Purpose and Scope

Groundwater-quality data, reports, and information are sparse for the counties in northwestern West Virginia that are experiencing intense development of oil, gas, and other hydrocarbons, and were not adequate to assess and characterize the quality of groundwater within the study area.

To address this data gap, 30 rural residential water wells were sampled in 2018 for a broad range of chemical constituents. This study focused on rural residential water supplies in areas with a high density of oil and gas drilling, production, and pipeline transport to provide a baseline dataset of groundwater quality for current assessment and future comparison.

The groundwater samples were analyzed for chemical and physical properties, including nutrients, major ions, trace elements and metals, indicator bacteria, radionuclides, and methane and other dissolved hydrocarbon gases. The groundwater-quality data and summary statistics are presented to assess current groundwater-quality conditions in the region and are compared to drinking-water standards. In addition, the data were analyzed using multivariate statistical methods to assess potential correlations with geology, topographic setting, well depth, and proximity to oil and gas wells. Mineral saturation indices (SIs) were calculated to better understand reduction-oxidation (redox) and other geochemical processes that control groundwater quality. Principal component analysis was used to identify processes affecting groundwater quality and associations among dissolved constituents in groundwater. Concentrations of methane and other dissolved hydrocarbon gases were analyzed to help determine potential sources of, and potential processes affecting, methane and other dissolved hydrocarbon gases in groundwater. Groundwater ages determined from various tracers are presented and a conceptual model for the evolution of groundwater quality is discussed.

Description of Study Area

The study area for this investigation is within the Marcellus Shale oil and gas play in northwestern West Virginia and includes parts of 8 counties (Wetzel, Tyler, Doddridge, Ritchie, Wirt, Calhoun, Gilmer, and Roane) covering approximately 2,498 mi² (fig. 1) and is completely within the Appalachian Plateaus Physiographic Province (Fenneman, 1938; Fenneman and Johnson, 1946). In 2017, the 8-county area produced 913,899 thousand cubic feet (MCF) of natural gas, 3,023,712 barrels (bbl) of oil, and 4,495,150 bbl of natural gas liquids (wet gas; table 1; West Virginia Geological and Economic Survey, 2020a). Oil and gas production is greater in the northern part (Wetzel, Tyler, Doddridge, and Ritchie Counties) than in the southern part (Wirt, Calhoun, Gilmer, and Roane Counties) of the eight-county study area.

The study area consists of rugged, moderately incised hilly terrain with uplifted plateaus capped by resistant layers of relatively flat-lying sandstone and shale. Approximately 57,452 oil and gas wells are present in the 8-county study area as of 2021, some of which are active and others which are no longer producing or have been abandoned (West Virginia Geological and Economic Survey, 2020b).

Average annual precipitation for the study area, based on 1991–2020 station normals (table 2) for the National Weather Service weather station in Middlebourne, West Virginia (town of Middlebourne is shown on fig. 1), is 47.27 inches per year (in/yr). Maximum and minimum monthly temperatures for the station are 84.1 degrees Fahrenheit (°F) in July and 19.4 °F in January (National Oceanic and Atmospheric Administration, 2022). Precipitation is typically distributed unevenly with respect to topography, with areas at higher elevations typically receiving higher average amounts of precipitation than areas at lower elevations.

Public supplies are the principal source of water for 61.7 percent of residents and businesses in the study area. These public water supplies serve 41,478 people and account for 4.86 million gallons per day (Mgal/d) of freshwater withdrawals (2.0 Mgal/d of groundwater and 2.86 Mgal/d of surface water) for residential and commercial use. The remaining water use consists of 3.02 Mgal/d of groundwater withdrawals for residential and commercial supply and serves an estimated 37,774 people who rely on private domestic wells or other unregulated sources such as springs (table 3; U.S. Geological Survey, 2020). Rural residential water systems, primarily from individual wells, supply 38.3 percent of all water used for residential and commercial use in the study area. The majority of water used for public supply in the region (58.8 percent) is derived from surface-water sources, primarily from stream withdrawals, and the remaining part (41.2 percent) is derived from groundwater withdrawals. For rural residential homeowners, however, almost 100 percent of their withdrawals are derived from groundwater (private wells or springs).

Hydrogeologic Setting and Groundwater Flow

The generalized hydrogeologic framework of the study area is based on regional topography, stratigraphy, and structure, with upland plateaus formed by resistant clastic rocks and valleys developed in zones of structural weakness. Pennsylvanian clastic rocks form the predominant units cropping out in the uplands and valley walls of southern and central West Virginia (Cardwell and others, 1968). Geologic nomenclature used in this report is that of the West Virginia Geological and Economic Survey (2022). The study area is underlain by Pennsylvanian and Permian stratigraphic units that consist of gently dipping, relatively flat lying, slightly folded, interbedded sandstone, conglomerate, siltstone, shale, and coal, with local beds of limestone and dolomite. Mapped geologic units within the study area (fig. 2) consist of three geologic groups or formations and include the Monongahela Formation and Conemaugh Group of Pennsylvanian age and the Dunkard Group of Permian and Pennsylvanian age (Cardwell and others, 1968). The bedrock units commonly are overlain by a relatively thin layer of regolith or alluvium (Berg and others, 1980). Older sedimentary rock formations of Pennsylvanian age, including the Allegheny and Kanawha Formations, have been mapped to the southeast of the study area (fig. 2).

Although the lithology of the various geologic units which form the fractured-bedrock aquifers within the study area are similar in composition to each other, there is some variability between them (Blake and others, 2002). The oldest rocks, which crop out in the southeastern part of the study area, are those of the Glenshaw and Casselman Formations of middle to upper Pennsylvanian age and comprise the Conemaugh Group (fig. 2). The Monongahela Formation crops out within the central part of the study area and is of upper Pennsylvanian age. The youngest rocks are those of the Waynesburg and Washington Formations of upper Pennsylvanian to Permian age and comprise the Dunkard Group, cropping out in the northwestern part of the study area (fig. 2). The proportion of shale, limestone, sandstone, coal, and other sedimentary rocks varies from one formation to another; it is this variability in lithologic composition that can result in varying groundwater quality within the study area.

In West Virginia, the Conemaugh Group consists of the Casselman Formation and underlying Glenshaw Formation, the border between units being the top of the Ames Limestone Member (Milici, 2004; U.S. Geological Survey and Association of American State Geologists, 2020b). The Monongahela Formation consists of the Gilboy Sandstone Member, Little Captina Limestone Member, McKeefrey Siltstone Member, Morningview Sandstone Member, Redstone Member, and Ritchie Red Beds (U.S. Geological Survey and Association of American State Geologists, 2020b). The Dunkard Group consists of interbedded sandstones, limestones, shales, and coal beds of the Washington and Greene Formations (Fedorko and Skema, 2013).

The proportion of limestone to other lithologies within the Monongahela Formation and Dunkard Group is greater than in the Conemaugh Group. There also are differences in the depositional sequence for the various formations. The older Pennsylvanian-age rocks in southern West Virginia were deposited in an area that received more paleorainfall and paleorecharge; thus, the sulfur content was diluted compared to the younger Pennsylvanian- and Permian-age rocks in northern West Virginia, which formed in a drier paleoclimate with higher ash, nutrient, and dissolved solids content that collectively resulted in higher sulfur content (Cecil and others, 1985). The younger rocks were deposited in marine environments that had greater contents of sulfur and calcium carbonate. There is also a difference in the depositional sequence for the limestone, with the limestones within the Monongahela Formation and Dunkard Group primarily deposited in non-marine brackish environments and the older rocks of the Conemaugh Group deposited in marine environments (Cardwell and others, 1968).

Groundwater-flow paths in the study area are relatively short and limited to two principal aquifers: (1) unconsolidated, unconfined, alluvial aquifers composed of sand, silt, clay, and gravel in valley settings, and (2) fractured-bedrock aquifers composed of siliciclastic and carbonate sedimentary rocks and associated coal (Puente, 1985). Because they tend to be shallow and thin, the alluvial aquifers within the study area typically are not used for water supplies but can combine with soil and regolith to provide shallow storage for recent recharge. Fractured-bedrock aquifers are the primary aquifers in the study area. Regolith is commonly thin with low permeability, providing little groundwater storage. Groundwater storage and flow in bedrock occurs through joints, fractures, and bedding-plane separations (Kozar and Mathes, 2001). In the study area, secondary permeability due to jointing and stress-relief fracturing accounts for most of the porosity and permeability in the bedrock because the original intergranular porosity commonly has been filled by calcium carbonate or silica cementation (Wyrick and Borchers, 1981).

Recharge to fractured-bedrock aquifers in the region occurs primarily as rainfall (Puente, 1985). Snowmelt is only an important source of recharge in areas of West Virginia at elevations above 3,000 feet (ft), but the maximum elevation in the study area is only 1,680 ft. Once precipitation falls on the surface, the part that does not run off to streams percolates into and through shallow soils and regolith and eventually recharges fractured-bedrock or alluvial aquifers. A decrease in hydraulic conductivity with depth in bedrock aquifers in the region has been documented by several researchers. For a mine site in West Virginia, the average hydraulic conductivity of an aquifer decreased from 3.3 × 10⁻⁴ ft per second (ft/s) at a depth of 150 ft to 3.3 × 10⁻⁸ ft/s at depths greater than 328 ft (Bruhn, 1985). For a site in Greene County, Pennsylvania about 60 miles from the study area, hydraulic conductivity decreased by an order of magnitude per 100 ft of depth to a depth of approximately 500 ft (Stoner, 1983). According to Callaghan and others (1998), the vast majority of groundwater circulation in Appalachian Plateau settings occurs at moderate depths of less than 300 ft.

In the Appalachian Plateaus, local fractured-bedrock aquifers of less than 5.0 mi² are defined by topographic valleys and boundary ridges, with total relief of only a few hundred feet from valley bottoms to ridge tops, on average. Each small valley may contain a locally distinct aquifer (Kozar and others, 2020) from which groundwater discharges to a nearby stream or to deeper subregional or regional aquifers (fig. 3). The ridges surrounding the valley define the lateral boundaries of the local aquifer and its principal recharge area. Subregional aquifers (generally from 100 to 500 ft below land surface [bls]) occur at intermediate depths between the shallow local aquifer (less than 500 ft bls) and deeper regional aquifers. Subregional aquifers are larger than local aquifers and may include several smaller local aquifers. Discharge of groundwater from subregional aquifers is primarily to tributary streams with drainage areas typically much larger than 5.0 mi². A small component of recharge to these subregional aquifers is from deeper regional aquifers that typically discharge to large rivers. Depth to saline water (brines) has been used to infer the base of regional aquifers (Callaghan and others, 1998). In West Virginia, the depth to brackish water ranges from more than 2,000 ft near the southern part of the study area to a minimum of less than 50 ft in areas bordering the Ohio River (Foster, 1980).

Within the local aquifers, groundwater typically flows from hilltops to valleys, approximately perpendicular to local tributary streams, through an intricate network of stress-relief fractures and interconnected bedding-plane separations, commonly in a stair-step pattern (fig. 3; Wyrick and Borchers, 1981; Harlow and LeCain, 1993). Vertical hydraulic conductivity can be negligible within the clastic sedimentary rocks comprising the aquifer, resulting in approximately horizontal groundwater flow parallel to bedding (especially within coal beds) that discharges as springs or seeps in hillsides and valleys (Harlow and LeCain, 1993). A small proportion of the groundwater flows deeper within the central core of the mountain or ridge, especially within coal beds and along bedding-plane separations, and likely reaches the valley to discharge locally to surface water or may recharge subregional and regional aquifers (Kozar and others, 2012). Groundwater flow in valleys primarily occurs in bedding-plane separations beneath valley floors and in vertical and horizontal stress-relief fractures along valley walls (Wyrick and Borchers, 1981). Enhanced permeability of bedrock in valleys may result in groundwater flow parallel to and beneath local tributary streams before ultimately discharging to surface-water bodies.

This study is focused on shallow groundwater quality in the wet gas dominated part of the Marcellus Shale oil and gas play in northwestern West Virginia (fig. 4), where the Marcellus Shale does not crop out but is present at depths greater than 7,000 ft bls. The Marcellus Shale ranges in depth from approximately 7,500 ft bls in Wetzel County, West Virginia, where rocks dip slightly, to 8,200 ft bls in the central trough of the Robinson Run Syncline in Doddridge County, based on published geologic cross sections (Ryder and others, 2009). The Marcellus Shale ranges from less than 20 ft thick in the western part of the study area to a maximum of 60 ft in the eastern portion of the study area (fig. 5). Multi-lateral horizontal production wells can produce very large quantities of gas from even thin shale reservoirs.

The yield of water from residential wells completed within the study area has long been postulated to be a function of topography, specifically the dominance of stress-relief fractures in the valleys and hillsides and the absence of such fractures on the hilltop settings (Wyrick and Borchers, 1981). Stress relief fracturing processes were documented for a study conducted at Twin Falls State Park in Wyoming County, West Virginia (Wyrick and Borchers, 1981) and are often referenced in hydrogeologic literature for the Appalachian region. For this study, well depth and well yield data for 482 water wells in the eight counties in northern West Virginia coincident with the geographic area for this study fully support the stress relief fracturing concepts of Wyrick and Borchers (1981), which were based on earlier work by Ferguson (1967). Data for the 482 wells were retrieved from the U.S. Geological Survey National Water Information System (NWIS) Groundwater Site Inventory (GWSI) database (table 4; U.S Geological Survey, written commun., 2019). Contact the USGS National Water Information System (NWIS) local data manager for the Virginia and West Virginia Science Center to request the quality control data referenced in this report. Those data and topographic assessment from digital elevation models (DEMs) show distinct differences with respect to topographic setting—wells in valleys have higher mean yields (35.6 gallons per minute [gal/min]) than wells on hillsides (29.0 gal/min) or hilltops (7.6 gal/min). Well depths were deeper on hilltops with an average depth of 173 ft compared to hillsides (average depth of 119 ft) or valley wells (average depth of 72 ft). Hilltop, hillside, and valley wells had median specific capacities of 0.72, 0.64, and 5 gallons per minute per foot (gal/min/ft) of drawdown, respectively. It should be noted that the data retrieved from the NWIS database (U.S Geological Survey, 2020) for the evaluation by Wyrick and Borchers (1981) are older and likely reflect past drilling practices that may not represent modern drilling methods. One example of this includes the fact that wells drilled with older cable tools typically are much shallower than wells drilled with modern air rotary methods. The data do, however, support the stress relief fracture theory, although overall trends are not as distinct with respect to the three topographic settings as was evident for similar data from southern West Virginia where topography is much steeper and overall relief from valley bottoms to hilltops is greater.

Table 4.    

Well depth, well yield, and specific capacity data for 482 wells in the 8-county study area comprising the wet gas dominated part of the Marcellus Shale oil and gas play in northwestern West Virginia.

[Data retrieved from the USGS Groundwater Site Information database (GWSI) database (U.S Geological Survey, written commun., 2019). ft, foot; bls, below land surface; gal/min, gallon per minute; gal/min/ft, gallon per minute per foot of drawdown]

Table 4.    Well depth, well yield, and specific capacity data for 482 wells in the 8-county study area comprising the wet gas dominated part of the Marcellus Shale oil and gas play in northwestern West Virginia.

Statistic	Well depth (ft bls)	Well yield (gal/min)	Specific capacity (gal/min/ft)
Valley wells
Count	280	280	24
Maximum	337	2,350	117.5
Minimum	16	0.07	0.02
Median	60	1.18	5
Mean	71.8	35.6	21.8
Hillside wells
Count	151	152	20
Maximum	2,300	700	440
Minimum	17	0.08	0.07
Median	90	2	0.635
Mean	119	29	29.8
Hilltop wells
Count	49	50	2
Maximum	650	300	0.93
Minimum	2	0.08	0.5
Median	135	0.88	0.715
Mean	173	7.63	0.715

Median groundwater ages determined by chlorofluorocarbon (CFC) analysis for hilltop, hillside, and valley bedrock wells in a similar hydrogeologic setting in the Monongahela River basin (which is immediately to the east of the study area), were typically on the order of 44, 31, and 35 years, respectively, whereas groundwater ages for wells sampled in West Virginia’s low-sulfur coal region in southern West Virginia were on the order of 18, 28, and 26 years, respectively (McAuley and Kozar, 2006). Relatively older groundwater is likely for wells sampled in the study area, which is adjacent to the Monongahela River basin, as compared to wells sampled in southern West Virginia, which has higher relief, steeper gradients, and much more distinct differences in topography. Longer residence time gives groundwater more time to interact with minerals in the bedrock, potentially affecting groundwater quality. The lack of clear topographic control on groundwater age in the study area may be related to the topography which is characterized as a rolling hummocky-type terrain, and therefore the distinction between hilltops, hillsides, and valleys is less prominent in northern West Virginia where total relief between ridge tops and streams is typically only 200–300 ft compared to southern West Virginia where total relief can be as much as 1,000 ft.

The Burning Springs Anticline in Wirt and Ritchie Counties is indicative of a location where hydrocarbons naturally discharge to the surface and is an area where hydrocarbons were historically skimmed from the surface of Oil Creek before modern well technology to drill deeper oil and gas wells was developed (Hennen, 1911; Eggleston, 2004). A series of freshwater and saline water maps for West Virginia (Foster, 1980) indicate that depth to saline water within the study area is typically very shallow, ranging from a minimum of 50-ft bls close to the western margin of the study area in Wetzel County bordering the Ohio River to a maximum of 750 ft in the eastern part of the study area in Calhoun County. Such shallow depths are within the depth range for rural residential water wells.

Land Cover and Land Use

Land cover in the study area is predominately forested or scrub shrub areas and represents 2,379 mi² (87 percent) of the study area. Residential and urban areas occupy approximately 15 mi² (0.5 percent). Barren land and developed open space account for 131 mi² (5 percent), 197 mi² (7 percent) is pasture, grassland, or cultivated crops, and open water and wetlands occupy 11 mi² (0.5 percent; Homer and others, 2015). The rural region has one dominant industry—oil and gas production. The study area has been developed extensively for oil and gas since 1859 (Eggleston, 2004). Approximately 57,452 oil and or gas wells have been drilled within the study area, including 1,626 multi-lateral horizontal Marcellus Shale oil and gas wells (fig. 6).

Recent data for the study area indicate that what little coal mining is occurring is primarily in Wetzel County, with a few permitted mines in Roane, Doddridge, and Gilmer Counties (West Virginia Geological and Economic Survey, 2020c). Many of these mines are inactive as indicated by 2019 coal production data, which indicates that coal mining is only active in Wetzel County within the study area (West Virginia Office of Miner’s Health Safety and Training, 2020). In 2019, underground coal mines in Wetzel County produced 6,497,707 tons of coal.

Previous Investigations

Recent investigations of groundwater quality within the study area are limited but include Chambers and others (2012, 2015) and Harkness and others (2017). Harkness and others (2017) conducted a comprehensive study of the geochemistry of naturally occurring methane and saline groundwater in an area of unconventional shale gas development and the results are similar to those of this study. In fact, the study areas for the two studies overlapped significantly and both studies collected data in Doddridge, Ritchie, Tyler, and Wetzel Counties, West Virginia. Harkness and others (2017) collected 145 groundwater samples from 105 wells in the aforementioned counties and in Harrison County, West Virginia. In addition to sampling a large number of wells over a 5-county area in northwestern West Virginia where legacy and more recent Marcellus Shale oil and gas production has occurred, that study also analyzed for a broad array of isotopic data that were not collected for our investigation. The Harkness and others (2017) study monitored geochemical variations of drinking-water wells before, during, and after the installation of nearby shale gas wells and concluded that there was no indication of groundwater contamination and subsurface impact from shale-gas drilling and hydraulic fracturing within the study area. The study also found that saline groundwater was common and indicative of an upward flow of Devonian-age brines migrating into shallow aquifers and were modified by water-rock interactions. Occurrence of ethane, propane, and carbon isotope ratios of ethane indicate that thermogenic gas contributes to the overall mixture of natural gas in shallow aquifers in the study area. Dissolved gas in shallow groundwater, however, predominantly contained microbial methane, even though both biogenic and migrated thermogenic gases occur in shallow groundwater in the study area but are unrelated to shale gas development. That study did document surface water contamination that occurred in Tyler County from two injection well sites and a flowback spill related to shale gas development.

A USGS statewide assessment of groundwater quality within West Virginia (Chambers and others, 2012) indicated that Pennsylvanian and Permian fractured-bedrock aquifers, which are typical of the current study area, can have elevated concentrations of radon-222, a carcinogenic and radioactive gas known to cause lung cancer. The highest concentrations of radon-222 documented in the statewide assessment were for wells sampled in the Blue Ridge Physiographic Province in Jefferson County in the easternmost part of West Virginia. However, median radon-222 concentrations also were commonly elevated in wells sampled in Permian and Pennsylvanian age bedrock aquifers and typically exceeded the EPA proposed 300-picocurie per liter (pCi/L) maximum contaminant level (MCL) drinking water standard. Other than radon, the statewide study did not find chemical constituents of concern for wells sampled within the general area of study for this investigation.

A water-quality assessment of stream base flow and groundwater in the Marcellus Shale oil and gas play within the Monongahela River basin, an area immediately to the east of the current study area, was conducted in 2011 and 2012 (Chambers and others, 2015). The 2011–12 study was conducted prior to high intensity development of the Marcellus Shale oil and gas play to provide a baseline of groundwater and stream base flow data for future comparison. The principal groundwater findings of the previous study were based on 39 wells and 2 springs sampled June through September of 2011.

Principal components analysis conducted for the 2011–12 study specify that the first principal component indicates gradients of redox conditions and dissolved solids concentrations with strong negative loadings for sodium, bromide, chloride, fluoride, barium, total dissolved solids, specific conductance, pH, and methane, which are inversely proportional to dissolved oxygen, carbon dioxide, argon, nitrogen, and redox potential. These gradients may reflect a continuum from deeper waters with higher concentrations of sodium, bromide, chloride, fluoride, barium and methane (constituents typically found in higher concentrations in deeper brines) to shallow, more dilute, more recent waters with higher concentrations of dissolved atmospheric gases. Whether this continuum reflects a typical gradient from recharge from shallow groundwater to deeper groundwater or indicates other pathways resulting in mixing of shallow groundwater and deeper brines is unknown.

The second principal component had strong positive loadings for calcium, magnesium, potassium, iron, manganese, sulfate, total dissolved solids, specific conductance, argon, nitrogen, carbon dioxide, and hardness, and negative loadings for geologic age, methane, redox potential, bromide, and dissolved oxygen (Chambers and others, 2015). The second component reflects a gradient from conditions typical of shallow groundwater in the Appalachian region, which have high concentrations of total dissolved solids, iron, manganese, calcium, magnesium, and sulfate, to conditions more typical of deeper groundwater. Carbonate dissolution and reduction of pyrite and siderite are dominant processes in the shallow aquifers of the region and can result in elevated concentrations of total dissolved solids, iron, manganese, calcium, and sulfate, whereas higher concentrations of chloride and bromide are more common in deeper waters.

Dissolved gas analyses for the 2011–12 study indicate that the majority of sites (38 of 41) sampled were from a shallow groundwater source. Analysis of groundwater data collected for the study with respect to geologic structure, including wells located near deep through-going faults, did not reveal a clear pattern linking methane concentrations to the presence of geologic structure.

Other prior work focused on hydrogeology, brines and salt in waters, areas affected by coal mining, or basin studies. An assessment of groundwater hydrology of the Little Kanawha River in West Virginia (Hobba, 1980) provided a preliminary assessment of groundwater quality and water-well yields in the basin. Well yields were classified by topographic setting and median well yields ranged from a minimum of less than 1 gallon per minute (gal/min) on hilltops to 14 gal/min in valleys adjacent to the stream side of wide terraces. The report included a map of specific capacity in gallons per minute per foot (gal/min/ft) of drawdown showing areas with groundwater development potential in three categories: less than 2, 2–5, and greater than 5 gal/min/ft. The map also summarized groundwater quality for 795 previously collected water-quality samples and 48 samples collected for the project. Water samples from the 48 sites were analyzed for pH, silica, calcium, magnesium, sodium, potassium, bicarbonate, carbonate, chloride, fluoride, sulfate, nitrate, aluminum, iron, manganese, hardness, and total dissolved solids (TDS), and included maps showing the suitability of groundwater for residential and public supply in the basin.

One of the more comprehensive assessments for a part of the study area was an assessment of groundwater hydrology of the alluvial and bedrock aquifers bordering the Ohio River between Chester and Waverly, West Virginia (Bader and others, 1997), an area immediately to the west of the current study area. Well yields in fractured-bedrock aquifers ranged from 1 to 300 gal/min, with a median of 6 gal/min. Groundwater samples were compared to U.S. Environmental Protection Agency (2019) drinking-water standards. Eight percent of the wells sampled contained chloride concentrations in excess of the 250-milligrams per liter (mg/L) U.S. Environmental Protection Agency (EPA) SMCL drinking water standard for aesthetic effects such as taste, odor, and color. Iron and manganese concentrations ranged from 3 to 52,000 μg/L and from less than 1 to 1,900 μg/L, respectively. Median iron concentrations were 50 micrograms per liter (μg/L) in alluvial aquifers and 16 μg/L in fractured-bedrock aquifers. Median manganese concentrations were 510 μg/L in alluvial aquifers and 56 μg/L in fractured-bedrock aquifers. Arsenic, barium, mercury or phenol were detected at concentrations exceeding EPA drinking-water standards at 7 of the 50 wells sampled. The hardness of the water exceeded 120 mg/L in 91 percent of the samples, which is characteristic of hard to very hard groundwater which can cause scaling of water lines, boilers, hot water heaters, and water distribution systems.

An inventory and assessment of salt water produced in conjunction with oil and gas development in the Little Kanawha River basin of West Virginia, which is immediately southeast of the study area, was conducted (Kirwan and Rauch, 1988) prior to the advent of horizontal well drilling methods. Based on data from 1981 to 1984, their study estimated that 4.2 × 10⁷ gallons (gal) of salt water had been produced as part of oil and gas development in the watershed, and of that amount, 2.6 × 10⁷ gal had been disposed of as surface discharge, either on land or in streams. The 2.6 × 10⁷ gal of saltwater approximated 11.9 percent of the chloride and 1.7 percent of the TDS load within the Kanawha River during the period assessed.

A report on Appalachian connate water (Heck and others, 1964) provides a discussion of connate brines in West Virginia, Pennsylvania, and Ohio. Two additional reports provide a discussion of oil and gas resources in Pleasants, Wood, and Ritchie Counties (Haught, 1955) and Doddridge and Harrison Counties (Haught, 1959) in West Virginia. None of the reports provided information on the quality or quantity of potable groundwater for residential or public groundwater supplies.

The first salt well completed in West Virginia was completed in Kanawha County in 1812. In 1815 at Charleston, West Virginia gas was first encountered while drilling for salt (Eggleston, 2004). In 1859, the Rathbone brothers bored a salt well on burning Springs Run, but rather than encountering salt, they hit petroleum at a depth of 200 feet—which produced 200 barrels a day (Eggleston, 2004). The first purposely drilled well for oil in West Virginia was completed along the Burning Springs Anticline in Wirt County in 1860 by the Rathbone brothers and is known as the Rathbone well (Hennen, 1911). The well was begun in the later part of 1859 and was completed in May of 1860 to a depth of 303 ft and produced oil at a rate of 100 barrels per day (Hennen, 1911). Prior to that, oil was skimmed off the surface of Oil Springs Creek. Discovery of oil and gas produced one of the earliest oil and gas booms in the United States. The natural occurrence of salt water and hydrocarbons at shallow depths in the basin is common, especially along the Burning Springs Anticline, and both oil and gas production and natural flow of brines and hydrocarbons have the potential to locally affect groundwater and surface-water quality within the basin. The report contains several illustrations depicting the conceptual understanding of groundwater flow and interaction of fresh and saline water within the basin.

A study of the hydrology of the U.S. Office of Surface Mining Reclamation and Enforcement (OSMRE) Area 8 Eastern Coal Province in West Virginia and Ohio (Friel and others, 1987), which includes the entire current study area, assessed the quality of groundwater and surface water in an area of limited coal mining activities. About 1 million tons of coal were mined from 26 surface mines and about 6.7 million tons from 6 underground mines in the Area 8 Coal Province in 1980. Coal mining has declined within the study area in recent years (West Virginia Office of Miner’s Health Safety and Training, 2020; West Virginia Geological and Economic Survey, 2020c) and oil and gas production, including prolific development of the Marcellus Shale oil and gas play has increased (West Virginia Geological and Economic Survey, 2020b). Based on samples collected from 158 streams within the Area 8 Eastern Coal Province study area, the median specific conductance of surface water was 260 microsiemens per centimeter (μS/cm) and ranged from 41 to 4,035 μS/cm; the median pH was 7.3 standard units and ranged from 2.9 to 8.1; alkalinity ranged from 4.8 to 2,845 mg/L, with a median of 35.5 mg/L; total iron concentrations ranged from 100 to 565,000 μg/L, with a median of 630 μg/L; and dissolved manganese concentrations ranged from 10 to 5,100 μg/L, with a median of 64 μg/L. Dissolved iron and sulfate concentrations were highest in streams draining mined areas. Water well yields assessed in the study ranged from less than 1 to 350 gal/min. Highest median iron concentrations were from streams draining coal mined areas and were 16 times higher than for streams draining unmined areas.

An assessment of water resources of the Little Kanawha River basin in West Virginia provided a comprehensive evaluation of both groundwater and surface water resources within the basin, both with respect to water quality and water quantity (Bain and Friel, 1972). Average annual streamflow of the river was 3,100 cubic feet per second (ft³/s) or 1.35 cubic feet per second per square mile (ft³/s/mi²) and recharge to groundwater was calculated to be between 5 and 7 in/yr. Water in the Little Kanawha River and its tributaries are a calcium carbonate type of water. Wells inventoried for that project ranged in yield from 1 to 8 gal/min, with median yields in the Dunkard Group, Monongahela Formation, and Conemaugh Group fractured-rock aquifers of 7.0, 6.5, and 4.0 gal/min, respectively. Potable groundwater exists to depths less than 200 ft in the western part of the basin and to depths between 500 and 1,000 ft in the eastern part of the basin. The 1972 report contains maps of chemical constituent distribution including pH, iron, chloride, and hardness.

Selection of Sampling Sites

Sampling site selection was based primarily on two criteria. First, sites were selected to provide data from areas of active or legacy oil and gas production in northwestern West Virginia in areas of current Marcellus Shale gas, oil, and natural gas liquids (NGL) production. Second, sites were selected in areas with sparse groundwater-quality data in northwestern West Virginia. Surficial geology, which commonly is used as a site selection criterion, was not an important factor for site selection in this study. Most of the study area has surficial geology within the Pennsylvanian to Permian age Dunkard Group (50 percent), the Pennsylvanian age Conemaugh Group (22 percent), or the Monongahela Formation (16 percent), all of which have similar lithologies. To a lesser extent, the study area’s surficial geology is within the Allegheny Formation (10 percent) and there are minor exposures of the Kanawha Formation and Quaternary alluvial deposits (1 percent each).

Sample Collection

Groundwater samples were collected from 30 residential water wells in the study area from June 11 to July 18, 2018, by the USGS using standard USGS methods (Wilde and others, 2004; U.S. Geological Survey, 2006). Prior to sampling, wells were purged to remove standing water from the well and ensure that representative water samples were collected. Wells were purged for a sufficient period to allow pH, dissolved oxygen, specific conductance, and water temperature to stabilize, and then water samples were collected and processed. Prolonged 3-volume purging of the wells was not required because the wells sampled were primarily rural residential wells with submersible pumps and are purged daily as part of routine use. Methods applicable for low-yield wells were utilized at a few wells to avoid pulling the water level down to the pump intake, which can cause turbidity issues, aerate the sample water, and potentially damage the well pump. Periodic water-level measurements were made and served as the criteria for length of the well purge in conjunction with close monitoring of dissolved oxygen, pH, water temperature, and specific conductance using a multiparameter water-quality sonde, which was calibrated daily.

For the wells sampled, plumbing and well-casing materials included steel, galvanized steel, polyvinyl chloride (PVC), and other plastics. The purging procedure minimized potential contamination from the well casing and plumbing, as non-contaminating Teflon sample tubing was connected as close to the wellhead as possible, usually at the pressure tank, prior to any treatment such as a water softener or chlorinator, and pumps were kept running as much as possible to prevent sample contamination from the plumbing or backflow from holding tanks. The sample line was then fed to a manifold with sampling ports and a port for connection of a multiparameter water-quality sonde. Field properties were measured in the flowthrough apparatus and monitored until parameters stabilized prior to sampling. To prevent environmental contamination, samples typically were collected and processed inside a mobile field laboratory or a portable processing chamber assembled near the closest spigot on the plumbing system to the well prior to any water treatment equipment such as chlorinators or water softeners, typically at the pressure tank.

Samples for bacterial analysis were collected and processed according to standard USGS methods (Myers and Sylvester, 2014) and processed using the Colilert system (IDEXX, 2019) according to established methods (American Public Health Association, American Water Works Association, and Water Environment Foundation, 2018), a defined substrate liquid-broth medium method for determination of total coliform bacteria and E. coli. Water samples for bacterial analysis were collected by cleansing the spigot on the pressure tank or water valve on the discharge line from the pump with isopropyl alcohol; the spigot then was allowed to thoroughly dry before filling a pre-sterilized 100-milliliter (mL) sample bottle. Bacteria samples were processed within 2 hours of collection and incubated for 24–28 hours, according to USGS standard protocols.

All samples from the 30 wells were analyzed for field properties (pH, water temperature, specific conductance, and dissolved oxygen), turbidity, alkalinity, major ions, metals, trace elements, total coliform and E. coli. bacteria, uranium, radium-226, radium-228, and gross alpha and gross beta radiation (table 5). Radon-222 was analyzed for samples from 28 of the 30 wells (two samples were lost in shipment). A subset of samples from 17 of the 30 wells were collected for analysis of dissolved hydrocarbons, sulfur hexafluoride (SF₆), CFCs, deuterium, oxygen-18, tritium, and helium as a methods development process for a secondary but related pilot study for a new USGS dissolved hydrocarbons and isotope laboratory established concurrent with this study in Reston, Virginia. Samples were processed according to standard USGS protocols (Wilde and others, 2004). Measurements of water temperature, specific conductance, dissolved oxygen concentration, pH, turbidity, and alkalinity were made on site at the time of sampling.

Quality Assurance and Quality Control

Three types of QA/QC samples were collected during the project—field blanks, equipment blanks, and replicates. Replicates were run on environmental samples to assess the reproducibility of analytical methods and assess bias that may have resulted in the laboratory analysis. Field blanks were run to assess any contamination that may have resulted from field sampling methods, to evaluate decontamination procedures between sites, and to detect contamination on sampling equipment during transit to or from the sampling site. Equipment blanks were collected in the laboratory at the USGS office in Charleston, West Virginia, and field blanks were run in the field to assess the potential of contamination from blank water used to process field and equipment blanks, the deionized water and detergent solutions used to decontaminate equipment between sampling sites, and to assess whether residual contamination was being carried over from site to site. All quality assurance data for the water samples collected for this study are stored in the quality assurance part of the USGS quality of water data (QWDATA) database for West Virginia (U.S. Geological Survey, written commun., 2022).

Variability for a replicate sample pair was quantified by calculating the relative percent difference (RPD) of the samples. The RPD was calculated using the following formula

CFC, SF₆, Tritium, Dissolved Gas, and Dissolved Hydrocarbon Sampling

Samples for CFCs, SF₆, tritium, and dissolved gas analysis were collected according to procedures described by the USGS Groundwater Dating Laboratory (https://water.usgs.gov/lab). The CFC samples were collected in 125-mL glass bottles with metal foil sealed caps. The SF₆ samples were collected in 1-L borosilicate amber glass bottles with polycone seal caps. Tritium samples were collected in 500-mL high density polyethene bottles with polycone seal caps. Dissolved gas samples were collected in pairs in pre-weighed 125-mL bottle pairs with butyl rubber septa. After sampling, the dissolved gas bottles were stored on ice and refrigerated until analysis to minimize bacterial activity. The CFC, SF₆, and dissolved gas bottles were purged and filled under water to prevent contamination from outside air.

Dissolved hydrocarbon samples were collected by filling a 1-L bottle to overflowing in a bucket then allowing a minimum of three volumes of sample water to flush through the sample bottle. The bottle was then removed from the water and three potassium hydroxide preservative tablets were added to the sample and the sample bottle was capped and taped shut. Samples were kept chilled in a cooler and then transferred to a refrigerator, prior to shipment to the USGS Groundwater Dating Laboratory in Reston, Va.

Analysis of Water Chemistry

Major ions, nutrients, metals, trace elements, and radon-222 were analyzed using standard EPA or USGS methods at the USGS National Water Quality Laboratory in Denver, Co. Trace elements and common ions, such as calcium, magnesium, sodium, potassium, iron, manganese, sulfate, chloride, bromide, and fluoride, were analyzed by inductively coupled plasma (ICP) mass spectrometry; nutrients (nitrate + nitrite, nitrite, total nitrogen, and orthophosphate) were analyzed by digestion or colorimetric methods; and radon-222 was analyzed by liquid scintillation methods. Reporting limits for these analyses are provided in table 5.

CFC, SF₆, and Dissolved Gas Analysis

CFCs (CFC-11, CFC-12, and CFC-113), SF₆, and dissolved gas samples for nitrogen, argon, methane, carbon dioxide, and oxygen analysis were shipped to the USGS Groundwater Dating Laboratory in Reston, Va. for analysis using gas chromatography methods (Busenberg and Plummer, 1992, 2000).

Tritium Analysis

Tritium samples were processed in the USGS Groundwater Dating Laboratory using Helium-3 (³He) ingrowth analysis using methods similar to those described by Schlosser and others (1988) and Papp and others (2012). For ingrowth, 500-gram (g) water samples were transferred in 1-L volume stainless steel vessels fitted with vacuum-tight copper tube stubs. The samples were on a vacuum manifold with a pressure of less than 1×10⁻⁴ torr, created by a stainless-steel liquid nitrogen trap and a rotary vane pump removing air-sourced ³He below the limits of detection. After degassing, the samples were clamped and then stored for at least 6 months to allow for tritium to decay to ³He. After the ingrowth interval, the canisters were attached to an automated vacuum manifold for ³He extraction and analysis on Thermo Scientific Helix SFT Split Flight Tube mass spectrometer. On the manifold, the ³He and any residual gases were extracted from the canisters, the gases were removed with cryogenic traps and reactive getters, and the ³He was quantitatively captured on a Gifford-McMahon cycle cryocooler with charcoal matrix at less than 10 Kelvin (K). The ³He from tritium decay was then expanded into the Helix SFT for analysis. The ³He samples were calibrated against samples of air of known ³He abundance and samples were corrected for any residual air-sourced ³He in the ingrowth canisters. Analysis of replicate samples and blind intercomparison showed method precision better that 1 percent relative standard deviation (RSD) and a lower reporting limit of 0.01 tritium unit (TU).

Dissolved Hydrocarbon Analysis

Dissolved hydrocarbon concentrations were analyzed by the USGS Dissolved Gas Laboratory. The dissolved C₁ to C₆ hydrocarbons (methane, ethane, ethene, ethyne, propane, propene, propyne, n-butane, isobutane, 1-butene, n-pentane, isopentane, 2- and 3-methylpentane, hexane, and benzene) were analyzed using a custom purge and trap gas chromatography-flame ionization detector and atomic emission detector (GC-FID/AED) system developed for quantifying trace volatile gases in water. This analytical method is sensitive to trace constituents in the picomole range, while maintaining enough dynamic range to enable quantification of samples with considerable amounts of dissolved gas and has been employed in numerous studies where trace hydrocarbon concentrations are of interest (Haase and others, 2014; Cozzarelli and others, 2017; Orem and others, 2017; Kozar and others, 2020). Duplicate samples for dissolved hydrocarbons were collected in 1-L borosilicate glass bottles, as discussed above, and refrigerated until analysis to inhibit bacterial activity and degassing. Dissolved hydrocarbon data for this study are published in a USGS data release (Haase and others, 2022).

The dissolved hydrocarbon analytical system was calibrated using a suite of certified gas standards with blend accuracies greater than 10 percent. Additionally, quality control samples consisting of homogenized tap water, nitrogen-purged water, and water purged with air containing parts-per-million to parts-per-billion by volume mixing ratios of hydrocarbon gases were collected in the laboratory in a manner identical to the field samples and analyzed to verify analytical performance (Haase and others, 2022). Methane was quantified exclusively on the flame ionization detector (FID) because concentrations are typically much higher than other hydrocarbons (Prinzhofer and others, 2000). The FID detection limit is 4.97 nanograms per kilogram (ng/kg) and the calibration precision is 0.9 percent RSD. The C₂ to C₆ hydrocarbons were simultaneously measured on the atomic emission detector (AED) and the FID, with the reported value coming from the highest sensitivity detector that was still in the range of linearity, which was typically the AED for most measurements of these compounds. The detection limits of the AED are typically in the range of 6.75 to 25.1 × 10⁻³ ng/kg, with calibration precisions between 1 and 14 percent, replicate quality control samples had concentration RSDs between 4 and 10 percent, and median percent differences between duplicate field samples were 2 to 56 percent.

Geospatial Analysis

Geospatial analysis for this project was conducted by incorporating requisite project data into ArcGIS version 10.6 (Esri, 2020). Data layers compiled or created for the ArcGIS Arc-Map project included but were not limited to (1) the locations of water wells sampled during the project, (2) locations of current and legacy vertical and more recent horizontal oil and gas wells, (3) a geologic map of the study area, and (4) significant geographic features such as locations of public water lines, roads, streams, cities, towns, county boundaries, and land use. The primary tasks of the spatial analysis were to determine (1) the extent of oil and gas well production near the sampled residential water wells (within either a 500-m or 1,000-m radius) in the study area, and (2) the geologic formation in which the sampled water wells were completed. Thus, GIS was used to determine the extent of oil and gas production and geology surrounding each residential water well sampled, which were the primary factors considered to potentially affect groundwater quality in the study area, although other factors, such as well depth and topographic setting, also were evaluated as part of the study.

Statistical and Graphical Analysis

Statistical and graphical techniques were used to summarize and compare water chemistry and field parameters among different sites according to spatial location, geology, proximity to oil and gas wells, and topographic setting. Statistical analyses were conducted and graphics were created with the R statistical computing environment version 3.4.0 (R Core Team, 2017). Non-parametric techniques were used for computing descriptive and multivariate statistics from water-quality data that were, in some instances, censored at multiple levels. Censored data are low-level concentrations of chemicals with values between zero and the laboratory’s reporting limit. The robust regression on order statistics (ROS) survival analysis method was used to calculate summary statistics for censored data because of the relatively small sample sizes and to avoid transformation bias of non-normal water-quality data (Helsel, 2012). Scatter plots were used to understand the relation between ion concentrations, mineral saturation indices, and pH. Tukey-type boxplots, censored at the highest reporting limit (Lorenz, 2018) were created to understand the relation between water quality and site characteristics, whereas trilinear Piper diagrams (Piper, 1944; Back, 1966) were used to show a graphical representation of major ion chemistry. Boxplots and Piper diagrams were created with the USGS smwrGraphs R package and, when necessary, the robust ROS method was employed to impute values for censored water-quality data (Lorenz and Diekoff, 2017). For statistical analysis, data for the wells sampled in the Conemaugh Group and Monongahela Formation were combined for multivariate statistical analysis because of the low sample size (only 7 and 4 wells sampled in the Monongahela Formation and Conemaugh Group, respectively), but were graphed as separate units in the boxplots and trilinear diagrams. Hillside and hilltop wells were also combined and labeled as upland wells for multivariate statistical and graphical analysis.

Prior to multivariate statistical analysis, and because of the presence of multiple censoring levels, censored data were re-coded to u-Scores with the codeU function in the USGS smwrQW package (Lorenz, 2018). The u-Score is the sum of the sign of the differences between each value and all other values and is equivalent to the rank but scaled so the median is equal to zero. Using u-Scores allows for the computation of multivariate relations without requiring censoring at the highest reporting limit and retains information at multiple reporting limits. In cases where a column of data has only one censoring level, the u-Scores are the same as ordinal methods of ranking for one reporting limit (Helsel, 2012).

Principal components analysis (PCA) was used to identify relations between the major chemical and hydrological processes that could explain dissolved element concentrations in the water-quality dataset. Principal component analysis is a multivariate statistical analysis method that allows rapid analysis of large datasets and extracts the eigenvectors and eigenvalues from a covariance matrix. It can be used to understand the intercorrelations of many variables and provide insight into underlying hydrogeochemical processes (Davis, 1986). PCA was computed with the principal function in the R psych package (Revelle, 2022), which first computes correlation coefficients (Spearman’s rho) for the raw u-Scores and then performs a PCA on the resulting correlation matrix (app. 1). Varimax rotation was applied to simplify the structure of the PCA model, which maximized the differences in components and aided in the interpretation of results. Water-quality variables that had missing values or were censored in more than 40 percent of the values were excluded from the PCA. The variable loadings from the varimax-rotated PCA were used to determine the master variables for each rotated component. Resulting loadings from the PCA and statistically significant correlations (p less than 0.01) from the correlation matrix were retained and used for further interpretation of the dataset.

The Wilcoxon Signed Rank Test (Helsel, 2012) was run to determine whether there were statistically significant differences between (1) the geologic formations, (2) the topographic setting (uplands or valleys) in which the water wells are present, (3) the depth of water wells sampled (less than 95 ft in depth or greater than or equal to 95 ft in depth), and (4) with respect to the number of oil and gas wells in proximity to the water wells sampled for the study (pre-1930 oil and gas wells, Marcellus Shale oil and gas play wells only, and oil and gas wells within 500 or 1,000 m of residential water wells sampled for the study). For the Wilcoxon Signed Rank Test, p less than 0.05 was used to determine if differences between groups of data are statistically significant. When p is less than 0.05, it indicates that groups are statistically different from one another, whereas if p is greater than 0.05, it indicates that differences between the two compared groups are not statistically significant.

Additional analyses were conducted by preparing trilinear diagrams showing the overall composition of the groundwater for the wells sampled with respect to the (1) geologic formations, (2) topographic setting (uplands or valleys) in which the water wells are present, and (3) depth of water wells sampled (less than 95 ft in depth or greater than or equal to 95 ft in depth). These factors commonly are found to be related to variability of groundwater quality in West Virginia. A similar approach for data analysis was used for a companion study of groundwater quality in areas of active and legacy coal mining in southern West Virginia (Kozar and others, 2020).

Finally, statistical tables of the mean, median, maximum, and minimum concentrations and the standard deviation of the data were developed for the data collected from the residential water wells sampled and assessed with respect to (1) overall groundwater quality as compared to EPA, USGS, OSMRE, and other drinking-water standards, and with respect to the (2) various geologic formations, (3) topographic setting (uplands or valleys) in which the residential wells are present, and (4) depth of wells sampled (less than 95 ft in depth or greater than or equal to 95 ft in depth).

Geochemical Modeling

Aqueous speciation and mineral saturation indices were computed with the geochemical modeling software PHREEQC (Parkhurst and Appelo, 2013). The mineral saturation index (SI) is a measure of whether a mineral has the potential to dissolve or precipitate depending on the conditions of the solution. The mineral SI is determined by dividing the ion activity product (IAP) by the thermodynamic solubility product (Ksp) and then taking the logarithm of the quotient [log(IAP/ Ksp)]. When a solution is at equilibrium, the SI is zero. In solutions where the SI is greater than zero (IAP is greater than Ksp), the solution is said to be supersaturated and the specified mineral, if present, is not likely to dissolve and may precipitate. In solutions where the SI is less than zero (IAP is less than Ksp), the solution is said to be undersaturated and the mineral, if present, could dissolve and would not precipitate (Benjamin, 2002).

Quality Assurance Results

The QA samples consisted of one replicate environmental sample pair for various analyses, one field blank, and one equipment blank. Analyte concentrations of a replicate sample pair typically differed by small amounts, less than 5 percent RPD. The only constituents that exceeded the 5-percent RPD threshold were radionuclides including gross alpha activity (one replicate pair with an RPD of 32 percent), radium-224 (one replicate pair with an RPD of 34 percent), radium-226 (one replicate pair with an RPD of 14 percent), uranium-234 (one replicate pair with an RPD of 12 percent), uranium-235 (one replicate pair with an RPD of 6 percent), and uranium-238 (one replicate pair with an RPD of 31 percent). However, the relatively large RPD for the replicate samples for radionuclides (excluding radon-222) reflects relatively greater uncertainty at low values near method reporting levels, as the field and replicate analyses for radium-224, radium-226, radium-228, and uranium were all at very low concentrations or less than the method detection limits. Of the remaining 39 constituents, the RPD could not be quantified accurately for nine constituents, as analytical results for both replicate samples were less than the method detection limits. The constituents where RPD could not be quantified accurately included beryllium, cadmium, chromium, cobalt, copper, mercury, nickel, silver, zinc, antimony, selenium, and uranium, principally due to a large number of non-detect concentrations and possibly to limitations in analytical methods.

Constituent concentrations for one field blank collected at the first site sampled generally were less than the method detection limit, indicating that field collection and processing procedures for samples were adequate to prevent cross contamination of environmental samples collected for the project. Trace metal concentrations for barium, cobalt, lead, manganese, and molybdenum indicate low level contamination or analytical bias with blank concentrations of 1.2, 0.04, 0.03, 0.717, and 0.064 µg/L, respectively. Due to the low concentrations and large number of non-detects for cobalt and lead, some bias in those data is possible, but environmental concentrations for barium, manganese, and molybdenum were orders of magnitude higher than the concentrations in the blank samples. For the replicate sample submitted for radionuclide analysis, concentrations of gross alpha activity, gross beta activity, radium-224, radium-228, uranium-235, and uranium-238 were all below the method detection limits, and radium-226 and uranium-234 were detected at concentrations of 0.022 and 0.043 pCi/L, respectively. Given that the replicate samples were sampled at the same time as the field blank and had similar concentrations, and concentrations of the radionuclide constituents were typically below the method detection limit, no significant variability with respect to analytical methods is evident.

Equipment blank data are QA samples collected in a controlled laboratory environment to assess the cleanliness of the equipment used, to test the quality of the water used for field and laboratory blanks, and to assess laboratory analytical contamination. One equipment blank was collected for the study and the data for common ion and trace metal analysis all were less than the method detection limits, indicating no bias with respect to the equipment, blank water, or laboratory analysis methods for these constituents. Equipment blank concentrations of gross alpha activity, gross beta activity, radium-224, radium-226, radium-228, uranium-235, and uranium-238 were all below the method detection limits, and uranium-234 was detected at a concentration of 0.032 pCi/L.

Overall, the QA samples indicate that data collected for the project were acceptable for the goals of the project, with minor potential field collection contamination with respect to cobalt and lead, with field blank concentrations of 0.040 and 0.030, µg/L, respectively.

Relation to Drinking-Water Standards

A primary impetus for this project was concern that trace metals, radionuclides, or other contaminants present in water from unregulated residential wells may pose a health threat to individuals that rely on these sources for their water supply. In addition, there is concern that legacy as well as current oil and gas development and production activities may adversely affect the quality of groundwater upon which residents in the study area rely for their primary source of water. As a result, groundwater-quality data collected for this study are compared to public drinking-water standards, even though those regulations are not enforceable for rural residential water supplies.

Analyte concentrations were compared to several drinking-water standards (table 7) including EPA’s maximum contaminant levels (MCLs), maximum contaminant level goals (MCLGs), health-based values (HBVs), surface water treatment rule (SWTR) treatment techniques (TT), proposed MCLs, alternate proposed maximum contaminant levels (AMCLs) for radon-222, and drinking water equivalent levels (DWEL). Full descriptions of EPA primary and secondary drinking-water standards and health advisories may be accessed at EPA web sites (U.S. Environmental Protection Agency, 2022a; 2022b). USGS health-based screening levels (HBSLs) and the OSMRE level of concern (LOC) and immediate action level (IAL) for methane in groundwater also are provided to assess the overall quality of groundwater in the study area. The EPA HBV of 20 mg/L for sodium for those on a sodium-restricted diet also is included. Full descriptions of the USGS HBSLs given by Norman and others (2018) may be accessed online at the USGS Health-Based Screening Levels website (https://water.usgs.gov/water-resources/hbsl/). Secondary maximum contaminant levels are standards that relate to aesthetic issues such as color, odor, taste, and staining of plumbing fixtures, rather than health-based criteria.

Overall, 21of 30 constituents analyzed exceeded drinking-water standards in one or more samples, including arsenic, turbidity, and bacteria that exceeded MCLs; pH, hardness, chloride, fluoride, TDS, iron and manganese that exceeded SMCLs; radon-222 that exceeded a proposed MCL; manganese that exceeded an HBSL; and sodium that exceeded the EPA HBV (table 7). Boxplots showing the distribution of some of the constituents that exceeded one or more of the drinking-water standards in 10 percent or more of the wells sampled are shown in figure 7.

Relation to Geologic Formation

The composition of the bedrock in which the water wells are completed can affect the quality of groundwater derived from the rural residential wells sampled for this study. There is substantial variability with respect to lithologic composition between the various geologic formations (fig. 8A, table 10) and even within individual strata that comprise the geologic formations (figs. 9, 10). There were statistically significant differences with respect to calcium, magnesium, potassium, silica, and turbidity (table 11). Calcium, magnesium, potassium, silica, and turbidity concentrations were statistically significantly higher in the Dunkard Group than in the Conemaugh Group or Allegheny Formation (tables 10, 11). Although not statistically significant, median sodium concentrations are highest in wells sampled in the Monongahela Formation (fig. 11). The surface geology does not allow for assessing from which specific strata (sandstone, limestone, siltstone, shale, or coal) a particular well may be deriving water. In fact, wells may obtain water from multiple intervals and therefore the following discussion is based on the major geologic groups and formations within the study area (Conemaugh Group, Monongahela Formation, or Dunkard Group).

Major Ions, Total Dissolved Solids, and Turbidity

An examination of the maximum, median, and mean concentrations for select constituents related to the various geologic groups or formations (table 10) indicate that there is variability in chemical constituent concentrations. The most significant trend in the statistical data with respect to geologic formation or group was to water hardness, calcium, magnesium, potassium, carbon dioxide, turbidity, dissolved oxygen, silica, nitrate, aluminum, iron, lead, manganese, strontium, selenium, uranium-234, uranium-238, radium-238, gross beta radiation, and the indicator bacteria total coliform and E. coli, which were all greater in water samples from wells completed in the Dunkard Group than in water samples from wells completed in either the Monongahela Formation or Conemaugh Group. Mean and median concentrations of specific conductance, alkalinity, bicarbonate, total dissolved solids, sodium, chloride, fluoride, molybdenum, gross alpha radiation, radium-224, radon-222, and uranium-235 were greater in water samples from wells sampled in the Monongahela Formation than from the Conemaugh or Dunkard Groups. Mean and median pH and concentrations of orthophosphate, radium-228, and ethane were greater in water sampled from wells in the Conemaugh Group than from wells sampled in either the Dunkard Group or the Monongahela Formation.

A trilinear diagram comparing major ion composition for the Conemaugh Group, Monongahela Formation, and the Dunkard Group (fig. 8A) does not show a substantial difference in water type for the various geologic groups or formations. This is not unexpected, as the trilinear diagram also shows variability in the composition of the major cations and anions comprising the waters sampled for the two dominant geologic groupings, and there are obvious differences in the overall composition as evidenced by the mean, median, and maximum cation and anion values (table 10). The dominant type of water for sites in all the geologic formations or groups varied between a calcium-magnesium bicarbonate water type (14 sites) to a sodium-potassium bicarbonate water type (13 sites), with additional sites being of a sodium-potassium sulfate-chloride water type (2 sites) or a calcium-magnesium sulfate-chloride water type (1 site).

A boxplot of the major cations that typically represent the majority of TDS in a water sample (fig. 9) shows higher median concentrations of calcium, magnesium, and potassium for water from wells sampled in the Dunkard Group than for wells completed in the Conemaugh Group or Monongahela Formation. Sodium concentrations were higher for wells sampled in the Conemaugh Group or Monongahela Formation than in the Dunkard Group.

There were statistically significant differences in concentrations of calcium, magnesium, potassium, silica, and turbidity for Dunkard Group wells compared with the combined data for the Conemaugh Group and Monongahela Formation. Results of the Wilcoxon Signed Rank Test of statistical comparison of populations (table 11) and statistical summaries of constituents by geologic group or formation (table 10) shows that the overall TDS, calcium, magnesium, potassium, and hardness concentrations are higher for wells sampled in the Dunkard Group than for wells sampled in the Conemaugh Group or Monongahela Formation (fig. 9). Concentrations of sodium are highest in groundwater sampled from the Conemaugh Group and Monongahela Formation (fig. 9).

Median concentrations of calcium, magnesium, potassium, silica and turbidity for wells sampled in the Dunkard Group (table 10) were 37.6, 7.55, 1.30, and 12.5 mg/L and 1.2 NTU, respectively, while median concentrations for the same constituents for wells sampled in the Monongahela Formation were 17.2, 3.55, 0.97, and 8.59 mg/L and 0.5 NTU, respectively, and for the wells sampled in the Conemaugh Group were 7.21, 1.57, 0.89, and 10.5 mg/L and 0.4 NTU, respectively. Further assessment of the geochemical processes that may be responsible for the differences in these and other chemical constituent concentrations and field measured parameters of dissolved oxygen, pH, specific conductance, and well depth will be discussed in the Geochemistry section of this report.

Iron, Manganese, pH, and Sulfate

The more problematic chemical constituents with respect to aesthetic concerns in groundwater in West Virginia are typically iron, manganese, sulfate, and pH (Chambers and others, 2012). Iron and manganese are not depicted in the trilinear diagrams comparing geologic formations (figs. 8A, 9) because they are not typically dominant ions with respect to the overall TDS of groundwater. Alkaline groundwater can induce corrosion of galvanized and copper water lines resulting in boiler scale deposits and imparting a poor taste to the water.

Boxplots of iron, manganese, sulfate, dissolved oxygen, and pH show that there are differences in the median concentrations for the sites in the various geologic formations (fig. 10) and are likely related to mineral composition of the bedrock within the various geologic groups or formations and to redox or other geochemical processes, as shown by higher median iron and manganese concentrations and lower mean pH in wells sampled from the Dunkard Group as compared to wells sampled in the Conemaugh Group and Monongahela Formation (fig. 10, table 10).

There are also differences in constituent concentrations between waters with pH characteristic of alkaline (pH greater than 7) and acidic (pH less than 7) waters (fig. 12A, B). Acidic waters consistently have higher concentrations of calcium, magnesium, potassium, and zinc. Alkaline (or basic) waters have consistently higher concentrations of bicarbonate, sodium, fluoride, and molybdenum. These relations partly reflect relative solubilities of the constituents under various pH conditions.

A trilinear diagram (fig. 8B) showing the distribution of major cations and anions classified with respect to pH indicates that geochemical processes exert strong influence on the distribution of dominant cations and anions, with waters of acidic composition being primarily a calcium-magnesium bicarbonate type of water and waters of an alkaline composition being primarily a sodium-potassium bicarbonate water type.

Results of the Wilcoxon Signed Rank Tests of statistical significance did not indicate a statistically significant difference in concentrations of iron, manganese, sulfate, or pH when comparing samples collected from wells in the Dunkard Group to samples collected from wells completed in the Conemaugh Group and Monongahela Formation, although the p value of statistical significance for pH (0.092) was close to the p value of 0.05 indicating statistical significance at a confidence interval of 95 percent (table 11). However, the lack of a trend with respect to geologic formation or group could have been due to limited data for the Conemaugh Group, which was combined for statistical analysis with data for the Monongahela Group.

Concentrations of iron, manganese, and dissolved oxygen were highest, and pH was lowest, in wells sampled in the Dunkard Group with median concentrations of 51.7 µg/L, 84.1 µg/L, 0.60 mg/L, and 7.2 standard units, respectively (table 10). Median concentrations of iron, manganese, dissolved oxygen, and median pH values for wells sampled in the Conemaugh Group were 10.0 µg/L, 5.28 µg/L, 0.30 mg/L, and 8.2 standard units, respectively (table 10). Median concentrations of iron, manganese, dissolved oxygen, and median pH values for sites sampled in the Monongahela Formation were 13.3 µg/L, 25.8 µg/L, 0.30 mg/L, and 7.6 standard units, respectively (table 10).

Median pH for water sampled from the various geologic formations and groups were all alkaline. Such high pH values are not common in groundwater in West Virginia (Chambers and others, 2012). Of special note was the median pH value of 8.2 for wells sampled in the Conemaugh Group. Limestones within the Conemaugh Group, as previously discussed in the Hydrogeologic Setting and Groundwater Flow section of this report, are primarily of marine deposition (U.S. Geological Survey and Association of American State Geologists, 2020a), which may indicate a higher solubility than the non-marine limestones, which are characteristic of the Monongahela Formation and the Dunkard Group.

Median sulfate concentrations (10.8 mg/L) are identical for wells sampled in the Monongahela Formation and Dunkard Group as compared to wells sampled in the Conemaugh Group, which has a median sulfate concentration of 8.45 mg/L. Overall sulfate concentrations for the various geologic groups and formations are all very low, with maximum concentrations for the Conemaugh Group, Monongahela Formation, and Dunkard Group of 13.9, 31.7, and 43.9 mg/L, respectively.

Radioactive Constituents

Boxplots of the uranium and radon-222 concentrations (fig. 13) and summary statistics showed little differences in the distribution of these constituents and other radionuclides (table 10) among the three geologic formations or groups. There were no statistically significant differences in radon-222, radium-226, or uranium between any of the geologic formations, as determined by the Wilcoxon Signed Rank Test.

A previous statewide study of groundwater quality in West Virginia (Chambers and others, 2012) showed that radon-222 concentrations were present in higher concentrations in younger Carboniferous units such as those in the study area—the Monongahela Formation and Conemaugh and Dunkard Groups—than in older Carboniferous units of southern West Virginia. Generally, the proportion of shale, siltstone, and mudstone varies with geologic age, with the younger Carboniferous geologic groups and formations composed of a higher proportion of dark shales than older Carboniferous geologic formations such as the Pocahontas and New River Formations, which have a higher proportion of sandstone. Radon-222 is commonly derived from uranium-bearing dark shales (Otton, 1992) such as are found in the study area within rocks of the Dunkard and Conemaugh Groups, the Monongahela Formation, and within the deeper Marcellus Shale, which is a natural gas reservoir of dark shale containing methane, ethane, and other dissolved hydrocarbons. It is likely that the radon-222 in the water samples is derived from the geologic sequences with a higher proportion of dark shales. However, there were not sufficient data collected for this study to fully assess the source of the radon-222, which often is found in concentrations exceeding the proposed 300-pCi/L MCL drinking water standard for all rural residential wells sampled for this study. Additional discussion of radionuclides and dissolved hydrocarbons will be presented in the Dissolved Hydrocarbon Gases and Geochemistry sections of this report.

Relation to Topographic Setting

A trilinear plot (fig. 8D) differentiating the wells sampled based on topographic position, either upland (hillside and hilltop) or lowland (valley), showed greater variability in major ion content and type of water for hillside and valley wells than for hilltop wells. Three hilltop wells have a calcium-magnesium bicarbonate water type signature, whereas hillside and valley wells varied from a calcium-magnesium bicarbonate water type to a sodium-potassium bicarbonate water type. Due to similar chemistry and a small number of hilltop (3 sites) and hillside (6 sites) wells, data for these sites were combined for statistical analysis and hereafter referred to as upland wells.

Although the majority of wells sampled were located in valley settings (21 sites) and only 9 sites were located on hilltops or hillsides (upland wells), there was a statistically significant difference in concentrations of several chemical constituents with respect to topographic setting when comparing samples from valley wells to samples from upland (hilltop and hillside) wells (table 11). There were statistically significant differences in valley and upland wells, with higher concentrations of manganese, radon-222, molybdenum, ammonia, methane, and ethane in samples from valley wells and higher concentrations of total coliform bacteria, calcium, magnesium, nitrate, uranium, copper, manganese, and selenium in samples from upland wells. The mean, median, and maximum values for constituents by topographic setting (table 12) show samples from upland wells appear to have higher dissolved oxygen concentrations than samples from valley wells, a finding supported by the nearly statistically significant Wilcoxon Signed Rank Test difference (p of 0.057) for dissolved oxygen between the two groups (table 11).

Examination of boxplots of constituents for upland and valley wells (fig. 14A, B) show that concentrations of dissolved oxygen, calcium, magnesium, nitrate, uranium, and copper were greater in upland wells compared to valley wells; ammonia, radon-222, manganese, and molybdenum were greater in valley wells as compared to upland wells. As was evident for chemical constituent concentrations for deep and shallow wells, the primary process likely responsible for the differences in groundwater quality, with the exception of total coliform bacteria, between upland and valley wells are redox processes. Redox and other geochemical processes affecting groundwater quality within the study area will be discussed more fully in the Geochemistry section of this report.

Median concentrations of dissolved oxygen, calcium, magnesium, nitrate, uranium, copper, and total coliform bacteria for upland wells were 1.3 mg/L, 40.1 mg/L, 13.3 mg/L, 0.199 mg/L, 0.397 µg/L, 4.30 µg/L, and 200 colonies per 100 mL of sample, respectively, and valley wells had median concentrations of 0.3 mg/L, 20.4 mg/L, 4.17 mg/L, less than the 0.040-mg/L method detection limit, 0.060 µg/L, 0.800 µg/L, and 12 colonies per 100 mL of sample, respectively (table 12). Median concentrations of ammonia, radon-222, iron, manganese, and molybdenum for uplands wells were less than the 0.010-mg/L method detection limit, 1,200 pCi/L, 10.0 µg/L, 5.00 µg/L, and 0.571 µg/L, respectively, and median concentrations of the same constituents for valley wells were 0.100 mg/L, 1,560 pCi/L, 51.7 µg/L, 84.1 µg/L and 2.18 µg/L, respectively (table 12).

Table 12.    

Statistical summary of median, mean, and maximum analytical data for select constituents analyzed in water samples from the 30 sites sampled in the wet gas dominated part of the Marcellus Shale oil and gas play in northwestern West Virginia segregated by topography.

[Data referenced in this table are available from the USGS National Water Information System (NWIS) database (U.S. Geological Survey, 2021). Upland, consists of hilltop and hillside wells; NA, not applicable; ft, foot; bls, below land surface; mg/L, milligrams per liter; std, standard; µS/cm, microsiemens per centimeter at 25 degrees Celsius; NTU, nephelometric turbidity units; <, less than; TDS, total dissolved solids; NO₃ + NO₂, nitrate plus nitrite; E. coli, Escherichia coli; MPN/100 mL, most probable number per 100 milliliters of sample; >, greater than; µg/L, micrograms per liter; pCi/L, picocuries per Liter; Th-230, thorium 230; Cs-137, cesium-137; mg/kg, milligrams per kilogram]

Table 12.    Statistical summary of median, mean, and maximum analytical data for select constituents analyzed in water samples from the 30 sites sampled in the wet gas dominated part of the Marcellus Shale oil and gas play in northwestern West Virginia segregated by topography.

Parameter	Unit	Median		Mean		Maximum
		Upland wells	Valley wells	Upland wells	Valley wells	Upland wells	Valley wells
Number of samples	NA	9	21	9	21	9	21
Site characteristics
Well depth	ft (bls)	116	85	158	96.8	460	180
Water level	ft (bls)	51	15.6	62.1	16.9	236	41.4
Field parameters
Dissolved oxygen	mg/L	1.3	0.3	2.2	0.74	9.1	3
pH	std unit	7.1	7.5	7.24	7.63	8.4	9.1
Specific conductance	µS/cm	470	459	601	577	1,110	1,580
Turbidity	NTU	1.4	0.8	8.2	1.1	60	3.5
Alkalinity	mg/L	212	190	242	196	483	334
Bicarbonate	mg/L	259	224	291	231	587	349
Carbonate	mg/L	<1	<1	2.43	4.47	14.7	31.8
Total dissolved solids and hardness
TDS	mg/L	277	267	358	331	656	863
Hardness	mg/L	157	69	182	76	381	184
Major ions
Calcium	mg/L	40.10	20.4	49.4	23.4	118	58.9
Magnesium	mg/L	13.3	4.17	14.1	4.13	28.5	11.1
Potassium	mg/L	1.37	1	1.29	1.05	1.71	2.03
Sodium	mg/L	33.4	61.4	62.7	98.1	483	328
Bromide	mg/L	0.039	0.051	0.065	0.363	0.25	3
Carbon dioxide	mg/L	29	13	34	23.8	70	139
Chloride	mg/L	7.50	21.4	33.4	55.2	170	353
Fluoride	mg/L	0.23	0.26	0.596	0.562	2.86	1.96
Silica	mg/L	13.8	10.4	13.3	11.3	20.2	17.9
Sulfate	mg/L	9.88	10.8	15.8	11.2	43.9	34.3
Nutrients
Ammonia	mg/L	<0.01	0.1	0.032	0.159	0.11	0.74
NO3 + NO2	mg/L	0.204	0.04	0.576	0.077	3.79	0.634
Nitrate	mg/L	0.199	<0.04	0.576	0.077	3.79	0.634
Nitrite	mg/L	<0.001	<0.001	<0.001	0.002	0.005	0.008
Orthophosphate	mg/L	0.009	0.021	0.014	0.046	0.036	0.181
Bacteria
E. coli	MPN/ 100 mL	<1	<1	3	22	10	220
Total coliform	MPN/ 100 mL	200	12	677	329	>2,400	>2,400
Trace metals
Aluminum	µg/L	9	5.4	15.2	10.9	70.5	101
Barium	µg/L	186	264	240	463	598	1,690
Beryllium	µg/L	<0.01	<0.01	0.026	0.019	0.06	0.05
Cadmium	µg/L	<0.03	<0.03	0.06	0.042	0.18	0.09
Chromium	µg/L	<0.5	<0.5	1.33	0.738	3	2.5
Cobalt	µg/L	0.056	0.031	0.102	0.05	0.306	0.15
Copper	µg/L	4.3	0.8	10.2	0.9	44.4	2.60
Iron	µg/L	10	51.7	112	351	487	5,150
Lead	µg/L	0.3	0.06	0.399	0.098	0.998	0.462
Manganese	µg/L	5	84.1	46.8	123	187	527
Mercury	µg/L	<0.005	<0.005	<0.005	<0.005	0.008	<0.005
Molybdenum	µg/L	0.571	2.18	1.58	3.82	7.22	16.9
Nickel	µg/L	<0.2	<0.2	0.61	0.332	1.8	1
Silver	µg/L	<1	<1	2	1.38	6	3
Strontium	µg/L	503	419	630	438	1,520	1,330
Zinc	µg/L	11.5	5.2	13.9	8.3	34	55.7
Antimony	µg/L	<0.06	<0.06	0.12	0.083	0.36	0.18
Arsenic	µg/L	0.65	2.5	4.59	6.95	26.4	31.4
Selenium	µg/L	<0.2	<0.05	0.180	0.091	0.34	0.25
Radionuclides
Gross alpha	pCi/L Th-230	4.6	1.4	4.82	2.12	19	7.3
Gross beta	pCi/L Cs-137	2.4	1.6	2.3	2.18	3.3	4.5
Radium-224	pCi/L	<0.03	0.2	0.103	0.286	0.55	1.3
Radium-226	pCi/L	0.14	0.23	0.167	0.44	0.45	1.6
Radium-228	pCi/L	0.42	0.48	0.383	0.583	0.77	1.6
Radon	pCi/L	1,200	1,560	1,080	1,640	2,300	2,910
Uranium	µg/L	0.397	0.06	0.649	0.08	2.63	0.23
Uranium-234	pCi/L	0.4	0.13	0.587	0.153	2.5	0.47
Uranium-235	pCi/L	0.03	0.02	0.022	0.016	0.05	0.038
Uranium-238	pCi/L	0.15	0.04	0.254	0.041	0.87	0.13
Dissolved hydrocarbons
Methane	mg/kg	0.002	0.088	0.02	1.74	0.087	17.9
Ethane	mg/kg	0.0000156	0.000321	0.00014	0.063	0.001	1.27

Relation to Well Depth

As discussed in the previous section, geology and topographic position can have an effect on the chemical composition of the groundwater sampled, but well depth also can be a factor. The dissolved oxygen content of shallow groundwater is typically higher because of near surface processes. For this study, well depths were grouped into two bins, one bin for wells less than 95 ft in depth and a second bin for wells greater than or equal to 95 ft in depth. These groupings were based on natural break points in the data and each bin had a sample size of 15. Unfortunately, the distribution of wells available for sampling were often older wells and information on the casing length and whether the wells were grouted adequately was not available for analysis. Therefore, this discussion is solely related to the chemical constituents commonly affected by redox processes, which are contingent on the dissolved oxygen content, or lack thereof, in the wells and aquifers sampled.

The depth of wells also can affect groundwater quality, as deeper wells typically contain lower dissolved oxygen levels than shallower wells or wells with short casings. Dissolved oxygen, or the lack thereof, is a primary factor controlling redox processes in groundwater. In this study, shallower wells (less than 95 ft deep) had slightly higher dissolved oxygen, iron, manganese, radon-222, and ammonia concentrations than deeper wells, which had slightly higher concentrations of calcium, magnesium, bicarbonate, and uranium (fig. 15A, B, table 13,). Median (50th percentile) concentrations of dissolved oxygen, iron, manganese, radon-222 and ammonia for shallow wells were 0.6 mg/L, 66.6 µg/L, 62.7 µg/l, 1,615 pCi/L, and 0.120 mg/L, respectively, whereas deeper wells had lower median concentrations of dissolved oxygen, iron, manganese, radon-222 and ammonia of 0.4 mg/L, 12.2 µg/L, 6.3 µg/l, 1,075 pCi/L, and 0.070 mg/L, respectively (table 13). Median concentrations of calcium, magnesium, bicarbonate, and uranium for shallow wells were 20.4, 3.55, 185 mg/L, and less than 0.033 µg/L, respectively, whereas deeper wells had slightly higher median concentrations of calcium, magnesium, bicarbonate, and uranium of 39.9, 7.43, 262 mg/L, and less than 0.166 µg/L, respectively (table 13).

Examination of the trilinear diagram of major ion content of the groundwater samples collected for the study did not show any obvious trends in overall types of groundwater with respect to well depth (fig. 8C) as there was variability in major ion content and water type for both shallow and deep wells. Water type generally varied from a calcium-magnesium bicarbonate type of water to a sodium-potassium bicarbonate type of water.

Wilcoxon Signed Rank Tests, computed to assess whether there were statistically significant differences in populations (table 11), confirmed statistically significant differences between deeper wells (greater than or equal to 95 ft in depth) and shallower wells (less than 95 ft in depth) for ammonia, radon-222, uranium, nickel, and bicarbonate similar to what is shown in the boxplots for these constituents grouped by depth of well (fig. 15A, B).

The primary process likely responsible for the differences in groundwater quality between deeper and shallower wells are redox processes. Redox and other geochemical processes affecting groundwater quality within the study area will be discussed more fully in the Geochemistry section of this report.

Relation to Density of Oil and Gas Wells

One primary objective of the study was to assess the water-quality data for the wells sampled with respect to the number of nearby oil and gas production wells. Processes associated with the drilling, production, maintenance, and upgrade of oil and gas wells have been postulated to possibly affect groundwater quality within the study area. There are also many older oil and gas wells within the study area that are no longer in production and have been abandoned, as well as other wells drilled prior to 1930 that were not regulated and did not require a permit and therefore may have a higher potential for failed or failing well casings. The effect, or lack thereof, of current Marcellus Shale horizontal production wells, current or legacy traditional vertical wells, and older, pre-1930, vertical wells is assessed within this report. Statistical analysis was conducted using the Wilcoxon Signed Ranks Test for the three different populations of oil and gas wells. The oil and gas wells’ spatial distributions were separated into two classes of lower and higher density for each of the three populations to assess whether there were any statistically significant differences among populations with respect to the constituents sampled for this study and land use (table 11). The first population—active oil and gas wells—was classified in two classes for multivariate statistical analysis with respect to the number of Marcellus Shale oil and gas wells (n) within 1,000 m of the rural residential wells sampled, with classes of n equal to zero (n equals 0) or n greater than or equal to 1 (n ≥ 1). The second population—older, pre-1930, oil and gas wells—was within a 1,000-m radius of the residential wells sampled, with classes of n (number of pre-1930 wells) less than (<) 3 and n ≥ 4. The third population—all oil and gas wells, regardless of type or date drilled—was within a 500-m radius of the rural residential wells sampled for the study, with classes of n (number of all oil and gas wells) ≤ 4 and n ≥ 5. The classes were determined by parsing the data into roughly equivalent numbers of observations based on natural breaks in the distribution of the data.

There were no statistically significant differences in the distribution of older, pre-1930, oil and gas wells or Marcellus Shale wells for any of the chemical constituents analyzed for the study when comparing lower and higher density of oil and gas wells within a 1,000-m radius of a sampled residential water wells (table 11). There was, however, a statistically significant difference in radium-226, barium, and ethane concentrations with respect to the density of oil and gas wells within a 500-m radius around the residential wells sampled for the study, with residential wells sampled that had four or fewer oil and gas wells surrounding the wells sampled having statistically lower concentrations of radium-226, barium, and ethane than for residential wells sampled that had five or more oil and gas wells in a 500-m radius of the residential wells sampled. Ethane is a common constituent in natural gas production, as are methane and other dissolved hydrocarbons. Barium is often associated with deep brines and has been linked to reductive weathering of shales and co-produced water during hydraulic fracturing of oil and gas reservoirs (Renock and others, 2016) but also can occur naturally in groundwater within the study area. Radium-226 is a radioactive decay product of uranium-238 and radium-226 decays to produce radon-222, a carcinogenic radioactive gas. Uranium-228, radium-226, and radon-222 are part of a natural radioactive decay cycle that eventually produces lead-208 (Otton, 1992). These radioactive constituents also are present in naturally occurring brines and flowback waters associated with unconventional Marcellus Shale gas production (Rowan and others, 2011).

Uranium, radium, and radon are commonly associated with dark shale such as the Marcellus Shale. Natural migration of hydrocarbons and associated brines into shallower aquifers has been documented within the study area, especially near the Burning Springs anticline (Hennen, 1911; Eggleston, 2004). Given the small sample size of the wells sampled for this study (30), there is still a statistically significant relation between the number of oil and gas wells within a 500-m radius of the residential wells sampled and higher concentrations of radium-226, ethane, and barium. It is not possible, however, given the data collected for this study, to definitively identify the source of these constituents in groundwater in the study area, whether derived from natural processes or derived from activities related to oil and gas development. Future work employing isotopic analysis of methane, ethane, and noble gases could provide additional insight into the source of ethane, radium-226, barium, and other constituents within the study area.

Chemical data for pertinent constituents related to oil and gas production such as barium, bromide, chloride, radium-222, methane, and ethane were plotted to assess if there was a distinct geographical bias within the data with respect to location of wells with higher constituent concentrations within groundwater in the study area. Only radium-226 showed a geographical bias with higher concentrations present in the northern part of the study and overall lower concentrations found in the southern part of the study area (fig.16). There was no apparent correlation between radium-226 and geologic formation; it is unclear what may be controlling the elevated radium-226 in the northern part of the study area.

Groundwater Age Tracers (CFCs, SF₆, and Tritium results)

Groundwater age tracers (CFCs, SF₆) were analyzed at 17 of the 30 wells sampled (Haase and others, 2022). The dissolved concentrations of the tracers were corrected for recharge temperature, excess air, and nitrification based on the nitrogen argon (N2/Ar) ratios using the Unsaturated Air (UA) model (Heaton and Vogel, 1981). Concentrations of all the age tracer results were greater than the analytical detection limits for all wells sampled indicating that water recharged after 1950 comprised a substantial fraction of each sample, as CFCs were not used substantially until the late 1940s. This was corroborated by tritium activities greater than 1 TU for all samples, indicating that the samples were predominately young (post 1950s) water (Lindsey and others, 2019). The most reliable age tracer is SF₆ because all the samples in this study had very low oxygen or were anoxic, and levels of nitrate, iron, and the presence of methane indicated reducing conditions that potentially degraded all of the CFC tracers to varying degrees. CFC-11 was degraded in most samples to levels that could not be used for dating. There were a few samples with tracer partial pressures greater than those of the atmosphere, or with apparent CFC-113 age younger than the CFC-12 apparent age, (CFC-113: Wet-0114, Wet-0140, and Dod-0070; SF₆: Dod-0070, and Rit-0128). The CFC-12 and CFC-113 piston flow ages are roughly correlated with each other and SF₆, though they are biased 20 to 30 years older than the SF₆, indicating that the CFC tracers in the study experienced similar decay rates. Several sites showed much larger differences in SF₆ concentrations (Tyl-0095, Dod-0068, and Tyl-0100), which could indicate locally higher CFC degradation rates or contributions from a non-atmospheric SF₆ source. Given that this is observed in other sites in the dataset, the apparent age of these sites is assigned from the CFC-12 age, subtracting the 27-year offset due to degradation. Overall, the combined observation of CFC degradation and non-atmospheric SF₆ makes fitting the age distributions to a specific mixing model impractical and only piston flow ages were used to determine relative differences in residence time.

Methane Geochemistry

There are several relationships in the geochemistry that are potentially relevant to the assessment of measured concentrations of methane in the groundwater in this study. The ratio of calcium to sodium in groundwater generally decreases as water equilibrates with aquifer materials and the concentration of TDS increases (fig. 17). Many of the samples in this study have high calcium to sodium (Ca/Na) ratios and low TDS concentrations, indicating the samples are dominated by recently recharged water, and that deep, long-transit-time sources of groundwater are minor.

Fifteen of the 30 samples had chloride concentrations less than 10 mg/L, limiting the ability to distinguish anion signatures that could characterize samples with halite dissolution, septic runoff, and deep Appalachian basin brine components. A subset of samples had chloride to bromide ratio (Cl/Br) mass ratios between 70 and 200 (fig. 18), indicating deep Appalachian brines mixed with the groundwater (Kessler and others, 1996; Llewellyn and others, 2015; Harkness and others, 2017). The green shaded area between 70 mg/L and 200 mg/L on figure 18 represents the ratios where Appalachian brines are expected to fall (Mullaney and others, 2009). These samples contrast with other common chloride to bromide (Cl/Br) ratios that are typical of halite dissolution (1,000–10,000 from natural and anthropogenic salt sources) and animal waste or sewage (300–1,000), which are the other probable major chloride and bromide sources to groundwater in the area (Mullaney and others, 2009).

Methane and Dissolved Hydrocarbon Composition and Origin

The abundance and sources of dissolved volatile hydrocarbons have long been a topic of study in West Virginia as the state’s abundant coal beds contribute to the presence of hydrocarbons in the state’s groundwater (Mathes and White, 2006; White and Mathes, 2006; Kozar and others, 2012; Chambers and others, 2015; Harkness and others, 2017; McAdoo and Kozar, 2017; Kozar and others, 2020). Natural gas development from the shale resources in the state have renewed interest in the origins of these compounds. Dissolved hydrocarbon concentration data in groundwater can provide information useful in determining the source and geochemical processes responsible for the presence of natural gas in shallow groundwater aquifers. Dissolved methane commonly occurs in groundwater and can originate from microbial activity in the aquifer system or from transport of thermogenic hydrocarbons. The presence of methane in West Virginia’s groundwater is well documented and generally associated with methane from coal beds (Mathes and White, 2006; White and Mathes, 2006; Kozar and others, 2012; Chambers and others, 2015; Harkness and others, 2017; McAdoo and Kozar, 2017; Kozar and others, 2020). As methane is a major component of natural gas recovered from thermally mature shales like the Marcellus Shale in West Virginia and the Utica Shale in Ohio, there is utility in assessing methane concentrations in groundwater before and during well development to establish the baseline conditions, extant sources of methane, and to help understand the relations that changes in water quality might have to surface activity associated with drilling and extraction. (Molofsky and others, 2016; Barth-Naftilan and others, 2018; Woda and others, 2018).

The presence of methane in groundwater could be an indication of contamination and other water-quality issues (Levêque and Burns, 2019; Turley and Caretta, 2020). Functionally, the presence of methane in groundwater has several effects: methane gas can accumulate in enclosed spaces where groundwater is discharged leading to a combustion hazard, it can create unpleasant qualitative characteristics (ebullition and reduced clarity), and it can potentially co-occur with other contaminants that can negatively impact the safety and utility of the water (Borch and Others, 2010). A histogram of the dissolved methane concentrations observed in this study is shown in figure 19 below. Methane concentrations in this study were generally very low, with the most probable concentration being between 0.01 and 0.1 mg/L. No samples exceeded 28 mg/L, but one sample, Rit-0119, exceeded 10 mg/L.

Methane and other dissolved hydrocarbons originate in groundwater from an array of possible biogenic pathways or from thermogenic sources. Biogenic gas can be generated in situ by multiple reductive pathways and both biogenic and thermogenic gas can be transported from their point of origin by groundwater flow or by resource extraction operations. During transport, compositions of dissolved gas can be modified by mixing with water of different geochemistry and exposure to different redox regimes, resulting in dissolved hydrocarbon compositions having unique signatures (Schoell, 1988; Colosimo and others, 2016). A defining characteristic of biogenic methane is that it does not usually co-occur with concentrations of higher chain hydrocarbons (C_n greater than 1), and therefore has C₁/C_n source ratios greater than 1,000. In contrast, thermogenic gas typically has lower C₁/C_n ratios, in the range of 10–1,000, with lower ratios representing low thermal maturity wet gas. This is because wet gas has a greater abundance of longer-chained hydrocarbons and higher ratios representing late-stage thermal maturity with lower abundances of higher hydrocarbons (C_n), but, as gas undergoes thermal maturation, the longer-chained hydrocarbons are decomposed to methane, which increases the C₁/C_n ratio, compared to that of higher thermal maturity dry gas (Schoell, 1983; Whiticar, 1999; Taylor and others, 2000). After generation, gases can be modified by oxidative processes, fractionation during transport, secondary methanogenesis, biodegradation, and methanotrophy. The cumulative processes result in signatures that can be used to infer the sources, type, and degree of modifications that have acted on a dissolved gas (Whiticar, 1999; Révész and others, 2012; Molofsky and others, 2013; Kozar and others, 2020).

For most of the samples in this study, the C₁ to C₆ hydrocarbons have characteristics that reflect a biogenic gas signature that has, to varying degrees, undergone methanotrophic oxidation processes during transport. To illustrate these processes, the concentrations of dissolved methane are plotted against the C₁/C₂ ratio and with the ratios of isobutane to n-butane (fig. 20) and isopentane to n-pentane (fig. 21). This graphic presentation allows the source, mixing, and redox characteristics to be assessed, as trends and groups in isomer fractionation (from approximate source ratios near unity (Lu and others, 2019)) from microbial action and transport are depicted in the same context as dryness and abundance (Molofsky and others, 2013; Kozar and others, 2020).

None of the samples show a characteristic thermogenic cracking pattern among the hydrocarbon ratios, with each successive pair of hydrocarbons (C₁/C₂, C₂/C₃, and so on) occurring in ratios in the percent range. Even samples with C₁/C₂ in the range of a thermogenic source did not have ratios of other (greater than C₂) hydrocarbons that indicated a thermogenic origin. Those samples with C₁/C₂ ratios in the range of 100 were generally of low concentration (less than 0.1 mg/kg), except for sample Cal-0134, which had a concentration of 8.0 mg/kg and had a C₁/C₂ ratio of 10 but was not accompanied by levels of higher hydrocarbons indicating a proximal thermogenic source of low thermal maturity. The ratios of greater than C₂ gases were also less than the range 0.1 ratio expected for thermogenic-sourced gas and were enriched with respect to branched isomers (high isoalkane to n-alkane ratios). This indicates that the gases in this sample had undergone considerable oxidative processing (Prinzhofer and others, 2000; Igari and others, 2007). Figure 20 shows there is no gradient for samples with a high methane and low methane/ethane ratio. The black lines represent the addition of biogenic methane with C₁/C₂ of 1,000 and 10,000 to background thermogenic gas with C₁/C₂ between 50 and 100. The green line represents the limit of detection of the measurements.

The plot in figure 21 shows no gradient in pentane isomer ratios for samples with high methane and low ethane/methane ratio. The black lines represent the addition of biogenic methane with C₁/C₂ of 1,000 and 10,000 to background thermogenic gas with C₁/C₂ between 50 and 100. The green line represents the limit of detection of the measurements.

The Cl/Br mass ratios of all the samples greater than 1 mg/kg methane are in the range of 70 to 200 (fig. 22), which resemble the anion signatures of Appalachian basin brines (Llewellyn and others, 2015), and coincide with the highest concentrations of methane observed in the study (shaded area on fig. 22). Additionally, all the samples with methane greater than 1 mg/kg were situated in valleys. This follows similar findings in previous studies that showed shallow aquifers in valley bottoms received brines that coincided with the presence of high concentrations of hydrocarbon gases, either from biogenic generation from long flow paths or from introduction into the aquifer by natural Appalachian basin brine movement (Heisig and Scott, 2013; Llewellyn and others, 2015; Colosimo and others, 2016; Molofsky and others, 2016; Harkness and others, 2017; Yan and others, 2017; Woda and others, 2018; Wen and others, 2019). Of the samples with methane concentrations greater than 1 mg/kg, the available groundwater age tracer data indicate that the apparent age was more than 40 years. Further, the low Ca/Na ratios (fig. 23) indicate that long flow paths can coincide with methane occurrence.

In contrast to methane and dissolved hydrocarbons collected in West Virginia’s southern coal field region (Kozar and others, 2020) where fractionation of the branched isomers increased with gas concentration at low C₁/C₂ ratios, there is not a clear trend in isomeric fractionation (isobutane to n-butane and isopentane to n-pentane) with either methane concentration or methane/ethane ratio, where the ratios of both compounds’ isomers are typically near 1. Though there is considerable variation in the butanes, samples with methane greater than 1 mg/kg tended to display enrichment in branched isomers, whereas samples with lower methane concentrations tended toward enrichment in straight-chain isomers, possibly indicating oxidation or formation pathways for some of the samples with concentrations greater than 1 mg/kg (fig. 24). No patterns were apparent for the pentane or hexane isomers.

Geochemical Evolution of Groundwater

Information on the geochemical processes controlling groundwater evolution in an aquifer system is gathered when data from numerous wells are analyzed together. Boxplots of selected major ions (fig. 25) show clear dominance of the bicarbonate anion and a mixture of calcium, magnesium, and sodium cations, which suggests multiple water-rock interactions in the shallow subsurface are altering solute concentrations from what would be expected in precipitation. Trilinear diagrams (figs. 7, 8B, 8C) also suggest two dominant water types, calcium plus magnesium bicarbonate and sodium plus potassium bicarbonate, with only a few samples showing higher proportions of sulfate for the dominant anion.

The conceptual model of geochemical evolution for groundwater in this area of West Virginia (fig. 26) starts with recharge from precipitation, is heavily influenced by water-rock interactions, and is classified by three possible redox categories. Precipitation in West Virginia has low dissolved solids, acidic pH, and is capable of dissolving carbonate minerals to buffer acidity during overland flow and infiltration. Precipitation percolates through the soil zone and recharges the saturated zone of the aquifer. Along this path, the chemically aggressive and oxygen-rich water dissolves carbonate minerals and oxidizes sulfides, which releases ions into the solution and increases TDS concentration in the water. Microbes likely mediate oxygen, nitrate, manganese, iron, and sulfate concentrations by preferential electron acceptor utilization through oxic and anoxic redox states (Ouyang and others, 2017).

The conceptual model of geochemical evolution in the shallow aquifer systems was evaluated by multivariate statistical analysis of the available chemical data for the 30 wells sampled using PCA, comparing significant Spearman’s correlation coefficients (app. 1), graphical analysis, and by computation of SIs for mineral species. The results of the PCA analysis (table 14) show four major hydrogeologic variables – carbonate dissolution, ion exchange, redox, and mixing – controlling the system with 76 percent of the total variance in the dataset explained by these first four principal components. These major hydrogeologic variables are discussed in separate sections that follow. Twenty-six of the most commonly detected variables are represented in the PCA by loadings, which are correlations of individual variables to specific principal components. Positive loadings indicate that as the value of one constituent increases, the value of the correlated constituent also increases, whereas negative loadings indicate that as the value of one constituent increases, the value of the correlated constituent decreases. Spearman’s rho correlation coefficients were found to be significant in this study for values greater than 0.463 and less than -0.463 at a confidence level of 99 percent (p less than 0.01) or between 0.362 and -0.362 at a confidence level of 95 percent (p less than 0.05; table 14; app. 1). Significant Spearman’s rho correlation coefficients for variables not assessed in the principal components model are shown in table 15.