ABSTRACT

Pike County, a 545 square-mile area in northeastern Pennsylvania, has experienced the largest relative population growth of any county in the state from 1990 to 2000 and its population is projected to grow substantially through 2025. This growing population may result in added dependence and stresses on water resources, including the potential to reduce the quantity and degrade the quality of groundwater and associated stream base flow with changing land use. Groundwater is the main source of drinking water in the county and is derived primarily from fractured-rock aquifers (shales, siltstones, and sandstones) and some unconsolidated glacial deposits that are recharged locally from precipitation. The principal land uses in the county as of 2005 were public, residential, agricultural, hunt club/private recreational, roads, and commercial. The public lands cover a third of the county and include national park, state park, and other state lands, much of which are forested. Individual on-site wells and wastewater disposal are common in many residential areas.

In 2007, the U.S. Geological Survey, in cooperation with the Pike County Conservation District, began a study to provide current information on groundwater quality throughout the county that will be helpful for water-resource planning. The countywide reconnaissance assessment of groundwater quality documents current conditions with existing land uses and may serve as a baseline of groundwater quality for future comparison.

Twenty wells were sampled in 2007 throughout Pike County to represent groundwater quality in the principal land uses (commercial, high-density and moderate-density residential with on-site wastewater disposal, residential in a sewered area, pre-development, and undeveloped) and geologic units (five fractured-rock aquifers and one glacial unconsolidated aquifer). Analyses selected for the groundwater samples were intended to identify naturally occurring constituents from the aquifer or constituents introduced by human activities that pose a health risk or otherwise were of concern in groundwater in the county. The analyses included major ions, nutrients, selected trace metals, volatile organic compounds (VOCs), selected organic wastewater compounds, gross alpha-particle and gross beta-particle activity, uranium, and radon-222. Analyses of the 20 samples were primarily for dissolved constituents, but six samples were analyzed for both dissolved and total metals.

Results of the 2007 sampling indicated few water-quality problems, although concentrations of some constituents indicated influence of human activities on groundwater. No constituent analyzed exceeded any primary drinking-water standard or maximum contaminant level (MCL) established by the U.S. Environmental Protection Agency. Radon-222 levels were greater than, or equal to, the proposed MCL of 300 picocuries per liter (pCi/L) in water from 15 (75 percent) of the 20 wells. Radon-222 levels did not exceed the alternative MCL of 4,000 pCi/L in any groundwater sample. Radon-222 is naturally occurring, and the greatest concentrations (up to 2,650 pCi/L) were in water samples from wells in members of the Catskill Formation, a fractured-rock aquifer. The dissolved arsenic concentration of 3.9 micrograms per liter (µg/L) in one sample was greater than the health-advisory (HA) level of 2 µg/L but less than the MCL of 10 µg/L. Recommended or secondary maximum contaminant levels (SMCLs) were exceeded for pH, dissolved iron, and dissolved manganese.

In six samples analyzed for dissolved and total concentrations of selected metals, total concentrations commonly were much greater than dissolved concentrations of iron, and to a lesser degree, for arsenic, lead, copper, and manganese. Concentrations of iron above the SMCL of 300 µg/L may be more widespread in the county for particulate iron than for dissolved iron. The total arsenic concentration in one of the six samples was greater than the HA level of 2 µg/L but less than the MCL of 10 µg/L. The total manganese concentration of 361 µg/L in one sample exceeded the HA of 300 µg/L for manganese in drinking water.

Chloride concentrations were above the estimated natural background levels of 1 to 5 mg/L in about half of the wells, indicating that human activities may have influenced groundwater quality. Nitrate concentrations were less than the estimated natural background level of less than 0.8 mg/L as N in all but two groundwater samples. Boron concentrations equal to or greater than 20 mg/L are above natural background levels and were measured in wells with elevated sodium and chloride levels, indicating probable association with septic effluent and (or) road salt linked to residential development.

Anthropogenic organic compounds were detected at low or trace levels in groundwater from 10 of the 20 wells, indicating human activities at the land surface have affected groundwater quality to some degree. These compounds included VOCs in 3 groundwater samples and a few organic wastewater compounds in 10 groundwater samples. The highest VOC concentration measured was 39 µg/L of Freon-11 and indicated local groundwater contamination by improper disposal of the compound. DEET (N,N-diethyl-meta-toluamide), an insect repellent, was the most frequently detected organic wastewater compound.

Concentrations of chloride and nitrite plus nitrate as nitrogen generally were greater in water from wells in the commercial and residential areas with on-site wastewater disposal than in undeveloped and sewered areas. The anthropogenic compound DEET was detected in water from wells in most land-use areas, including undeveloped. Most other organic wastewater compounds and VOCs, except for the one well sample with Freon-11 contamination, were detected in water from wells in commercial and residential areas with onsite wastewater disposal.

Age-dating by measurement of chlorofluorocarbons (CFC-11, CFC-12, CFC-113) and sulfur hexafluoride (SF₆) in groundwater in samples from three wells yielded a range of ages from 6 to 54 years before 2007, with younger water in the glacial aquifer and older water in the bedrock aquifer. These findings indicate that groundwater in both aquifer types probably is susceptible to contamination by human activities at or near the land surface. Comparison of chloride and nitrate concentrations in the 2007 samples with samples collected previously in five wells since 1982 showed that concentrations of these constituents remained similar through time in samples from some wells but increased up to three-fold, especially for chloride, in others. The two wells with the largest increases in chloride were in a residential and commercial area along Route 209, a major transportation corridor in eastern Pike County.

Overall, based on this reconnaisance study, groundwater quality in Pike County is relatively good with no constituents exceeding any primary water-quality standards. The low levels of anthropogenic organic compounds detected and elevated concentrations of chloride and related constituents (boron and nitrate) relative to background levels indicate that human activities have influenced groundwater quality in some parts of the county.

First posted July 1, 2009