a monthly time scale; a positive index value indicates that an area has received more precipitation over the previous 1-month time step, whereas a negative index value indicates that less precipitation fell compared to the previous month. Although drought conditions can lead to less runoff infiltrating into aquifers and lower water levels in aquifers, wetter conditions may increase infiltration rates and groundwater flow, leading to greater concentrations of dissolved constituents. A total of 162 samples collected between 2015 and 2018 were used in this analysis; computed PDSI values were not available for 2019. Several dissolved nutrients, major ions, and trace elements were compared to PDSI values from the month the sample was collected in, as well as the PDSI values from each of the 9 months prior to sample collection. Values that are below the reporting limit are replaced with normally distributed random values below the reporting limit for each constituent to account for missing values.
For each constituent and each time lagged PDSI value, a Kendall correlation was computed to determine if PDSI was an accurate indicator of constituent concentrations on a networkwide scale (
Table 12. Correlation matrix between selected water-quality constituents and time-lagged Palmer Drought Severity Index values collected from 28 wells within the Pennsylvania Groundwater Monitoring Network, 2015–18.
Parameter | Palmer Drought Severity Index | |||||||||
0-month lag | 1-month lag | 2-month lag | 3-month lag | 4-month lag | 5-month lag | 6-month lag | 7-month lag | 8-month lag | 9-month lag | |
Ammonia | –0.03 | –0.03 | –0.03 | –0.06 | –0.04 | –0.04 | –0.05 | –0.03 | –0.03 | 0 |
Nitrate | –0.01 | –0.02 | –0.04 | –0.07 | –0.08 | –0.07 | –0.09 | –0.08 | –0.07 | –0.08 |
Phosphate | –0.06 | –0.08 | –0.07 | –0.07 | –0.02 | –0.04 | –0.04 | –0.05 | –0.05 | –0.05 |
Orthophosphate | –0.03 | –0.05 | –0.09 | –0.09 | –0.10 | –0.18 | –0.20 | –0.21 | –0.20 | –0.18 |
Calcium | –0.04 | –0.04 | –0.05 | –0.09 | –0.10 | –0.09 | –0.11 | –0.08 | –0.07 | –0.08 |
Magnesium | –0.04 | –0.06 | –0.06 | –0.10 | –0.08 | –0.08 | –0.09 | –0.07 | –0.07 | –0.05 |
Sodium | 0.03 | 0.01 | 0.00 | –0.02 | –0.04 | –0.07 | –0.07 | –0.08 | –0.09 | –0.09 |
Potassium | –0.09 | –0.10 | –0.09 | –0.12 | –0.10 | –0.07 | –0.08 | –0.05 | –0.04 | –0.03 |
Chloride | 0.09 | 0.07 | 0.07 | 0.04 | 0.01 | –0.01 | –0.02 | –0.02 | –0.04 | –0.04 |
Sulfate | 0.14 | 0.13 | 0.13 | 0.11 | 0.10 | 0.08 | 0.06 | 0.06 | 0.05 | 0.03 |
Fluoride | 0.07 | 0.05 | 0.07 | 0.08 | 0.12 | 0.10 | 0.12 | 0.11 | 0.10 | 0.11 |
Silica | 0.03 | 0.00 | –0.02 | –0.04 | –0.02 | –0.08 | –0.10 | –0.11 | –0.11 | –0.11 |
Barium | 0.03 | 0.02 | 0.02 | 0.00 | 0.01 | –0.03 | –0.04 | –0.06 | –0.08 | –0.06 |
Strontium | 0.05 | 0.02 | 0.01 | –0.04 | –0.06 | –0.10 | –0.11 | –0.11 | –0.13 | –0.13 |
Lithium | 0.09 | 0.08 | 0.08 | 0.06 | 0.04 | 0.02 | 0.02 | –0.01 | –0.02 | –0.03 |
Well clusters produced from hierarchical clustering (
Seasonal differences in selected constituent concentrations in wells VE 57 (
Figure 22. Boxplots of seasonal differences in selected constituent concentrations in wells VE 57 and WR 50.
Biannual sampling results were examined to look for changes in constituent concentrations that may be attributed to the sampling season that the sample was collected in. Wells within the GWMN generally followed a pattern of higher winter and spring water levels and lower summer and fall water levels (
Wilcoxon rank-sum tests were used to compare several constituents that have been sampled throughout the GWMN over the course of several spring and fall sampling seasons. Tests were completed for all wells in the GWMN and on a subset of wells that had sufficient data to examine the seasonal differences on an individual scale. Of the 221 sampling events that have occurred since the inception of the GWMN, 100 samples have been collected in the spring (generally in April and May) and 121 samples have been collected in the fall (generally in October and November). Exact Wilcoxon rank-sum tests were applied for constituents that had values above the reporting limit for more than 50 percent of samples during both the spring and fall sampling periods, with data from all available samples from wells in the GWMN. With a significance level of P = 0.05, there are no filtered or unfiltered parameters that show a significant difference between the spring and fall sampling seasons, suggesting that there are no statewide patterns of constituent increases or decreases based on sampling season.
The second set of exact Wilcoxon rank-sum tests on seasonality examined samples from 16 of the 28 wells in the GWMN that have been sampled at least 8 times, with at least 4 spring and fall samples each. For these wells, 45 parameters with a sufficient number of results were used to perform Wilcoxon rank-sum tests between fall and spring sampling for at least one well. Results based on the number of significant differences (P <0.05) using a two-sided alternative hypothesis are presented for individual parameters (
Table 13. Counts of Wilcoxon rank-sum tests with significant results comparing spring and fall sampling seasons for selected constituents from 16 wells with long term records within the Pennsylvania Groundwater Monitoring Network, 2015–19.
[wu, unfiltered water sample; wu,recov, unfiltered water sample, recovered; wf, filtered water sample; °C, degrees Celsius; fixedEP,lab:fixed endpoint, laboratory; NephRatio, nephelometric turbidity ratio units; --, not applicable or none]
Constituent | Count of wells showing a significant seasonal difference |
Sodium, wf | 4 |
Sodium, wu,recov | 4 |
Barium, wf | 3 |
Barium, wu,recov | 3 |
Calcium, wf | 3 |
Calcium, wu,recov | 3 |
Dissolved solids dry at 180 °C | 3 |
Magnesium, wf | 3 |
Magnesium, wu,recov | 3 |
Specific conductance at 25 °C | 3 |
Sulfate, wf | 3 |
Sulfate, wu | 3 |
Alkalinity, wf,fixedEP,lab | 2 |
Chloride, wf | 2 |
Chloride, wu | 2 |
Hardness, water | 2 |
Manganese, wf | 2 |
Nitrate, wu | 2 |
Silica, wf | 2 |
Turbidity, NephRatio | 2 |
Dissolved oxygen | 1 |
Iron, wf | 1 |
Iron, wu,recov | 1 |
Manganese, wu,recov | 1 |
Potassium, wf | 1 |
Potassium, wu,recov | 1 |
Silica, wu | 1 |
Strontium, wf | 1 |
Strontium, wu,recov | 1 |
Table 14. Counts of Wilcoxon rank-sum tests with significant results comparing spring and fall sampling seasons for 16 wells with long term records within the Pennsylvania Groundwater Monitoring Network, 2015–19.
Well | Count of constituents showing a significant seasonal difference |
VE 57 | 16 |
WR 50 | 16 |
UN 51 | 8 |
WN 64 | 8 |
TI 470 | 3 |
CE 118 | 2 |
CN 1 | 2 |
LA 1201 | 2 |
CB 104 | 1 |
FO 11 | 1 |
LB 372 | 1 |
PI 522 | 1 |
PO 72 | 1 |
SQ 61 | 1 |
AG 700 | 0 |
CU 2 | 0 |
Increases or decreases in constituents over an extended period of time may indicate that aquifers are experiencing long-term changes to ambient groundwater quality. Changes may be the result of a variety of factors including but not limited to, precipitation patterns, road salt application, nutrient runoff from agricultural areas, and growth of developed areas that are proximal to sampling sites. The Mann-Kendall trend test was used to determine whether there are positive or negative trends among constituents in wells that have been sampled for at least four spring and four fall sampling seasons (
Table 15
[wu, unfiltered water sample; wu,recov, unfiltered water sample, recovered; wf, filtered water sample; Statistic, Z quantile of the standard normal distribution. Positive values indicate an increasing trend over time and negative values indicate a decreasing trend over time]
Parameter | P-value | Statistic |
Magnesium, wu,recov | 0.03 | 2.15 |
Beryllium, wu,recov | 0.05 | 1.96 |
Cadmium, wf | 0.05 | 1.96 |
Thallium, wf | 0.05 | 1.95 |
Thallium, wu | 0.05 | 1.95 |
Barium, wu,recov | 0.06 | 1.89 |
Magnesium, wf | 0.07 | 1.79 |
Manganese, wf | 0.07 | –1.79 |
pH | 0.08 | 1.72 |
Dissolved oxygen | 0.11 | 1.61 |
Phosphorus, wu | 0.15 | 1.45 |
Copper, wf | 0.15 | 1.45 |
Iron, wf | 0.15 | –1.44 |
Redox potential, relative to standard hydrogen electrode | 0.15 | –1.43 |
Zinc, wu,recov | 0.16 | –1.39 |
Generally, large increasing or decreasing trends were not observed on a statewide scale, in part owing to the diversity of settings of the GWMN wells and the relatively short period of current monitoring (2015–19). The only parameter that exhibited a significant change (P <0.05) over the period of record was total magnesium (P = 0.032). Both total and filtered magnesium (P = 0.074) showed an increasing trend over time, as did several other constituents including sodium and potassium. Decreasing trends over time were observed for constituents including calcium, iron, and manganese, although none of these trends are considered significant. Although changes were not apparent on a statewide scale, trends (significant or not) are more pronounced at individual sampling sites where factors affecting constituent levels have a direct impact on the water chemistry of a particular well. Forty-two parameters exhibited a significant change (P <0.05) over the period of record in at least 1 well (
Table 16. Mann-Kendall trend test results for individual wells within the Pennsylvania Groundwater Monitoring Network, 2015–19.
[N, nitrogen; °C, degrees Celsius; --, no data or not applicable]
Parameter | Type | CB 104 | CE 118 | CN 1 | CU 2 | FO 11 | LA 1201 | LB 372 | PI 522 | PO 72 | SQ 61 | TI 470 | UN 51 | VE 57 | WR 50 | WN 64 |
Alkalinity | Dissolved | -- | -- | -- | -- | -- | 0.04 | 0.14 | -- | -- | b0.01 | -- | -- | -- | 0.12 | -- |
Aluminum | Total | a0.05 | -- | 0.59 | 0.06 | 0.76 | -- | -- | -- | 0.64 | -- | -- | -- | 0.72 | -- | -- |
Ammonia + organic-N | Dissolved | 0.66 | -- | 1.00 | 0.43 | -- | a0.05 | -- | 0.49 | -- | 0.43 | 0.43 | -- | -- | -- | 0.43 |
Ammonia | Total | -- | 0.16 | -- | 0.43 | 0.06 | 0.33 | -- | a0.04 | -- | -- | -- | -- | -- | 1.00 | -- |
Arsenic | Total | -- | -- | 0.20 | 0.06 | 0.64 | 1.00 | 0.81 | 0.16 | 0.15 | 0.80 | -- | 1.00 | 0.30 | b0.05 | 0.15 |
Beryllium | Total | -- | 0.16 | 0.05 | -- | a0.05 | -- | 0.15 | 0.05 | a0.05 | 10.05 | a0.05 | -- | 0.18 | a0.05 | 0.15 |
Cadmium | Dissolved | 0.55 | 0.24 | a0.04 | 0.81 | a0.05 | 0.19 | 0.35 | 0.05 | a0.05 | a0.05 | 0.05 | 0.15 | 0.26 | a0.05 | 0.15 |
Cadmium | Total | -- | 0.16 | -- | -- | -- | -- | -- | 0.05 | -- | a0.03 | a0.05 | -- | -- | -- | 0.15 |
Calcium | Dissolved | a0.02 | 0.47 | 0.65 | 0.81 | 0.27 | 1.00 | b0.04 | 0.21 | 0.16 | a0.02 | 0.88 | -- | 0.13 | 1.00 | 0.12 |
Calcium | Total | a0.05 | 0.37 | 0.86 | 0.48 | 0.16 | 0.44 | 0.28 | 0.18 | 0.16 | a0.04 | 0.88 | -- | 0.15 | 0.70 | 0.24 |
Chromium | Total | -- | -- | -- | -- | -- | a0.05 | -- | -- | -- | -- | -- | -- | -- | -- | -- |
Copper | Dissolved | 0.71 | 0.60 | 0.70 | b0.01 | 0.06 | 0.17 | 0.44 | 0.85 | 1.00 | 0.35 | a0.05 | 1.00 | 0.85 | 1.00 | 0.11 |
Copper | Total | 0.54 | 0.72 | 0.86 | a0.02 | 1.00 | 0.87 | 0.94 | 0.78 | 0.64 | 0.81 | 0.35 | 0.57 | 0.79 | 0.74 | 0.80 |
Dissolved oxygen | -- | 0.54 | a0.02 | -- | 0.35 | -- | -- | 0.58 | 0.24 | 0.21 | 0.39 | 0.87 | 0.69 | 1.00 | -- | a0.02 |
Hardness | -- | a0.03 | 0.37 | 0.84 | 0.22 | 0.13 | 0.39 | 0.12 | 0.21 | 0.14 | a0.04 | 0.94 | -- | 0.16 | 0.70 | 0.35 |
Iron | Total | -- | -- | 0.86 | b0.00 | 0.64 | 0.14 | 0.06 | 0.80 | a0.03 | 0.81 | 1.00 | -- | 0.47 | 0.25 | 0.31 |
Lead | Total | 0.07 | 0.49 | 0.86 | b0.03 | 0.74 | 0.84 | 0.05 | 0.30 | -- | 0.44 | 0.12 | 0.24 | 1.00 | 0.81 | -- |
Magnesium | Dissolved | 0.02 | 0.37 | 0.37 | 0.58 | 0.12 | 0.75 | 0.21 | a0.05 | 0.12 | 0.14 | 0.44 | -- | 0.21 | 0.58 | 0.39 |
Magnesium | Total | 0.13 | 0.24 | 1.00 | 0.58 | 0.16 | 0.48 | 0.21 | 0.37 | 0.31 | a0.04 | 0.39 | -- | 0.13 | 0.53 | 0.35 |
Manganese | Dissolved | 0.90 | 0.49 | 1.00 | 0.76 | 0.35 | a0.01 | -- | 0.30 | 1.00 | 0.06 | 0.21 | b0.04 | 0.72 | 0.12 | 0.70 |
Manganese | Total | a0.03 | 0.49 | 0.86 | 0.88 | 0.88 | a0.01 | 0.65 | 0.91 | 0.58 | a0.04 | 0.76 | 0.16 | 0.59 | 0.39 | 1.00 |
NH3+orgN | Total | -- | 0.49 | 0.80 | 0.43 | 0.15 | a0.05 | -- | 0.49 | 0.43 | 0.43 | 0.43 | -- | -- | -- | 0.43 |
Nitrate | Dissolved | 0.16 | 0.65 | 0.25 | 0.94 | -- | -- | 0.64 | 0.59 | 0.12 | -- | 0.76 | -- | 0.28 | -- | a0.04 |
pH | -- | -- | 0.71 | 0.61 | 0.52 | 0.47 | 0.93 | 0.86 | 0.17 | 0.53 | 0.56 | 0.08 | 0.26 | 0.12 | 0.63 | 0.52 |
Phosphorus | Dissolved | -- | -- | 0.93 | -- | 0.11 | a0.02 | -- | 0.22 | -- | 1 | 0.87 | -- | -- | 0.01 | -- |
Phosphorus | Total | -- | -- | 0.40 | 0.15 | 0.69 | a0.01 | -- | 0.47 | 0.10 | 0.81 | 0.75 | 0.64 | -- | 0.21 | -- |
Potassium | Dissolved | a0.04 | a0.05 | 0.53 | 0.12 | 1.00 | 0.28 | 0.31 | 0.59 | 0.88 | 0.87 | 1.00 | -- | 0.37 | 0.58 | 0.48 |
Potassium | Total | a0.02 | 0.07 | 0.86 | a0.03 | 0.53 | 1.00 | 0.31 | 0.47 | 1.00 | 0.75 | 0.64 | -- | 0.72 | 0.16 | 0.64 |
Redox potential | -- | -- | 0.15 | -- | 0.35 | -- | 0.09 | -- | 0.15 | 0.21 | b0.00 | b0.03 | 0.16 | 0.11 | 0.28 | 0.35 |
Silica | Dissolved | a0.00 | 0.06 | 0.86 | 0.81 | 0.76 | 0.88 | 0.48 | 0.06 | 0.64 | 0.48 | 1.00 | -- | a0.04 | 0.24 | 0.09 |
Silica | Total | a0.01 | 0.24 | 0.79 | 0.04 | 0.44 | 0.09 | 0.31 | 0.58 | 1.00 | 0.53 | 0.58 | -- | 0.07 | 0.39 | 0.06 |
Sodium | Dissolved | 1.00 | a0.01 | 0.78 | 0.28 | 0.21 | 0.64 | b0.01 | a0.02 | a0.04 | 0.16 | 0.18 | -- | 0.53 | 1.00 | a0.01 |
Sodium | Total | 0.32 | a0.01 | 0.86 | 0.18 | 0.09 | 0.81 | b0.01 | 0.07 | a0.03 | 0.27 | 0.12 | -- | 0.42 | 0.35 | 0.07 |
Specific conductance at 25 °C | -- | 0.17 | 0.10 | -- | 0.43 | b0.01 | -- | 0.12 | 0.12 | a0.05 | 0.12 | 0.24 | 0.62 | 0.28 | 0.39 | 0.09 |
Strontium | Total | a0.02 | 0.71 | -- | -- | 0.33 | 0.36 | 0.81 | 0.18 | 0.08 | 0.24 | -- | -- | -- | 0.94 | 0.21 |
Temperature | -- | 0.90 | b0.02 | 0.20 | 0.87 | 0.57 | 0.94 | 0.81 | 0.20 | 0.43 | 0.94 | 0.10 | 0.53 | 0.16 | 1.00 | 0.58 |
Thallium | Dissolved | 0.10 | a0.04 | 0.05 | a0.03 | 0.14 | 0.14 | a0.03 | 0.05 | a0.03 | a0.03 | a0.03 | a0.02 | 0.05 | a0.03 | a0.03 |
Thallium | Total | 0.10 | a0.04 | 0.05 | a0.03 | -- | 0.14 | a0.03 | 0.05 | a0.03 | a0.03 | a0.03 | a0.03 | 0.05 | a0.03 | a0.03 |
Significant result (P <0.05) with a pattern of increasing concentrations over time.
Significant result (P <0.05) with a pattern of decreasing concentrations over time.
Ongoing and future sampling and expansion of wells in the present GWMN will help gain a more comprehensive understanding of ambient groundwater quality in the State of Pennsylvania. Additionally, new wells may be considered for addition to the GWMN, while sampling at others may be curtailed or suspended. This, along with further suggestions for maintaining data quality and data management practices, are explained below.
Considerations for changes and adaptations of the sampling interval and protocol for the GWMN are largely focused on more frequent data collection of constituents that are only sampled once or periodically. Currently, radiochemicals, hydrocarbons, and VOCs are only collected during the first sampling event at a well. Although many of the samples collected for these constituents are below the established reporting limits, a modified sampling frequency (in other words, once every 5 to 10 years), particularly at wells with previous measurable VOC concentrations, may be useful for determining if more frequent monitoring is necessary for specific wells or constituents. Conversely, as the GWMN is sampled only twice per calendar year, short-term changes that may occur in the GWMN are not currently well-understood. Sampling of a subset of the GWMN at an increased frequency over a period of time may help to determine if events like heavy precipitation affect water quality at a time scale that semiannual sampling does not capture; this information could be used to identify the best times to sample particular wells to get the most representative samples of ambient groundwater quality while also enumerating the time scale at which the water chemistry of wells changes and then recovers to ambient conditions (
Expanding the range of parameters that are collected may also benefit the interpretation of GWMN sampling results. Extended sampling of hydrocarbons (including isotopic signature ratios), particularly at wells with detectable levels of methane, may be useful for determining whether the origin of methane is biogenic or thermogenic. In turn, this information could be useful as a proxy for the age of groundwater, as thermogenic gases are commonly produced at depth, whereas the biological production of methane from anaerobic decomposition of organic materials typically occurs at shallower depths (
Analysis of differences between total and dissolved concentrations of constituents including major ions, trace elements, and nutrients showed that some parameters are more likely than others to have differences between samples collected with and without filtration. Major ions, which are largely present in solution in the GWMN, do not show statistical differences between total and dissolved samples. Several trace elements, as well as phosphorus, show statistical differences between total and dissolved concentrations because of the lower solubility of these elements, making the collection of both values more important when evaluating water quality of wells within the GWMN. Taken together, these results can inform on updating or modifying sampling schedules to reduce to the duplicative collection of samples.
Plans for monitoring the GWMN over a long period of time allow for analysis of trends on both a network and an individual well scale; maintaining the active sampling status of the wells in the GWMN allows for greater power of statistical analysis. Currently, 16 of the wells have 8 or more samples that were collected on a semiannual schedule, a number that was used as a threshold for inclusion in long-term trend analyses; continued monitoring of this subset of GWMN will provide additional information for characterizing long-term patterns and changes in ambient groundwater quality.
Results of the current study indicate that future interpretations of data from the GWMN would likely benefit from (1) ensuring that the sample collection procedures and equipment at each well are of similar design, and (2) choosing wells that produce water indicative of ambient conditions in a given part of the State of Pennsylvania. Some of the wells currently sampled in the GWMN have water chemistry that differs greatly from the general concentration distributions of constituents across the GWMN; these wells may be affected by local factors that do not represent ambient groundwater quality on a larger scale within the State. Wells BR 202 and BR 203 are not sampled through dedicated pumps and inert tubing as the rest of the GWMN is; rather, the water collected for these samples arrives at the sampling point after travelling through plumbing that contains copper and other metals. Copper concentrations in well BR 203 considerably exceed values measured at every other well in the GWMN; the presence of DOC may facilitate the dissolution of copper from the plumbing fixtures in the well. BR 202 may not be representative of ambient groundwater quality, as it contains elevated levels of several elements including sodium, chloride, barium, and fluoride. This well is located in proximity to a water treatment facility, which may impact local groundwater quality; additionally, a local source of brine may be present. Further investigation would be needed to appropriately determine the source of water in this well.
In general, some wells as currently configured may not meet ambient groundwater-quality monitoring criteria for a variety of reasons including plumbing design deficiencies and anomalous water-quality constituent data that are not reflective of ambient water quality. Removing these wells from future GWMN sampling events may be appropriate despite these wells having been included in several rounds of sampling owing to the aforementioned deficiencies. Although previously collected data are valuable for understanding local conditions, the design differences at these wells may make comparisons to other wells in the GWMN more difficult owing to the known deficiencies that exist; furthermore, changes to sampling equipment would necessitate differentiating between samples that were collected before or after equipment changes were made.
The analyses presented in this report indicate that interpretations of the data from the GWMN would benefit if future additions to the GWMN included an attempt to use consistent well-sampling designs to remove the variability that is associated with changes in the sampling mechanisms in wells. This could include using similar pumps, tubing, and pipe fittings to ensure that any background levels of constituents that are the result of the equipment used are constant across the GWMN as well as performing QA/QC procedures that ensure the quality of this equipment. Ideally, prospective new wells would be pumped to ascertain field parameters and the specific capacity of the well before it is added to the GWMN.
Underrepresented aquifers in the State could be prioritized when seeking to add wells. These areas include the various surficial, siliciclastic, and carbonate aquifers in the Great Lakes, Pittsburgh Low Plateaus, Appalachian Mountains, and Great Valley physiographic sections of the State. Including new wells in these regions would allow for more detailed statistical analysis to compare water types between various aquifers and physiographic provinces, thus furthering the goal of characterizing ambient groundwaters in spatially diverse parts of the State. Future additions to the GWMN could also be considered for continuous (1-hour data collection interval) monitoring of water levels; currently, a subset of wells in the GWMN do not have continuous water-level monitoring, making it difficult to analyze the effect that rising or falling water levels might have on constituent concentrations. Water-level data would supplement the examination of both long-term trends of constituent concentrations as well as provide more insight on the recharge patterns and cycles of the wells in the GWMN.
The Pennsylvania Groundwater Monitoring Network (GWMN) was initiated in 2014 with 17 wells located in largely rural areas across the State of Pennsylvania through coordination with the Pennsylvania Department of Environmental Protection (PaDEP), with a goal of characterizing the ambient groundwater quality at several fixed stations around the State. Additional sampling points added to GWMN following its inception have brought the total number of wells sampled semiannually to 28, spread across 27 counties. Wells are typically sampled once in the spring and once in the fall each year for a range of constituents including filtered and unfiltered major and trace elements, nutrients, and organic compounds. Additional sampling that occurs during the initial sampling event at each well includes volatile organic compounds, radionuclides, and hydrocarbons.
Summary statistics for 221 groundwater quality samples from the GWMN collected between 2014 and 2019 are presented and are grouped by constituent type. Nitrate concentrations consistently exceeded the U.S. Environmental Protection Agency (EPA) maximum contaminant level (MCL) in well LB 372. Iron, manganese, and aluminum exceeded the secondary maximum contaminant level (SMCL) in wells including AG 700, CF 321, CW 2417, FO 11, LA 1201, LY 796, SO 854, WR 50, and WY 197. SMCL exceedances were also recorded for pH field readings, with many wells having a pH consistently lower than the recommended 6.5–8.5 range. Two wells (CU 2 and UN 51) contained radon levels greater than the higher EPA action level of 4,000 picocuries per liter (pCi/L), and 18 of the wells contained radon levels higher than the lower 300 pCi/L EPA action level.
Principal components analysis (PCA) was performed on 200 of 221 groundwater-quality samples to determine the driving factors of water quality within the GWMN. Results of the PCA were correlated to additional parameters not included in the PCA to understand relations between principal components (PCs), various parameters, and categorical variables that are not expressed in the model. Three PCs (dissolved solids, redox, and sodium-chloride) collectively explained 74.5 percent of variance seen in GWMN samples. Hierarchical clustering of samples based on PCs 1 and 2 indicate four distinct clusters of wells that have similar chemical and geological characteristics.
Nonparametric Wilcoxon rank-sum tests were employed to test for differences between paired filtered and unfiltered constituents, and differences between samples taken in the spring and in the fall. Rank-sum tests were used for these comparisons to account for the non-normal spread that is common in environmental data. Results showed that there are minimal differences between filtered and unfiltered constituent values except in cases where a low-solubility constituent is more likely to be present in suspension, such as iron and manganese. Correlations between time-lagged Palmer Drought Severity Index values and constituent concentrations were not seen on a networkwide scale, but relations were identified between wells in cluster 2 for chloride, lithium, potassium, fluoride, phosphorus, and orthophosphate.
Comparisons between spring and fall sampling were completed on both a statewide and an individual well scale. Results from the statewide analysis indicate that there are no constituents that show appreciable differences between spring and fall sampling. For individual well seasonal analysis, only the 16 wells with a long-term record (at least 4 spring and 4 fall samples) were included; wells WR 50 and VE 57 showed the most significant differences between spring and fall sampling, with higher concentrations of calcium and magnesium being measured during the spring sampling seasons. Analysis of filtered and unfiltered sodium showed significant differences in 4 (25 percent) of the 16 wells, and many other major ion constituents, including calcium, magnesium, chloride, and fluoride showed a seasonal difference in at least one well.
Long-term constituent trends were evaluated using the nonparametric Mann-Kendall trend test. Median values for selected field parameters and major ion, trace element, and nutrient constituents for 15 wells with a long-term record (at least 8 samples beginning with the spring 2015 sampling season) were computed for each sampling season as well as for the individual wells. Results of the trend tests indicate that although there are statistically significant concentration increases and decreases over time for a range of constituents, the majority of these trend changes are small in magnitude and do not indicate major changes to water quality in the wells. Trend tests were limited by the small sample size; however, future sampling will be helpful for such analysis of an expanded dataset.
Considerations for adaptive monitoring are presented to discuss ways that the sampling and GWMN design can be modified to strengthen the quality of data collected in future GWMN groundwater quality samples. Future additions to the GWMN would benefit from focusing on sampling in physiographic regions of the State that are not currently sampled, with efforts being made to ensure that well and sampling design are comparable to current GWMN specifications. The sampling of additional parameters, specifically emerging contaminants that have not been extensively studied in the ambient aquifers of the State, as well as the resampling of parameters that were sampled during the initiation of the GWMN has the potential to provide greater detail about how these aquifers are changing over time with respect to human and natural influences.
Supplemental information for
[Redox processes calculated using guidelines from
Well name | County | USGS station ID | Latitude | Longitude | Redox process | Well cluster | Dissolved oxygen, in mg/L | Specific conductance, in µS/cm | pH, in standard units | |||||||||
Minimum | Median | Maximum | Range | Minimum | Median | Maximum | Range | Minimum | Median | Maximum | Range | |||||||
AD 146 | Adams | 395846077040601 | 39.97928 | 77.06917 | O2 Reduction | 3 | 3.5 | 3.8 | 4 | 0.5 | 372 | 383 | 388 | 16 | 7.3 | 7.55 | 7.6 | 0.3 |
AG 700 | Allegheny | 403734080063001 | 40.62619 | 80.10839 | -- | 4 | 0 | 0.2 | 0.4 | 0.4 | 575 | 615 | 634 | 59 | 7.5 | 7.75 | 8 | 0.5 |
BR 202 | Bradford | 414815076391801 | 41.80486 | 76.65644 | O2 Reduction | 4 | 0.1 | 0.5 | 7 | 6.9 | 888 | 1,040 | 1,160 | 272 | 8 | 8.3 | 8.4 | 0.4 |
BR 203 | Bradford | 414748076403901 | 41.79646 | 76.67693 | O2 Reduction | 3 | 1.5 | 1.9 | 3.1 | 1.6 | 402 | 420 | 445 | 43 | 6.9 | 7 | 7.2 | 0.3 |
CB 104 | Carbon | 410123075425401 | 41.02314 | 75.71464 | O2 Reduction | 1 | 8.9 | 10.7 | 11.8 | 2.9 | 108 | 114.5 | 134 | 26 | 4.2 | 4.7 | 5 | 0.8 |
CE 118 | Centre | 404518077575501 | 40.75558 | 77.96617 | O2 Reduction | 3 | 10.1 | 10.5 | 13.9 | 3.8 | 227 | 242 | 273 | 46 | 7.6 | 7.9 | 8.2 | 0.6 |
CF 321 | Clearfield | 410627078313601 | 41.10744 | 78.52647 | -- | 2 | 0 | 0.1 | 0.1 | 0.1 | 199 | 206 | 208 | 9 | 6 | 6 | 6.1 | 0.1 |
CH 10 | Chester | 395450075485401 | 39.91372 | 75.81419 | O2 Reduction | 3 | 7.1 | 7.95 | 8.6 | 1.5 | 286 | 298 | 330 | 44 | 7.4 | 7.5 | 7.5 | 0.1 |
CN 1 | Clinton | 411424077462201 | 41.24022 | 77.77278 | O2 Reduction | 1 | 7.9 | 8.85 | 10.9 | 3 | 18 | 18 | 20 | 2 | 4.9 | 5.55 | 5.8 | 0.9 |
CU 2 | Cumberland | 400209077183301 | 40.03592 | 77.30887 | -- | 1 | 6.2 | 8.4 | 10 | 3.8 | 32 | 37 | 55 | 23 | 4.6 | 5 | 5.7 | 1.1 |
CW 2417 | Crawford | 413658079572601 | 41.61604 | 79.95735 | -- | 4 | 0 | 0 | 0 | 0 | 289 | 289 | 289 | 0 | 7.4 | 7.4 | 7.4 | 0 |
FO 11 | Forest | 412823079030601 | 41.47312 | 79.05143 | -- | 2 | 0.1 | 0.2 | 0.4 | 0.3 | 109 | 127.5 | 132 | 23 | 6.1 | 6.5 | 7 | 0.9 |
LA 1201 | Lawrence | 410538080280801 | 41.09395 | 80.46868 | -- | 4 | 0 | 0.2 | 0.6 | 0.6 | 430 | 439.5 | 461 | 31 | 6.7 | 6.8 | 6.9 | 0.2 |
LB 372 | Lebanon | 402207076180801 | 40.36883 | 76.30181 | O2 Reduction | 3 | 2.9 | 6 | 7 | 4.1 | 614 | 640 | 742 | 128 | 7 | 7.2 | 7.3 | 0.3 |
LY 796 | Lycoming | 411640077215802 | 41.2779 | 77.36616 | -- | 2 | 0.1 | 0.3 | 1 | 0.9 | 115 | 128 | 154 | 39 | 6.1 | 6.4 | 7.1 | 1 |
NU 579 | Northumberland | 404218076351501 | 40.70504 | 76.58755 | O2 Reduction | 3 | 0.3 | 0.6 | 1.2 | 0.9 | 197 | 200.5 | 201 | 4 | 7.6 | 7.7 | 7.8 | 0.2 |
PI 522 | Pike | 411833075133601 | 41.30919 | 75.22686 | O2 Reduction | 1 | 1.5 | 7.3 | 9.1 | 7.6 | 61 | 71 | 87 | 26 | 5.6 | 5.9 | 6.3 | 0.7 |
PO 72 | Potter | 414640077493801 | 41.77784 | 77.82694 | O2 Reduction | 1 | 9.8 | 10.4 | 11.2 | 1.4 | 36 | 46 | 61 | 25 | 5.9 | 6.2 | 6.9 | 1 |
SO 854 | Somerset | 395920079021501 | 39.98883 | 79.0375 | -- | 2 | 0 | 0 | 0.1 | 0.1 | 109 | 112 | 132 | 23 | 5.5 | 5.6 | 5.6 | 0.1 |
SQ 61 | Susquehanna | 415323077451301 | 41.88997 | 75.75325 | Mn(IV) Reduction | 4 | 0 | 0.2 | 0.4 | 0.4 | 244 | 252 | 258 | 14 | 7.5 | 7.8 | 8 | 0.5 |
SU 169 | Sullivan | 412403076234802 | 41.40092 | 76.39675 | O2 Reduction | 1 | 5.6 | 6 | 8.5 | 2.9 | 80 | 104 | 547 | 467 | 7.2 | 9 | 11.8 | 4.6 |
TI 470 | Tioga | 414634077235801 | 41.77608 | 77.39925 | O2 Reduction | 1 | 7.3 | 8.3 | 9.4 | 2.1 | 72 | 77 | 97 | 25 | 6.1 | 6.4 | 6.8 | 0.7 |
UN 51 | Union | 405928077115501 | 40.99014 | 77.19772 | O2 Reduction | 1 | 9.6 | 10 | 10.9 | 1.3 | 20 | 22 | 227 | 207 | 5.7 | 6 | 6.3 | 0.6 |
VE 57 | Venango | 411958079540202 | 41.33158 | 79.90022 | O2 Reduction | 1 | 8.6 | 9.4 | 9.9 | 1.3 | 27 | 32 | 48 | 21 | 4.7 | 4.9 | 5.5 | 0.8 |
WE 300 | Westmoreland | 402138079031802 | 40.36047 | 79.05583 | -- | 4 | 0.1 | 1.7 | 5.5 | 5.4 | 258 | 266 | 275 | 17 | 7 | 7.1 | 7.3 | 0.3 |
WN 64 | Wayne | 414333075153201 | 41.72522 | 75.25858 | O2 Reduction | 3 | 4.8 | 6.4 | 11.1 | 6.3 | 179 | 228.5 | 284 | 105 | 6.4 | 6.8 | 7.1 | 0.7 |
WR 50 | Warren | 414159079213601 | 41.69953 | 79.35983 | -- | 4 | 0 | 0.1 | 0.7 | 0.7 | 271 | 288 | 324 | 53 | 7 | 7.3 | 7.5 | 0.5 |
WY 197 | Wyoming | 412708076150201 | 41.45228 | 76.25053 | -- | 2 | 0 | 0.4 | 2.3 | 2.3 | 192 | 193 | 196 | 4 | 6.9 | 6.9 | 7.2 | 0.3 |
U.S. Environmental Protection Agency testing methods and references used by the Pennsylvania Department of Environmental Protection Bureau of Laboratories (
[mg/L, milligrams per liter; µg/L, micrograms per liter; pCi/L, picocuries per liter; µs/cm, microsiemens per centimeter; °C, degrees Celsius; SMEWW, Standard Methods for the Evaluation of Water and Wastewater; EPA, U.S. Environmental Protection Agency; PaDEP, Pennsylvania Department of Environmental Protection; BOL, Bureau of Laboratories; USGS, U.S. Geological Survey; NFM, National Field Manual; SMEWW, Standard Methods for the Examination of Water and Wastewater; --, not applicable]
PaDEP standard analysis code | Parameters sampled | Reporting units | Method | Analytical method short reference |
METH | Hydrocarbon gases | mg/L | PaDEP BOL6019 | PaDEP, 2012 |
RAD91 | Radium-228 | pCi/L | ||
RAD91 | Gross alpha and beta radioactivity | pCi/L | EPA 900.0 | EPA, 2018b |
RAD91 | Radium-226 | pCi/L | EPA 903.1 | EPA, 2018c |
RAD91 | Radon-222 | pCi/L | SM 7500-Ra B | |
SAC200 | Dissolved organic carbon | mg/L | SM 5310 C | |
SAC997 | Trace elements | µg/L | EPA 200.7 | EPA, 1994a |
SAC997 | Major ions - cations | mg/L | EPA 200.7 | EPA, 1994a |
SAC997 | Silica | mg/L | SM 4500 | |
SAC997 | Trace elements | µg/L | EPA 200.8 | EPA, 1994b |
SAC997 | Mercury | µg/L | EPA 245.1 | EPA, 1994c |
SAC997 | Major ions - anions | mg/L | EPA 300.0 | EPA, 1993a |
SAC997 | Bromide | µg/L | EPA 300.1 B | EPA, 1997 |
SAC997 | Ammonia | mg/L | EPA 350.1 | EPA, 1993b |
SAC997 | Kjeldahl nitrogen | mg/L | EPA 351.2 | EPA, 1993c |
SAC997 | NO3, NO2 | mg/L | EPA 353.2 | EPA, 1993d |
SAC997 | PO4, Ortho-P | mg/L | EPA 365.1 | EPA, 1993e |
SAC997 | Cyanide | mg/L | EPA KELADA-01 | EPA, 2001 |
SAC997 | Alkalinity | µg/L | SM 2320B | |
SAC997 | Total hardness | mg/L | SM 2340 B | |
SAC997 | Specific conductance | µs/cm | SM 2510B | |
SAC997 | pH | Standard units | SM 4500 H+B | |
SAC997 | Water temperature | °C | SM 4500 H+B | |
SAC997 | Total organic carbon | mg/L | SM 5310 C | |
SAC997 | Total dissolved solids | mg/L | USGS I-1750 | |
VOADW | Volatile organic compounds | µg/L | EPA 524.3 | EPA, 1995 |
Field | Static water level, pumping rate | -- | ||
Field | Various field parameters | -- | USGS NFM chapter A6 |
Distributions of continuous variables were compared among different sample classifications using boxplots (
Selected constituents grouped by pH categories of ≤5.5, 5.6–6.5, 6.6–7.5, and ≥7.6 for 221 water samples collected from 28 wells within the Pennsylvania Groundwater Monitoring Network, 2015–19.
Figure 3.1. Boxplots of selected constituents grouped by pH categories of ≤5.5, 5.6–6.5, 6.6–7.5, and ≥7.6 for 221 water samples collected from 28 wells within the Pennsylvania Groundwater Monitoring Network, 2015–19.
Selected constituents grouped by specific conductance categories of ≤50, 51–250, 251–500, and ≥500 for 221 water samples collected from 28 wells within the Pennsylvania Groundwater monitoring Network, 2015–19.
Figure 3.2. Boxplots of selected constituents grouped by specific conductance categories of ≤50, 51–250, 251–500, and ≥500 for 221 water samples collected from 28 wells within the Pennsylvania Groundwater monitoring Network, 2015–19.
Selected constituents grouped by reduction-oxidation (redox) state categories of anoxic, mixed, and oxic for 221 water samples collected from 28 wells within the Pennsylvania Groundwater monitoring Network, 2015–19.
Figure 3.3. Boxplots of selected constituents grouped by reduction-oxidation (redox) state categories of anoxic, mixed, and oxic for 221 water samples collected from 28 wells within the Pennsylvania Groundwater monitoring Network, 2015–19.
Selected constituents grouped by physiographic province categories of Appalachian Plateau, Piedmont, and Ridge and valley for 221 water samples collected from 28 wells within the Pennsylvania Groundwater monitoring Network, 2015–19.
Figure 3.4. Boxplots of selected constituents grouped by physiographic province categories of Appalachian Plateau, Piedmont, and Ridge and valley for 221 water samples collected from 28 wells within the Pennsylvania Groundwater monitoring Network, 2015–19.
Selected constituents grouped by major aquifer type categories of carbonate, crystalline, and siliciclastic for 221 water samples collected from 28 wells within the Pennsylvania Groundwater monitoring Network, 2015–19.
Figure 3.5. Boxplots of selected constituents grouped by major aquifer type categories of carbonate, crystalline, and siliciclastic for 221 water samples collected from 28 wells within the Pennsylvania Groundwater monitoring Network, 2015–19.
Selected constituents grouped by topographic position categories of canyon, ridge, and slope for 221 water samples collected from 28 wells within the Pennsylvania Groundwater monitoring Network, 2015–19.
Figure 3.6. Boxplots of selected constituents grouped by topographic position categories of canyon, ridge, and slope for 221 water samples collected from 28 wells within the Pennsylvania Groundwater monitoring Network, 2015–19.
Selected constituents grouped by sample season categories of spring and fall for 221 water samples collected from 28 wells within the Pennsylvania Groundwater monitoring Network, 2015–19.
Figure 3.7. Boxplots of selected constituents grouped by sample season categories of spring and fall for 221 water samples collected from 28 wells within the Pennsylvania Groundwater monitoring Network, 2015–19.
Selected constituents grouped by hierarchical cluster designation based on principal components analysis results for 221 water samples collected from 28 wells within the Pennsylvania Groundwater monitoring Network, 2015–19.
Figure 3.8. Boxplots of selected constituents grouped by hierarchical cluster designation based on principal components analysis results for 221 water samples collected from 28 wells within the Pennsylvania Groundwater monitoring Network, 2015–19.
Parameter | Cluster | Palmer Drought Severity Index | |||||||||
0-month lag | 1-month lag | 2-month lag | 3-month lag | 4-month lag | 5-month lag | 6-month lag | 7-month lag | 8-month lag | 9-month lag | ||
Ammonia | 1 | –0.13 | –0.12 | –0.11 | –0.15 | –0.10 | –0.03 | –0.03 | 0.01 | 0.08 | 0.06 |
2 | 0.14 | 0.12 | 0.15 | 0.10 | 0.22 | 0.28 | 0.25 | 0.16 | 0.20 | 0.06 | |
3 | 0.01 | 0.03 | 0.01 | 0.01 | 0.06 | 0.09 | 0.14 | 0.06 | 0.06 | 0.04 | |
4 | –0.09 | –0.08 | –0.11 | –0.17 | –0.19 | –0.14 | –0.18 | –0.12 | –0.10 | –0.08 | |
Barium | 1 | –0.09 | –0.07 | –0.03 | –0.10 | –0.12 | –0.14 | –0.19 | –0.09 | –0.11 | –0.10 |
2 | 0.11 | 0.10 | 0.18 | 0.23 | a0.37 | a0.34 | 0.24 | 0.26 | 0.27 | a0.42 | |
3 | –0.02 | –0.02 | –0.01 | –0.01 | –0.04 | –0.05 | 0.00 | –0.04 | –0.05 | –0.03 | |
4 | 0.08 | 0.05 | 0.01 | 0.00 | –0.02 | –0.10 | –0.11 | –0.19 | –0.20 | –0.22 | |
Calcium | 1 | 0.08 | 0.08 | 0.09 | 0.06 | –0.01 | –0.01 | 0.01 | 0.01 | –0.01 | –0.09 |
2 | –0.09 | –0.09 | –0.14 | –0.21 | c–0.32 | –0.26 | –0.20 | –0.20 | –0.19 | c–0.42 | |
3 | –0.15 | –0.19 | –0.19 | –0.27 | –0.25 | –0.20 | –0.19 | –0.19 | –0.12 | –0.14 | |
4 | –0.10 | –0.07 | –0.06 | –0.07 | –0.03 | 0.02 | 0.00 | 0.08 | 0.12 | 0.12 | |
Chloride | 1 | 0.07 | 0.05 | 0.06 | –0.02 | –0.10 | –0.11 | –0.12 | –0.05 | –0.11 | –0.10 |
2 | b0.52 | b0.53 | b0.56 | b0.52 | a0.50 | a0.45 | a0.35 | 0.29 | 0.23 | 0.26 | |
3 | –0.02 | –0.02 | 0.03 | 0.05 | 0.05 | 0.13 | 0.19 | 0.23 | 0.22 | 0.20 | |
4 | 0.17 | 0.14 | 0.09 | 0.06 | 0.00 | –0.04 | –0.05 | –0.09 | –0.11 | –0.15 | |
Fluoride | 1 | –0.07 | –0.08 | –0.09 | –0.02 | –0.09 | –0.07 | –0.03 | –0.02 | –0.01 | –0.01 |
2 | c–0.38 | c–0.41 | c–0.34 | –0.26 | –0.04 | –0.08 | –0.02 | 0.09 | 0.13 | 0.23 | |
3 | 0.12 | 0.11 | 0.11 | 0.14 | 0.15 | 0.20 | 0.20 | 0.14 | 0.13 | 0.14 | |
4 | 0.20 | 0.18 | 0.17 | 0.19 | 0.20 | 0.15 | 0.19 | 0.20 | 0.19 | 0.22 | |
Lithium | 1 | a0.31 | a0.32 | 0.27 | 0.19 | 0.12 | 0.10 | 0.07 | 0.04 | 0.03 | –0.05 |
2 | b0.76 | b0.76 | b0.76 | b0.69 | b0.61 | b0.54 | a0.37 | 0.23 | 0.16 | 0.10 | |
3 | a0.33 | 0.28 | 0.24 | 0.17 | 0.11 | 0.00 | 0.03 | –0.03 | –0.08 | –0.12 | |
4 | 0.15 | 0.14 | 0.10 | 0.09 | 0.03 | 0.02 | 0.03 | –0.03 | –0.05 | –0.10 | |
Magnesium | 1 | –0.03 | –0.07 | –0.06 | –0.10 | –0.13 | –0.12 | –0.06 | 0.00 | –0.06 | 0.00 |
2 | d–0.55 | d–0.58 | d–0.52 | d–0.55 | c–0.33 | c–0.31 | –0.21 | –0.07 | –0.03 | 0.10 | |
3 | –0.09 | –0.14 | –0.16 | –0.22 | –0.20 | –0.14 | –0.15 | –0.18 | –0.12 | –0.13 | |
4 | –0.11 | –0.10 | –0.08 | –0.08 | –0.04 | 0.01 | –0.02 | 0.10 | 0.12 | 0.12 | |
Nitrate | 1 | 0.04 | 0.07 | 0.08 | 0.09 | 0.11 | 0.11 | 0.07 | 0.05 | 0.05 | 0.07 |
2 | 0.20 | 0.14 | 0.18 | 0.17 | 0.17 | 0.19 | 0.11 | 0.20 | 0.25 | 0.23 | |
3 | –0.03 | –0.07 | –0.07 | –0.17 | –0.14 | –0.09 | –0.12 | –0.12 | –0.07 | –0.11 | |
4 | 0.01 | 0.00 | –0.03 | –0.03 | 0.03 | 0.06 | 0.07 | 0.09 | 0.11 | 0.09 | |
Orthophosphate | 1 | 0.03 | 0.03 | –0.03 | 0.00 | –0.13 | –0.19 | –0.21 | –0.27 | c–0.31 | c–0.35 |
2 | d–0.52 | d–0.54 | d–0.51 | c–0.50 | c–0.32 | c–0.33 | c–0.31 | –0.11 | –0.01 | 0.17 | |
3 | 0.09 | 0.01 | –0.07 | –0.14 | –0.18 | –0.28 | c–0.37 | c–0.38 | c–0.38 | c–0.46 | |
4 | 0.02 | 0.03 | –0.02 | –0.02 | –0.03 | –0.13 | –0.13 | –0.18 | –0.12 | –0.07 | |
Phosphorus | 1 | 0.10 | 0.12 | 0.07 | 0.08 | –0.05 | –0.10 | –0.15 | –0.19 | –0.22 | –0.25 |
2 | d–0.65 | d–0.67 | d–0.63 | d–0.57 | c–0.33 | –0.29 | –0.23 | –0.05 | 0.02 | 0.17 | |
3 | 0.18 | 0.07 | 0.01 | –0.09 | –0.16 | –0.24 | –0.27 | –0.29 | –0.29 | c–0.36 | |
4 | –0.15 | –0.15 | –0.16 | –0.14 | –0.07 | –0.10 | –0.10 | –0.18 | –0.16 | –0.14 | |
Potassium | 1 | –0.08 | –0.07 | –0.07 | –0.08 | –0.05 | –0.06 | –0.07 | –0.07 | –0.05 | –0.05 |
2 | c–0.41 | c–0.45 | c–0.38 | c–0.50 | c–0.36 | c–0.33 | c–0.32 | –0.22 | –0.20 | –0.16 | |
3 | –0.18 | –0.22 | –0.22 | –0.29 | –0.27 | –0.19 | –0.17 | –0.17 | –0.13 | –0.13 | |
4 | –0.07 | –0.04 | –0.04 | –0.04 | –0.03 | 0.04 | 0.02 | 0.09 | 0.12 | 0.09 | |
Silica | 1 | –0.17 | –0.18 | –0.16 | –0.13 | –0.05 | –0.02 | –0.04 | –0.05 | 0.01 | –0.01 |
2 | c–0.33 | c–0.40 | c–0.33 | –0.26 | –0.12 | –0.12 | –0.06 | 0.04 | 0.07 | 0.26 | |
3 | a0.32 | 0.23 | 0.15 | 0.03 | –0.04 | –0.22 | –0.27 | –0.26 | –0.27 | c–0.35 | |
4 | –0.08 | –0.09 | –0.07 | –0.03 | 0.03 | –0.01 | –0.01 | –0.06 | –0.05 | –0.04 | |
Sodium | 1 | –0.02 | –0.04 | –0.02 | –0.12 | –0.17 | –0.17 | –0.19 | –0.11 | –0.17 | –0.15 |
2 | a0.33 | a0.33 | 0.29 | 0.26 | 0.03 | 0.03 | –0.01 | –0.08 | –0.12 | c–0.31 | |
3 | –0.10 | –0.13 | –0.10 | –0.14 | –0.13 | –0.05 | 0.02 | 0.07 | 0.10 | 0.08 | |
4 | 0.10 | 0.07 | 0.03 | 0.00 | –0.06 | –0.11 | –0.12 | –0.16 | –0.19 | –0.22 | |
Strontium | 1 | 0.12 | 0.13 | 0.13 | 0.09 | 0.01 | 0.02 | 0.03 | 0.01 | 0.00 | –0.11 |
2 | –0.15 | –0.18 | –0.15 | –0.27 | –0.25 | –0.24 | –0.17 | –0.11 | –0.13 | –0.22 | |
3 | 0.16 | 0.10 | 0.06 | –0.04 | –0.09 | –0.17 | –0.14 | –0.18 | –0.19 | –0.23 | |
4 | 0.04 | 0.00 | –0.04 | –0.07 | –0.12 | –0.17 | –0.20 | –0.20 | –0.23 | –0.24 | |
Sulfate | 1 | 0.18 | 0.18 | 0.22 | 0.26 | 0.24 | 0.19 | 0.22 | 0.18 | 0.12 | 0.14 |
2 | 0.10 | 0.11 | 0.11 | 0.27 | 0.22 | 0.18 | 0.08 | 0.04 | 0.07 | –0.01 | |
3 | 0.29 | 0.21 | 0.20 | 0.11 | 0.05 | –0.01 | –0.02 | –0.07 | –0.12 | –0.19 | |
4 | 0.11 | 0.14 | 0.15 | 0.16 | 0.16 | 0.21 | 0.19 | 0.24 | 0.29 | 0.27 |
Weak (0.3 < R < 0.5) positive relation.
Moderate (0.5 < R < 0.7) positive relation.
Weak (–0.3 > R > –0.5) negative relation.
Moderate (–0.5 > R > –0.7) negative relation.
Boxplots of seasonal differences for water-quality constituents measured in 15 of 28 Pennsylvania Groundwater Monitoring Network wells with long-term records of at least four sampling events in both spring and fall time frames (
Differences in water-quality attributes from spring and fall samples, Pennsylvania Groundwater Monitoring Network, 2015–19.
Figure 5.1. Boxplots comparing differences in water-quality attributes from spring and fall samples, Pennsylvania Groundwater Monitoring Network, 2015–19.
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U.S. Geological Survey
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