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Despite the reliance on groundwater by approximately 2.4 million rural Pennsylvania residents, publicly available data to characterize the quality of private well water are limited. As part of a regional effort to characterize groundwater in rural areas of Pennsylvania, samples from 54 domestic wells in Clinton County were collected and analyzed in 2017. The samples were evaluated for a wide range of constituents and compared to drinking-water health standards and geochemical characteristics. The sampled wells were completed to depths ranging from 46 to 500 feet in bedrock that was of predominantly sandstone, shale, or carbonate lithology. Results of this study show that the sampled groundwater quality in Clinton County generally met most drinking-water standards that apply to public water supplies. However, a percentage of samples exceeded drinking-water maximum contaminant levels (MCLs) for total coliform bacteria (57.4 percent), Escherichia coli (E. coli) (25.9 percent), nitrate (1.9 percent), and arsenic (1.9 percent); and secondary maximum contaminant levels (SMCLs) for pH (31.5 percent), manganese (29.6 percent), iron (13 percent), total dissolved solids (7.4 percent), aluminum (1.9 percent), and chloride (1.9 percent). Sodium concentrations exceeded the U.S. Environmental Protection Agency drinkingwater advisory recommendation in 16.7 percent of the samples. Radon-222 activities exceeded the proposed drinking-water standard of 300 picocuries per liter (pCi/L) in 59.3 percent of the samples. The only volatile organic compounds (VOCs) detected were acetone and methyl ethyl ketone in two separate samples; neither constituent exceeded drinking-water standards. Higher median nitrate concentrations were found in the carbonate (3.26 milligrams per liter [mg/L]) versus shale (less than 0.04 mg/L) and sandstone (0.27 mg/L) aquifer subsets. Most of the elevated nitrate concentrations were associated with E. coli detections in the carbonate aquifers, where transmissive bedrock can facilitate groundwater contamination by human activities at the land surface. The median pH of groundwater from the sandstone aquifers (6.53) was less than those for the shale aquifers (7.31) and carbonate aquifers (7.43). Generally, the lower pH samples had greater potential for elevated concentrations of dissolved metals, including beryllium, copper, lead, nickel, and zinc, whereas the higher pH samples had greater potential for elevated concentrations of total dissolved solids, sodium, fluoride, boron, and uranium. Near-neutral samples (pH 6.5 to 7.5) had greater hardness and alkalinity concentrations than other samples with pH outside this range. Many samples from the shale or sandstone aquifers, particularly those with pH less than 6.5, were identified as having serious potential corrosivity based on the combination of the calcite saturation index and the chloride to sulfate mass ratio; however, none of the samples from the carbonate aquifers was identified as seriously corrosive. Groundwater from 3.7 percent of the wells had concentrations of methane greater than the Pennsylvania action level of 7 mg/L, and 48 of the 54 wells (88.9 percent) had detectable concentrations of methane greater than the 0.0002 mg/L detection limit. Greater methane concentrations were found more frequently in groundwater sampled from the shale aquifers than the carbonate or sandstone aquifers in the study area. Most of the samples containing elevated methane (greater than 0.2 mg/L) were located outside the area of the Appalachian Plateaus. The elevated concentrations of methane generally were associated with suboxic groundwater (dissolved oxygen less than 0.5 mg/L) that had near-neutral to alkaline pH and were correlated with concentrations of iron, manganese, ammonia, sodium, lithium, barium, fluoride, and boron. The stable carbon and hydrogen isotopic compositions of methane in two of four samples analyzed for isotopes were consistent with compositions reported for mud-gas logging samples from gas-bearing geologic units (thermogenic gas) in the Appalachian Plateaus region, whereas two others were consistent with methane of microbial origin or a mixture of microbial and thermogenic gas. Forty-two percent of samples had chloride concentrations greater than 20 mg/L with variable bromide concentrations. Corresponding chloride/bromide ratios are consistent with low-bromide sources such as road-deicing salt and septic effluent or animal waste, or, in a few cases, high-bromide brine. Brines characterized by relatively high bromide are naturally present in deeper parts of the regional groundwater system and, in some cases, may be mobilized by gas drilling. The chloride, bromide, and other constituents in road-deicing 2 Groundwater Quality in Relation to Drinking Water Health Standards for Wells in Clinton County, Pennsylvania salt or brine solutions tend to be diluted by mixing with fresh groundwater in shallow aquifers used for water supply. One of the four groundwater samples with methane concentrations greater than 4 mg/L had chloride and bromide concentrations and a chloride/bromide ratio that indicates mixing with a salinity source such as road-deicing salt, whereas the chloride and bromide concentrations and ratios for the other three high-methane samples indicate mixing with a small amount of brine (0.03 percent or less). In two other eastern Pennsylvania county studies where gas drilling is absent, groundwater with comparable chloride/bromide ratios, bromide, and chloride concentrations plus other element associations have been reported. Additional sampling and analysis, such as isotopic analysis of the dissolved gas, fracture analysis, and more detailed evaluation of surrounding land uses, may be warranted to better understand the origin of the methane and brine constituents in groundwater at specific locations. Introduction Pennsylvania has the second highest number of residential wells of any state in the Nation with approximately 2.4 million residents that depend on groundwater for their domestic water supply. Despite the widespread reliance on groundwater in rural areas of the state, publicly available data to characterize the quality of private well water are sparse (Low and Chichester, 2006; Giddings, 2014; Johnson and Belitz, 2017). In Clinton County, approximately 20 percent of the residents use groundwater from private domestic-supply wells as their primary drinking source (Pennsylvania Bureau of Topographic and Geologic Survey, written commun., 2017). The U.S. Environmental Protection Agency (EPA) has established human-health water-quality standards classified as maximum contaminant levels (MCLs), secondary maximum contaminant levels (SMCLs), and action levels for some constituents in drinking water (U.S. Environmental Protection Agency, 2012a). MCLs generally are set because elevated concentrations of these constituents may cause adverse health effects. SMCLs generally are set for aesthetic reasons; elevated concentrations of these constituents may impart an undesirable taste or odor to water. These MCLs and SMCLs may be used as a guideline for private well owners but are only enforced for public drinking-water supplies. Groundwater from more than one in five private domestic-supply wells in the Nation potentially exceeds a human-health water-quality standard and yet, the potential health risks associated with domestic groundwater supplies are understudied in comparison to the research and testing of public water supplies (DeSimone, 2009). Unlike public water-supply systems, managing domestic-supply wells in Pennsylvania is solely the responsibility of the homeowner. Drinking water health concerns can be associated with naturally occurring chemicals derived from the surrounding bedrock and possibly combined with anthropogenic (manmade or human derived) inputs from sewage, agricultural activities, and chemical waste spills or disposal. Acute gastrointestinal illnesses have been attributed to parasitic, viral, and bacterial pathogens in rural private well water from human and animal waste (Raina and others, 1999; Murphy and others, 2016). Metals such as copper and lead can be present in the soil and rock or leached from plumbing components by corrosive groundwater (Belitz and others, 2016). Long-term exposure to excess copper can cause liver and kidney damage, and children exposed to lead can have delayed mental and physical development (Brown and Margolis, 2012; U.S. Environmental Protection Agency, 2012a). The presence of naturally occurring iron and manganese in drinking water can be a nuisance owing to unpleasant taste, odor, and color, and the associated staining and clogging of household plumbing fixtures (Penn State Extension, 2017). Long-term exposure to arsenic, which is another potentially naturally occurring contaminant in groundwater (Chapman and others, 2013), has been linked to skin and internal cancers (National Research Council, 1999). Various naturally occurring radioactive elements and manmade volatile organic compounds (VOCs) that may be present in drinking water can be carcinogenic, mutagenic, or teratogenic (Otton, 1992; National Research Council, 1998; Hopke and others, 2000). Elevated levels of dissolved methane and associated hydrocarbon gases can pose an explosive safety risk if wells or water tanks are not properly vented and the gas is ignited (Swistock and Sharpe, 2006). The quality of well water used for domestic supply is influenced by the regional and local setting, including the topography, geology, and land use, plus well-construction features and household plumbing. Wells drilled in the bedrock of northern Pennsylvania are typically cased from the surface down to competent bedrock without grouting and consist of an open hole that intersects water-bearing zones in the rocks (aquifer), whereas those constructed in unconsolidated sand/ gravel may utilize open-ended casing or be screened in the water-bearing zone

scientific investigations report

Groundwater quality in relation to drinking water health standards and geochemical characteristics for 54 domestic wells in Clinton County, Pennsylvania, 2017