National Water-Quality Assessment--Potomac River Basin: Assessment of Ground-Water Quality in the Great Valley Carbonate Subunit, Potomac River Basin Study Unit, National Water-Quality Assessment Program

Abstract

One approach used to assess the ground-water quality in the Potomac River Basin study unit of the U.S. Geological Survey's National Water-Quality Assessment (NAWQA) program is the land-use study, which focuses on the factors that affect the quality of shallow ground water that underlies major categories of land use. The Great Valley carbonate subunit of the Potomac River Basin was selected for a land-use study in 1993, because of the potential effects of intensive agricultural practices on water quality of the basin.

Thirty primary sampling locations and three alternatives were randomly selected by use of an equal-area cell grid. Domestic wells in each of the 30 cells were selected, using criteria that included well depth, casing material, construction method, age, type of pump installed, accessibility, and surrounding land use. Water samples from these 30 wells were collected from June to September 1993 and analyzed for pH, dissolved oxygen, temperature, alkalinity, specific conductance, major ions, nutrients, pesticides, dissolved organic carbon, radon, and uranium. Water samples from 17 of the 30 wells were analyzed for volatile organic compounds (VOC's).

Nitrogen in the form of nitrate plus nitrite was detected in measurable quantities in samples from 29 of 30 wells. Concentrations of nitrate plus nitrite exceeded the U.S. Environmental Protection Agency's (USEPA) Maximum Contaminant Level (MCL) of 10 milligrams per liter (mg/L) as nitrogen in about 25 percent of the samples; the highest value was 29 mg/L. Ammonia was also detected in water samples from 29 of the 30 wells; concentrations ranged from 0.01 to 1.50 mg/L.

Pesticides were detected in approximately 90 percent of the samples. The triazine herbicides atrazine and simazine and the atrazine metabolite deethylatrazine were detected in more than 50 percent of the samples. The herbicides metolachlor and prometon were detected in about 33 percent of the samples. The herbicide terbacil was detected in one sample; the herbicide EPTC and the DDT metabolite p,p'-DDE were detected at or near the detection limit in samples from several wells. No other pesticide for which analysis was conducted was found above the detection limit. None of the pecticide concentrations exceeded USEPA MCL's.

VOC's (specifically chloroform and toluene) were detected in only one sample in concentrations of 0.4 and 0.5 micrograms per liter, respectively. Radon was detected in concentrations that exceeded the USEPA's proposed MCL of 300 picocuries per liter (pCi/L) in 24 of 30 wells; the highest concentration was 1,700 pCi/L.

Citation

Ferrari, M. J., 1994, Assessment of ground-water quality in the Great Valley carbonate subunit, Potomac River Basin study unit, National Water-Quality Assessment Program [abs.] : Abstract Booklet, Mid-Atlantic Highlands Area Environmental Monitoring and Assessment Conference, Hershey, Pennsylvania, February 23-25, 1994, p. 50.