Purpose and Scope

The primary objectives of this study were to assess changes in the water quality as it pertains to human health and the environment and to identify potential effects of land use on water quality. This investigation provides an update on the water quality of the sand and gravel aquifers from the countywide network of groundwater monitoring wells and selected surface-water sites. The purpose of this report is to summarize and describe the findings from the analysis of water-quality samples collected during June and July of 2020 from the MCGMN, plus four NAWQA monitoring wells and three surface-water sites, and to complete the following:

Description of Study Area

McHenry County, Ill. (611 square miles), is 35 miles northwest of the Chicago metropolitan area. As described in Gahala (2017), the county is generally geographically split by type of land use. The western half of the county is dominated by agricultural land use, and the eastern half is primarily urban land use. Geologically, the land is described as hilly with moraines, kames, eskers, and outwash plains resulting from several glacial advances and retreats during the Quaternary Period (Hackett and McComas, 1969; Berg and others, 1997; Stiff and Hansel, 2004). McHenry County is divided by two hydrologic drainage basins, the Fox River drainage basin, which drains towards the east, and the Kishwaukee River drainage basin, which drains towards the west (Nicholas and Krohelski, 1984; Berg and others, 1997; Illinois State Water Survey, 2022; Gahala, 2017, fig. 3).

Precipitation ranged from 27.07 inches (in.) in 2012 to 49.9 in. in 2018, with an average of 44.51 in. over the decade (2010–20) according to the Global Historical Climate Network observation in Woodstock, Ill. (Woodstock 5NW) (National Centers for Environmental Information, 2022a). The legacy climatological period from 1981 to 2010 from weather observation station Woodstock 5NW, in McHenry County, Ill., indicates an average annual precipitation of 36.62 in. (National Centers for Environmental Information, 2022b). The most recent standard climatological period (1991–2020) indicates an average annual precipitation of 38.68 in. (National Centers for Environmental Information, 2022c), and the latest 15-year period from 2006 to 2020 indicates an average annual precipitation of 41.18 in. (National Centers for Environmental Information, 2022d). The average annual precipitation, calculated from the daily values from 2010 to 2020 from data downloaded for the observation station Woodstock 5NW is slightly greater than the 2006–20 average annual precipitation (44.51 in.) for the decade. Increases in precipitation can affect water quality by increasing the amount of runoff and infiltration of various land-use inputs through mobilization of constituents into nearby surface waters (Hatfield and Prueger, 2004) and potentially imparting geochemical changes and transport within an aquifer.

Aquifers used to supply drinking water in McHenry County were mapped and described in detail by Meyer and others (2013). Drinking water supplies for McHenry County are pumped from deeper bedrock aquifers such as the Cambrian-Ordovician, Silurian dolomite, and the shallow sand and gravel aquifers within glacial drift deposits (Woller and Sanderson, 1976). Directions of groundwater flow were determined by Meyer and others (2013) and Gahala (2017) and are briefly summarized in this section. Groundwater is replenished (recharge) from precipitation that falls onto the land surface and infiltrates through the soil and sediment to the water table, where it flows downward and laterally through hydraulically connected sediments over time. Some of the groundwater flows laterally and discharges into local surface-water bodies. Deeper parts of the sand and gravel aquifers that are overlain by a low-permeability clay (till) can receive recharge through downward transport through saturated clays or through preferential, permeable flow paths made by thin or discontinuous clay deposits.

This study focuses on the water quality within the shallow sand and gravel aquifers. The aquifer units within the shallow sand and gravel aquifers that the monitoring wells represent are described in detail in Meyer and others (2013) and Gahala (2017) (table 1). Two wells are screened in the upper 10 ft of the Shallow Bedrock Aquifer. The source of the water is recharged from the sand and gravel aquifers.

The MCGMN and NAWQA wells are used to monitor for the effects of various land uses and changes. Samples collected from the MCGMN and NAWQA wells give a snapshot of water quality at shallow, intermediate, and deep depths. The depths of the MCGMN and NAQWA monitoring wells range from 20.3 to 344.4 ft and are generally comparable to the depth range of production and private (residential) wells extracting drinking water from the shallow sand and gravel aquifers. According to the Illinois State Water Survey, estimates from the ILWATER database indicate about 58 percent of production wells are extracting groundwater from the sand and gravel aquifers at depths of 16 to 272 ft. About 69 percent of private wells are extracting groundwater from sand and gravel aquifers at depths from 8 to 372 ft (Daniel Abrams, Illinois State Water Survey, written commun., March 23, 2022). It is important to note that these estimates from the ILWATER database are uncertain because the database does not account for wells that have not been abandoned (sealed) or categorized as both bedrock and unconsolidated (Daniel Abrams, Illinois State Water Survey, written commun., March 23, 2022).

The 2020 population estimate for McHenry County was 310,229, compared to the 2010 population of 308,760 (U.S. Census Bureau, 2021). The USGS has estimated water use for the United States every 5 years since 1950 at the State level, and since 1985 at the county level. The most recent estimate of water use was for 2015 (U.S. Geological Survey, 2020a). The water-use compilation includes data from production wells, domestic-supply wells, irrigation wells, and other sources (industrial, mining, livestock, and aquaculture) and is reported at the county level. The population served and water-use estimates for 2015 indicate that total groundwater withdrawals have decreased by about 9 percent from 2010 (U.S. Geological Survey, 2020a; table 2). The USGS is transitioning from previous approaches of 5-year annual compilations to a nationally consistent modeling platform to improve temporal and spatial discretization of water use that is planned to allow forecasting capability. Water-use data were not available for 2020 at the time of this publication.

As of 2009, land use in the county was 61 percent agriculture, 11 percent open spaces, and 16 percent residential, and the remaining land use consisted of government/institutional, commercial retail, industrial, earth extraction, and some designated as “vacant” (McHenry County Planning and Development, 2020). It is estimated that by 2030, agricultural land use will be reduced to 45 percent, and residential, open spaces, and spaces designated as “environmentally sensitive” together will account for 51 percent of the land use (McHenry County Planning and Development, 2020).

Previous Water-Quality Investigations

Countywide water quality of the sand and gravel aquifers of McHenry County was assessed from sampling of municipal supply wells in 1976 (Woller and Sanderson, 1976) and nearly quarterly sampling of residential wells from April 1979 through April 1980 (Nicholas and Krohelski, 1984), which included major ions and nutrients. Results from these studies indicated concentrations of major ions and nutrients were not substantially variable and were generally consistent across seasons. Dissolved iron had the greatest amount of fluctuation but could be affected by other factors, such as age of the residential well and piping. Health and aesthetically based water-quality benchmarks were exceeded for sodium, nitrates, iron, manganese, and dissolved solids in residential wells sampled (Nicholas and Krohelski, 1984). In 2010, 41 of 42 monitoring wells in the MCGMN plus 5 NAWQA monitoring wells were sampled for field properties, major ions, metals, trace elements, and nutrients (Gahala, 2017). Health and aesthetically based water-quality benchmarks were exceeded for arsenic, sodium, nitrate (as nitrate plus nitrite), manganese, dissolved solids, chloride, and iron.

Water-Quality Sampling

The MCGMN consists of 42 monitoring wells, of which 37 are equipped with pressure transducers that continuously monitor the groundwater levels (1 reading every 15 minutes). These data are transmitted using satellite telemetry to the USGS National Water Information System database (U.S. Geological Survey, 2020b) Water-quality samples were collected from 41 of the 42 monitoring wells in the MCGMN and 4 NAWQA monitoring wells during June–July 2020 (fig. 1, table 1). The NAWQA monitoring wells were included to assess effects from urbanization. Two monitoring wells (2–ALD–D from MCGMN and 45N9E–7.6a from NAWQA) were planned to be sampled in 2020 but were not sampled because of site conditions and well integrity issues. All groundwater monitoring wells were sampled for field properties, major ions, trace metals, and nutrients. Field properties included dissolved oxygen, pH, dissolved solids, specific conductance, water temperature, turbidity, and alkalinity. Additionally, 12 monitoring wells were sampled for the analysis of CECs. Water-quality results can be retrieved from the USGS National Water Information System database using the station identifiers in table 1. The chloride-bromide ratio analysis from Gahala (2017) was used to help select which wells would be sampled for CECs. Based on the chloride-bromide ratio analyses, 9 of the 12 wells were identified as potentially affected by sewage, and the remaining 3 wells were considered control wells and were not identified as being affected by sewage (Gahala, 2017). Water-quality sampling procedures generally adhered to the USGS “National Field Manual for the Collection of Water-Quality Data” (U.S. Geological Survey, variously dated).

Surface-water samples also were collected in 2020 to preliminarily assess the presence of CECs downstream from wastewater-treatment plants and other industrial discharges. Five grab samples were collected along the cross-sectional transect of each stream and composited with a precleaned and prerinsed churn. This approach slightly deviated from the recommended method described in the National Field Manual (U.S. Geological Survey, variously dated), where streams greater than 5 ft in width should have a minimum of 10 sample points. Cross-sectional field properties were measured within the stream with a multiparameter device at intervals of 2–5 ft at each surface-water-quality monitoring location to determine if the stream was well mixed at that location. Water-quality properties (major ions, trace elements, and nutrients) were added at two of the three surface-water locations to assess general water quality in the two streams that contribute to downstream (beyond McHenry County) drinking water sources.

Water-quality samples were collected following the methods described in the USGS “National Field Manual for the Collection of Water-Quality Data” (U.S. Geological Survey, variously dated), and field alkalinities were completed in the field using the inflection-point method. Major ions, trace metals, and nutrients were analyzed by the methods described in Fishman (1993), Fishman and Friedman (1989), and Garbarino and others (2006). WICs are a general category for anthropogenically derived chemicals that include some pharmaceuticals, pesticides, and some volatile organic compounds all quantified in parts per billion (micrograms per liter). Pharmaceuticals are quantified in parts per trillion (nanograms per liter). Pharmaceuticals and WICs were analyzed by the methods described in Furlong and others (2008 and 2014) and Zaugg and others (2006). Water-quality samples were shipped with appropriate sample preservation to the USGS National Water Quality Laboratory (NWQL) in Denver, Colorado, for analysis. The NWQL uses method reporting conventions for establishing the minimum concentration greater than which a quantitative measurement can be made (Oblinger Childress and others, 1999). These reporting conventions are the method reporting level and the laboratory reporting level. The method reporting level is defined by the NWQL as the smallest measured concentration of a substance that can be reliably measured using a given analytical method. The method detection level is the minimum concentration of a substance that can be measured and reported with 99-percent confidence that the concentration is greater than zero (Oblinger Childress and others, 1999; Barr and Davis, 2010). The laboratory reporting level is the interim reporting level, which is typically two times the method detection level to minimize false negative risks and is the smallest concentration of a chemical that can be reported by the laboratory (Williams and others, 2015). QAQC results are summarized in appendix 1 and are groundwater focused. No QAQC samples were collected for the preliminary CEC assessment in the surface water. QAQC samples include equipment blanks, field blanks, and replicates (replicate of the primary environmental sample) and were collected almost weekly throughout the duration of the sampling effort. For a total of 12 QAQC samples, 2 equipment blanks, 5 field blanks, and 5 replicates were collected. The full results of the QAQC samples and the full list of CECs sampled for are summarized in Gahala and Gruhn (2022).

The water-quality results of the 2020 data are compared to health-based and aesthetically based benchmarks in water-quality samples established by the U.S. Environmental Protection Agency (EPA), such as the drinking water advisories (DWAs) and established maximum contaminant level (MCL) and the secondary maximum contaminant level (SMCL) (U.S. Environmental Protection Agency, 2009a, b, 2018). Aesthetically based benchmarks are derived from EPA SMCLs, which are nonregulated, recommended criteria that establish taste and odor quality. Surface-water-quality results also were compared to aquatic ambient water-quality criteria established by the EPA to assess suitability for aquatic life (U.S. Environmental Protection Agency, 2021b).

Statistical Analysis

To identify changes in water quality between 2010 and 2020, two analyses were completed. Using paired samples, differences between 2010 and 2020 were assessed across all wells for each constituent. For constituents that did indicate a statistically significant difference across all wells, differences between 2010 and 2020 also were assessed for three groups of wells in either deep, intermediate, or shallow aquifers. A total of 20 constituents were measured in 43 wells in 2010 and 2020. The wells also were grouped according to aquifer depth; 10, 19, and 14 wells were designated as “shallow” (13.6–58.7 ft below ground surface), “intermediate” (62.3–120.6 ft below ground surface), and “deep” (139–344 ft below ground surface), respectively. Of the constituents, nine had at least one censored (less than the reporting level) observation. An alpha level of 0.05 was used for determining statistical significance. All statistical analyses were completed using the R statistical software program (R Core Team, 2019), specifically the NADA (Lopaka and Helsel, 2020). NADA2 (Julian and Helsel, 2021) and Wilcox (R Core Team, 2021) R packages, which include statistical tests for censored data.

The applicable statistical test depended on the presence of censored values in the dataset. The WSR test (Helsel, 2012) was used to assess changes when the data were all uncensored values. The PPW test was used when data were a mix of censored and uncensored values. Both tests are nonparametric, require no assumptions about the distribution of the data, and have a null hypothesis that the difference between paired samples is zero. Rejection of the null hypothesis indicates samples collected in 2020 differ from those collected in 2010. The PPW test is a score test and allows for left-censored values to have multiple reporting levels, meaning recensoring data to the highest reporting level is not required (Helsel, 2012). These two tests also were used when testing for differences in concentration between 2010 and 2020 for each aquifer group. Because three comparisons (one for each aquifer group) were made for each parameter, probability values (p-values) from the WSR and PPW tests were adjusted using the Benjamini and Hochberg correction, which controls the false discovery rate when completing multiple comparison tests (Benjamini and Hochberg, 1995). The combination of testing for differences between 2010 and 2020 across all wells and testing for differences within each aquifer group helped to identify at what depth changes were likely.

Field Properties

Results for field properties are presented in table 4. Dissolved oxygen concentrations at groundwater wells ranged from 0.0 to 7.0 milligrams per liter (mg/L) with a median 0.20 mg/L but were typically (80 percent) less than 1.0 mg/L. According to McCallum and others (2008), denitrification becomes more favorable at a dissolved oxygen concentration of less than 2.0 mg/L. Dissolved oxygen at surface-water sites ranged from 8.4 to 14.3 mg/L.

The pH collected at groundwater wells ranged from 6.5 to 7.6 standard pH units with a median of 7.2, and field alkalinities ranged from 268 to 429 mg/L as calcium carbonate with a median of 340 mg/L as calcium carbonate. Surface-water sites had higher pH values than wells and ranged from 8.0 to 8.9; no field alkalinities were measured at the surface-water sites.

Specific conductance measured at groundwater sites ranged from 489 to 2,343 microsiemens per centimeter at 25 degrees Celsius (µS/cm) with a median of 719 µS/cm, and specific conductance at surface-water sites ranged from 688 to 803 µS/cm. Some of the groundwater sites had elevated measurements of specific conductance and may be affected by the application of salt and salt brines on roads that can infiltrate into the groundwater. Elevated specific conductance was especially noticeable at shallow well sites that are closer to roadways.

Water temperature in groundwater wells ranged from 11.8 to 25.3 degrees Celsius (°C) with a median of 12.85 °C. Most wells were in the 11–16 °C range, but two wells were considerably higher and more similar to surface-water sites (17–ALG–S and 4–RCH–S), potentially from heating of the pump discharge line and flow-through chamber because of incomplete shading on sunny days or from the lower pumping rates allowing the water to warm within the discharge lines. Surface-water sites had water temperatures ranging from 21.3 to 28.2 °C, which is typical of many Midwest streams and rivers in June and July.

Turbidity at groundwater sites was generally low and ranged from 0.4 to 77.0 nephelometric turbidity units (NTU), and the median was 3.4 NTU. No turbidity measurements were made at surface-water sites.

Major Ions

Major ions were analyzed in the groundwater samples and two surface-water samples to compare to applicable drinking water standards, determine changes in groundwater sources, and identify potential land-use effects. Constituents included calcium, magnesium, potassium, chloride, sodium, dissolved solids, bromide, fluoride, silica, and sulfate. Results from the analysis of these samples are presented in table 5 and further described in the following paragraphs.

Calcium in monitoring wells ranged from 41.8 to 184 mg/L with a median of 85.9 mg/L, and at the two surface-water sites, results were 44.5 mg/L and 72.5 mg/L. Magnesium in wells ranged from 27.8 to 95.0 mg/L with a median of 44.9 mg/L, and concentrations in surface-water sites were 34.8 mg/L and 37.9 mg/L. Calcium and magnesium are dissolved from soils, sediments, and rock, particularly limestone and dolomite; these constituents can cause water hardness and scale formation on pipes and plumbing (National Ground Water Association, 2010).

Potassium in groundwater wells ranged from 0.83 to 8.00 mg/L with a median of 1.54 mg/L, and concentrations in surface-water sites were 2.39 mg/L and 2.73 mg/L. Potassium is not only commonly detected in clays, rocks, and soils, but also can be detected in fertilizers (Mullaney and others, 2009). High concentrations of potassium (greater than 500 mg/L) are usually indicative of a road salt source, especially if potassium chloride is applied as a deicer (National Ground Water Association, 2010; Gahala, 2017).

Sodium ranged from 4.3 to 180 mg/L with a median of 14.64 mg/L, and concentrations in surface-water sites were 29.3 mg/L and 52.3 mg/L. High occurrences of sodium were often incidental with high occurrences of chloride, which could indicate that sodium chloride is being used as a deicer for roadways.

Bromide in the groundwater wells ranged from an estimated 0.011 to 0.254 mg/L with a median of 0.031 mg/L, and concentrations at the two surface-water sites were 0.032 mg/L and 0.036 mg/L. Fluoride in groundwater ranged from 0.05 to 0.68 mg/L with a median of 0.29 mg/L, and concentrations at the surface-water sites were 0.14 mg/L and 0.16 mg/L. Silica in groundwater ranged from 8.8 to 25.3 mg/L with a median of 19.96 mg/L, and concentrations at the surface-water sites were 1.1 mg/L and 15.9 mg/L. Bromide is a trace element in seawater, or naturally dissolved from sedimentary rocks such as evaporites, carbonates, and shales (Salameh and others, 2016). Anthropogenic sources of bromide can come from septic tanks, deicers, agricultural chemicals, solvents, and gasoline additives (Davis and others, 1998). Fluoride in groundwater is from weathering and leaching of fluoride-bearing minerals from rocks and sediments (Jha and others, 2013). Anthropogenic sources of fluoride can include coal burning, oil refining, steel productions, brick-making industries, and phosphatic fertilizer plants (Jha and others, 2011).

Sulfate from wells ranged from 0.03 to 97.7 mg/L with a median of 29.7 mg/L, and concentrations at the two surface-water sites were 24.5 mg/L and 30.7 mg/L. Hydrogen sulfides commonly are associated with a “rotten egg” odor and can impart a foul smell at higher concentrations (Agency for Toxic Substances and Disease Registry, 2011). Sniff tests at the time of sampling can often determine if there is a presence of hydrogen sulfides. Additional discussion later in this report relates these values to health-based benchmarks, implications to drinking water suitability, and aquatic life criteria for surface water.

Results for major ions did not exceed any health-based benchmarks except sodium, which exceeded the DWA of 20 mg/L in 40 percent (18 of 45 groundwater sites) of samples (fig. 2, table 5). Sources of sodium include dissolution of sediments and rocks, road salt, and septic-system leachate solutions (U.S. Environmental Protection Agency, 2002; Panno and others, 2006). Excess ingestion of sodium can increase blood pressure in at-risk individuals (Centers for Disease Control and Prevention, 2022). DWAs are not legally enforceable, and the 20 mg/L benchmark applies only to people on a 500-milligram-per-day restricted sodium diet (U.S. Environmental Protection Agency, 2018).

Dissolved solids are a measure of the dissolved ion concentrations in water. Groundwater samples ranged from 272 to 1,358 mg/L with a median of 417 mg/L, and concentrations at surface-water sites were 385 and 403 mg/L. The EPA has established an SMCL for dissolved solids because they can aesthetically affect groundwater, leave objectionable tastes, and have laxative effects when consumed at concentrations greater than 1,000 mg/L (table 3). Additionally, higher concentrations of dissolved solids can cause foaming and corrosion of metals. Dissolved solids can distinguish types of aquifers or indicate contamination from landfill leachate or road salt (Clark and Piskin, 1977). Shallower wells generally having higher dissolved solids concentrations than deeper wells (fig. 3); 29 percent of the groundwater wells (13 of 45 sites) exceeded the SMCL of 500 mg/L and 4 percent (2 of 45 sites) exceeded the 1,000-mg/L concentration (fig. 4). The two surface-water sites had similar concentrations of dissolved solids and did not exceed any thresholds of concern.

Chloride in groundwater ranged from 0.55 to 517 mg/L with a median of 16.9 mg/L, and concentrations at surface-water sites were 52.8 mg/L and 89.6 mg/L (table 5). Chloride was variable across wells, and the highest concentrations were near roadways. High concentrations of chloride are often attributed to road salt being applied as a deicer during winter weather events. Chloride has an SMCL of 250 mg/L, and greater concentrations can impart a salty taste on drinking water and can contribute to the corrosion of infrastructure (Shi and others, 2009; Pieper and others, 2018). Detections of chloride were variable across the MCGMN, and the highest concentrations were detected near roadways. A total of 4 percent of the wells (2 of 45 wells) exceeded the SMCL, and several shallow wells have elevated concentrations of chloride (fig. 5).

Trace Metals

Metals and trace elements are naturally occurring in the environment, although natural concentrations can vary depending on local geology and contributions from anthropogenic activities (U.S. Geological Survey, 2019). Some metals are essential nutrients, but all metals can be toxic at some level, and for some metals, it only takes small amounts to cause toxicity (U.S. Environmental Protection Agency, 2022a). During the 2020 sampling campaign, a variety of metals and trace elements were analyzed for, and these results are presented in table 6. These results are discussed in relation to health-based benchmarks and compared to the 2010 results later in this report to determine suitability of the groundwater and surface water for drinking and the potential implications on aquatic life in surface water. In general, most metals and trace elements were less than the reporting level or detected at low concentrations and did not exceed any health-based benchmarks (table 3); however, arsenic, iron, and manganese did exceed a benchmark and are discussed individually in this section.

Results for arsenic in groundwater and surface water were variable and ranged from less than 0.20 to 70.3 micrograms per liter (µg/L). Arsenic is a naturally occurring element that is detected in sediments and rocks, and it dissolves in anoxic groundwater (Hem, 1985; Lin and Puls, 2000). Chronic ingestion of arsenic at concentrations greater than the EPA MCL of 10 µg/L has been linked to increased risk of lung, bladder, skin, liver, kidney, and prostate cancer (U.S. Environmental Protection Agency, 2018; table 3). Arsenic concentrations exceeded the MCL in about 24 percent (11 of 45) of the monitoring wells, and most locations had concentrations similar to what was measured in 2010, with slight increases in 17–ALG–D from 16.1 µg/L in 2010 to 20.7 µg/L in 2020 and in 1–CHE–D from 62 µg/L in 2010 to 70.3 µg/L in 2020 (fig. 6). The screen interval at monitoring well 1–CHE–D is open to the top of the limestone bedrock aquifer and had low dissolved oxygen (0.2 mg/L). The geochemically reducing conditions may increase the dissolution of arsenic-bearing minerals within the limestone aquifer (Renard and others, 2015). MARN–10–03 (10.1 µg/L) and MARN–10–04 (16.3 µg/L) were sampled in 2020 but were not available to sample in 2010; both wells had detections of arsenic greater than the MCL. Detections for arsenic at the two surface-water sites that were sampled did not exceed the MCL for arsenic.

Iron concentrations in groundwater ranged from less than 10 to 3,478 µg/L with a median of 1,739 µg/L. Groundwater in McHenry County has naturally elevated iron from the dissolution of glacial aquifer material and is consistent with previous water-quality investigations and this 2020 decadal update (Groschen and others, 2008; Gahala, 2017). Iron has an SMCL of 300 µg/L, and concentrations exceeding the SMCL can cause staining of household fixtures and laundry (National Ground Water Association, 2010). Concentrations greater than 1,800 µg/L can impart a metallic taste in water (Fetter, 1980, p. 355). A total of 87 percent (39 of 45 wells) exceeded the 300-µg/L SMCL, and 40 percent (18 of 45 wells) exceeded the 1,800-µg/L concentration (fig. 7). Iron in surface-water samples was less than the reporting level of 10 µg/L.

Manganese ranged from 0.5 to 740 µg/L with a median of 25.6 µg/L (table 6). Manganese naturally occurs in aquifer sediments but also can have anthropogenic sources such as industrial effluent, acid-mine drainage, sewage, and landfill leachate (Nádaská and others, 2010; World Health Organization, 2011). Long-term exposure to DWA exceedances of manganese can cause toxicity to the nervous system (World Health Organization, 2011). Manganese has a DWA of 300 µg/L (exceeded at one well) and an SMCL of 50 µg/L. Similar to iron, manganese is naturally abundant in groundwater in McHenry County because of dissolution of manganese-oxide coatings on glacial aquifer material (Groschen and others, 2008). Concentrations greater than the SMCL can cause dark brown or black staining on household fixtures and laundry. About 29 percent (13 of 45 wells) exceeded the SMCL, and the highest concentration was nearly 15 times the SMCL (740 µg/L at 10–MAR–S; fig. 8; table 3). Manganese concentrations in surface water did not exceed the aesthetic or health benchmarks.

Nutrients

Nitrogen (nitrate plus nitrite as nitrogen, hereafter referred to as “nitrate”) and phosphorus naturally occur in the environment but also can come from anthropogenic sources such as fertilizers, animal manure, and sewage (Gahala, 2017). Anthropogenic sources of nutrients, such as fertilizers, have increased the concentrations of nitrate in many streams and aquifers (U.S. Geological Survey, 1999; Dubrovsky and others, 2010; Domagalski and Johnson, 2012). In total, 8 of 45 wells, or about 18 percent of the sampled monitoring wells, had detections for nitrate (0.08 to 5.33 mg/L) and all were shallow (less than a 50-ft depth) (table 7). Nitrite ranged from less than the reporting level of 0.001 to 0.054 mg/L with a median of less than 0.001 mg/L as nitrogen. Orthophosphate ranged from less than the reporting level of 0.004 to 0.141 mg/L as phosphorus with a median of 0.010 mg/L as phosphorus. Ammonia ranged from less than 0.01 to 2.04 mg/L as nitrogen with a median of 0.34 mg/L as nitrogen. Nippersink Creek and Fox River were the two surface-water sites sampled for nutrients (table 7). Nitrate, nitrite, and orthophosphate were detected at Nippersink Creek, but the concentration of ammonia was less than the reporting level of 0.01 mg/L as nitrogen.

No detections of nitrate exceeded the MCL of 10 mg/L in 2020. Nitrate concentrations ranged from less than the reporting level to 5.33 mg/L as nitrogen. Although no nitrate samples exceeded the MCL of 10 mg/L, a few sites did indicate slight increases. Well 17–ALG–S increased from 0.71 mg/L as nitrogen in 2010 to 1.57 mg/L as nitrogen in 2020 and 10–MAR–S increased from 0.23 mg/L as nitrogen in 2010 to 1.52 mg/L as nitrogen in 2020. Nitrite concentrations ranged from less than the reporting level of 0.001 to 0.054 mg/L as nitrogen and did not exceed the MCL of 1 mg/L for the health-based benchmark in any samples. Nitrite is unstable in aerated water and is generally considered to be an indicator of contamination from sewage or organic waste (Hem, 1985) if present. Results for all nutrients are summarized in table 7.

A dissolved oxygen concentration of 2.0 mg/L is a common threshold for denitrification to exist; below this concentration, denitrification becomes favorable (McCallum and others, 2008). In all but one well, detections of nitrate were coincident with dissolved oxygen concentrations greater than 2.0 mg/L, and most of these wells were shallow wells (20–48 ft). The one exception is well 17–ALG–S, where the dissolved oxygen concentration was 0.4 mg/L and the nitrate concentration of 1.57 mg/L as nitrogen was relatively high. One potential explanation for the lower dissolved oxygen and elevated nitrate concentration may be that groundwater can move quickly through sand and gravel aquifer materials along a preferential flow path or that dissolved oxygen decreases at a faster rate than denitrification.

Contaminants of Emerging Concern

CECs are pharmaceuticals, detergents, natural and synthetic hormones, pesticides, and other chemicals such as WICs, plastic components, surfactant metabolites, and fire retardants (U.S. Environmental Protection Agency, 2009b; Tomasek and others, 2012) that are not typically treated for in wastewater-treatment plants. CECs that may be removed at wastewater-treatment plants are incidental to biological treatment and disinfection processes (U.S. Environmental Protection Agency, 2009b; U.S. Geological Survey, 2018). The release of CECs to the environment is a growing concern because of the potential to cause adverse effects in humans and other nontargeted organisms (Kolpin and others, 2002), and the use of pharmaceuticals has increased by 2.8 percent since 2015 (Martin and others, 2019; Centers for Disease Control and Prevention, 2021). The most commonly prescribed medications include analgesics, antihyperlipidemic agents, and dermatological agents (Centers for Disease Control and Prevention, 2021). WICs are a general category for anthropogenically derived chemicals that include some pharmaceuticals, pesticides, and some volatile organic compounds all quantified in parts per billion (micrograms per liter). Pharmaceuticals were analyzed in parts per trillion (nanograms per liter) and include the most commonly used prescription drugs for antibiotics, opioids, diabetes, anti-inflammatories, and high blood pressure and cholesterol (statins) medication. Atrazine is also used as an antihistamine and is referred to in the results as “pharmaceutical atrazine,” and thiabendazole is an antiworm medication (Drugs.com, 2021; WebMD, 2021).

In 2020, a subset consisting of 12 monitoring wells and 3 surface-water sites was sampled and analyzed for CECs, including pharmaceuticals, pesticides, and WICs. Chloride-bromide ratio analyses completed in Gahala (2017) identified wells that plotted near the sewage mixing curve or indicated a mix of sewage and dilute road salt. Monitoring wells with chloride-bromide ratio results in 2010 that plotted near the sewage mixing curve or mixture of sewage and dilute road salt were HEBR–09–03, 15–COR–I, 10–MAR–S, 9–MCH–S, MARN–09–01, 8–GRN–I, HARV–09–01, 4–RCH–S, and HEBR–08–02. Designated background wells were selected based on their chloride-bromide ratio plotting near high road salt mixing curves (7–HRT–S) or within the dilute groundwater range or with no clear proximity towards a particular mixing curve (8–GRN–D and WOOD–08–01).

A total of 172 CECs were analyzed for in the groundwater monitoring wells, and only 5 percent of the CECs were detected in either low or estimated concentrations, indicating a lack of widespread presence of CECs in the shallow groundwater resource and supporting the overall good quality of the shallow groundwater resource (table 8). A total of 9 CECs were detected at low concentrations in groundwater samples from 8 of the 12 (about 66 percent) monitoring wells (fig. 9; table 8); however, 3 of the 9 CECs also were detected in at least 1 blank QAQC sample (appendix 1, table 1.2; Gahala and Gruhn, 2022). These CECs are formatted in bold italics and flagged as a potential but not verified detection (table 8). The most common uses of the pharmaceuticals are provided in the parentheses in table 8 and were obtained from searching the database available at https://www.drugs.com/ (Drugs.com, 2021). Of the 111 pharmaceuticals analyzed, 2 (2 percent) pharmaceuticals were detected in groundwater samples; caffeine was detected in 1 well and nicotine was detected in 6 wells (table 8). Of the 61 WICs analyzed, 7 (about 11 percent) were detected in five groundwater samples, 10–MAR–S and 9–MCH–S had detections of 2 WICs. A total of three WIC detections are flagged because they also were detected in at least one blank QAQC sample (appendix 1, table 1.2; Gahala and Gruhn, 2022). N,N-Diethyl-m-toluamide (commonly known as DEET) was detected in every groundwater sample and surface-water sample, including all the QAQC samples (appendix 1, table 1.2; Gahala and Gruhn, 2022). This compound was removed from the results tables and discussions because of its ubiquitous detection and questionable source. The low level and often estimated concentrations not only indicate the vulnerability of groundwater with respect to sewage and septic effluents but also highlight the potential capacity of the aquifer to degrade or mitigate some of the detected CECs.

The surface-water sampling and analysis of CECs provide a general indication of the presence of CECs at locations downstream from multiple or specific wastewater-treatment discharges. The results are based on a single sample collection at three surface-water sites and do not constitute a robust analysis of the sources, range in concentrations, seasonality, and persistence. Of the 172 CECs analyzed, 35 (about 20 percent) CECs were detected in at least 1 surface-water sample and 18 were detected at all 3 surface-water sites. Of the 111 pharmaceuticals analyzed, 20 were detected in the surface-water samples, including the pharmaceutical atrazine. Of the 61 WICs analyzed, 15 were detected in surface-water samples, including the pesticide atrazine (table 8). Surface waters indicated the highest concentrations and most frequent detection of CECs. These locations were selected based on proximity to wastewater-treatment plant effluent, and the detections highlight the limits of the treatment systems.

The analytical methods for pharmaceutical compounds, in nanograms per liter, and WICs, in micrograms per liter, naturally create high variability in the quantified results of detections. For detailed discussion of the groundwater QAQC results, refer to appendix 1. The groundwater results discussed consider only the constituents that were detected at low or estimated quantities in the sample or replicate and omit consideration or discussion of constituents detected in the blank samples. No equipment blanks were collected for the surface-water sample collection. Surface-water results are considered acceptable for discussion of general presence and frequency of detections among the three samples.

Pharmaceuticals are used by individuals for health reasons and include over-the-counter medication such as aspirin and pseudoephedrine, and medications prescribed by a physician such as Lipitor, albuterol, amoxicillin, and many others (U.S. Environmental Protection Agency, 2009b). Most ingested pharmaceuticals are not fully metabolized, so parts are excreted in human waste. Metabolized and unmetabolized pharmaceuticals are discharged in domestic sewage (U.S. Environmental Protection Agency, 2009b; U.S. Geological Survey, 2021a, b). Carbamazepine is an anticonvulsant used to prevent and control seizures (Drugs.com, 2021) and was detected (2.9 nanograms per liter [ng/L]) in the replicate sample from 9–MCH–S, but was not detected in the primary environmental sample (appendix 1, table 1.2; Gahala and Gruhn, 2022). Caffeine was detected in HEBR–09–03, estimated at 5.5 ng/L. Pharmaceutical atrazine was detected in the replicate of 10–MAR–S at 5.03 ng/L (appendix 1, table 1.2; Gahala and Gruhn, 2022) but was not detected in the environmental sample at 10–MAR–S. Nicotine was detected at low and estimated concentrations in 50 percent (6 out of 12) of the groundwater samples. The number of CEC detections in the MCGMN are shown in figure 9.

Fox River and Kishwaukee River had the greatest number of pharmaceuticals detected (18 in each), whereas Nippersink Creek had 16 pharmaceutical detections. The highest concentrations detected in the three surface-water sites were for methylbenzotriazole, hexamethylenetetramine, fexofenadine, metformin, sulfamethoxazole, carbamazepine, and caffeine. The Kishwaukee River had the higher detections of lidocaine, methocarbamol, tramadol, metoprolol, venlafaxine, famotidine, and desvenlafaxine than the other two surface water sites. Nippersink Creek had higher concentrations of atrazine, cotinine, and acetaminophen than the Fox River or Kishwaukee River. Of the 13 pharmaceuticals detected in all 3 surface-water samples, concentrations were generally highest in Fox River and Kishwaukee River.

Wastewater indicators are chemical compounds that indicate the presence of domestic and industrial waste in water sources such as streams and aquifers (U.S. Geological Survey, 2021b). Domestic WICs include caffeine and cotinine (a byproduct of nicotine metabolism in the liver; Bradley and others, 2007). Industrial wastewater indicators include compounds used in plastic components, surfactant metabolites, fire retardants, and many others (Tomasek and others, 2012). Several WICs were detected in the groundwater samples from the MCGMN, and many more were detected in surface-water samples from Nippersink Creek, Fox River, and the Kishwaukee River. The following paragraphs discuss the results of WICs that are classified as “organic other” and do not fall under the pharmaceutical or pesticide/herbicide categories discussed in the previous sections.

FYROL FR 2 (0.05 µg/L) was detected in the groundwater at 10–MAR–S and is a common additive flame retardant in commercially used flexible and rigid polyurethane foams (Chemical Book, 2021). Indole (0.08 µg/L) was detected in the groundwater at 8–GRN–D and has many uses (cleaning agent, food additive, veterinary drug, fragrance, and more; PubChem, 2021a). A common food additive and fragrance (PubChem, 2021b), 2,6-Dimethylnapththalene (0.01 µg/L), was detected in the groundwater at HEBR–09–03.

Detections of WICs in surface water were most frequent and at greater concentrations in the Fox River but were generally ubiquitous in all three surface-water locations (table 8). WICs (organics/other, not including pesticides) detected in samples from all three surface-water locations include β-sitosterol (cosmetics, detergent, veterinary drug; PubChem, 2021c) and diethyl phthalate (plasticizer, solvent, household products, and more; PubChem, 2021d). No endocrine disrupting compounds such as 4-nonylphenols were detected at concentrations greater than reporting level in any of the surface-water samples. Endocrine disrupting compounds are known to cause various hormone and reproductive effects on aquatic life (U.S. Environmental Protection Agency, 2022b).

Pesticides are chemicals used to prevent, destroy, or repel living organisms from where they are not wanted (U.S. Environmental Protection Agency, 2009b). Pesticides are frequently used to increase crop yields and improve the quality of the crops; however, they can be transported to groundwater and surface water (U.S. Geological Survey, 1999, 2021a). Results for pesticide analyses are presented in table 8.

Four pesticides (prometon, atrazine, metolachlor, and pentachlorophenol) were detected in the water samples collected in 2020. Prometon is an herbicide used to control the emergence of most annual and many perennial broadleaf weeds and grasses in nonagricultural areas (U.S. Environmental Protection Agency, 1990). Prometon was detected in 9–MCH–S (estimated 0.04 ng/L; table 8) and also was detected in the associated replicate sample (0.04 ng/L; table 1.2). No detections of prometon were in the surface-water samples.

Atrazine is a common and widely used herbicide that can be applied before and after planting to control broadleaf and grassy weeds (U.S. Environmental Protection Agency, 2021a). Atrazine is used primarily in agriculture, but to a lesser extent, it is sometimes used on residential lawns and golf courses (U.S. Environmental Protection Agency, 2021a). As indicated in table 8, atrazine also can be used as antianxiety medication; therefore, atrazine has two results, the pharmaceutical method analyzed concentrations in parts per trillion (nanograms per liter), and the WIC analysis measured concentrations in parts per billion (micrograms per liter). Atrazine was not detected in any environmental samples from groundwater wells, although it was detected in the replicate sample for 9–MCH–S (3.29 µg/L) and the replicate sample for 10–MAR–S at the parts per trillion concentration of 5.03 ng/L (table 1.2). This inconsistency in detection between the parts per billion and parts per trillion concentrations and between the environmental and replicate samples highlights the variability of detections at these low levels with the laboratory methods applied as described in the “Methods of Study” section and detailed in the cited references.

All three surface-water sites had detections for atrazine (table 8). Atrazine was highest in Nippersink Creek with a concentration of 299 ng/L, as detected with the pharmaceutical method, and also was detected at 0.31 µg/L (315 ng/L) with the WIC analytical method. Consistent concentrations of atrazine between the pharmaceutical analysis and WIC analytical method also were noted for the Fox River and Kishwaukee River. Metolachlor is a broad-spectrum herbicide used for general weed control in numerous agricultural crops, lawns and turf, ornamental plants, trees, shrubs, right of ways, fence and hedge rows, and in forestry (U.S. Environmental Protection Agency, 1995). Metolachlor was not detected in any of the groundwater wells, although it was detected at all surface-water sites. One surface-water site (Fox River) had a detection for pentachlorophenol (estimated 0.13 μg/L), but no other detections of this pesticide were in surface water or groundwater.

Suitability of Water for Drinking

The largest concern for causing negative effects to the quality of groundwater as a source of drinking water is for arsenic, iron, manganese, and dissolved solids, which are naturally occurring in the sand and gravel aquifers in McHenry County (table 3). The MCL of 10 µg/L for arsenic was exceeded in about 24 percent of the wells, and chronic ingestion of arsenic in exceedance of the MCL has been linked to increased risk of lung, bladder, skin, liver, kidney, and prostate cancer (U.S. Environmental Protection Agency, 2018; table 3). Manganese, which can cause toxicity to the nervous system, has a health-based drinking water advisory of 300 µg/L, and that was exceeded at one location (U.S. Environmental Protection Agency, 2018; table 3). Aesthetically, the SMCL of 500 mg/L for dissolved solids was exceeded at 13 locations (about 29 percent). Dissolved solids in exceedance of the SMCL can cause objectionable taste and, at concentrations greater than 1,000 mg/L, can have laxative effects (U.S. Environmental Protection Agency, 2018; table 3). Two sites had dissolved solids concentrations that exceeded 1,000 mg/L (4–RCH–S and 43N8E–3.7d) (table 5). Aside from health concerns, the main concern with most of these constituents is that treatment would be needed to prevent damage to infrastructure and staining of household appliances and laundry. Another major concern for drinking water is nutrient concentrations, especially nitrate and nitrite, which have an MCL of 10 mg/L and 1 mg/L, respectively. No exceedances of the MCL were measured for either nitrate or nitrite. In general, the groundwater in McHenry County is suitable for drinking water, and some of the aesthetic issues could be mitigated with home water-treatment systems such as water softeners. The concentrations of many of these constituents could also be reduced with an at-home water-treatment system such as a carbon filtration, ion-exchange systems, and reverse-osmosis systems (Thomas and Eckberg, 2015).

The implications of low concentrations of CECs in surface water on drinking water and aquatic life are unknown, and it is important to reiterate that these results are a single sample at three sites and do not constitute a robust understanding of the distribution and persistence of these CECs. These pharmaceuticals, pesticides, and low concentrations of WICs are potentially ingested by consumers of stream-sourced drinking water and aquatic biota within the streams (Wilson and others, 2011). In a study by Benson and others (2017), the potential long-term toxicity of CECs to humans for the most frequently detected and highest concentrations of WICs and pharmaceuticals were quantified with a risk-assessment analysis. The study concluded that the exposure to these CECs is not likely to pose a public health concern; however, additional toxicity data would be helpful to improve the calculated risks (Benson and others, 2017). Also, studies have determined that many of these CECs may degrade naturally in the streams (Bradley and others, 2007) whereas others persist miles downstream (Barber and others, 2013), including some of the contaminants detected in this study (sulfamethoxazole and carbamazepine). The persistence, seasonality, sources, and range in concentrations and effects of CECs in surface water were not investigated as part of this study. Frequent testing by water-treatment facilities provides the data on exposure levels and baseline understanding for potential effects.