GROUND-WATER QUALITY IN REGIONAL, AGRICULTURAL, AND URBAN SETTINGS IN THE PUGET SOUND BASIN, WASHINGTON AND BRITISH COLUMBIA, 1996-1998
Emily L. Inkpen, Anthony J. Tesoriero, James C. Ebbert, Steven R. Silva, and Mark W. Sandstrom
U.S. Geological Survey, Water Resources Division, 1201 Pacific Avenue, Suite 600, Tacoma, Washington 98402
A regional study, a study focused on agricultural raspberry production, and a study focused on post-1970 urban residential areas were conducted from 1996-98 as part of the U.S. Geological Survey's National Water-Quality Assessment Program to characterize the quality of shallow ground water in the unconfined portions of the Fraser aquifer in the Puget Sound Basin, Washington and British Columbia. Relations between water quality, land use, and natural and anthropogenic factors were also investigated using results for primarily nutrients, volatile organic compounds, pesticides, and radon.
In the regional study, or study-unit survey, the quality of ground water was generally good; no samples had concentrations of pesticides or volatile organic compounds that exceeded any drinking-water guidelines or health advisories, and only one concentration of nitrate exceeded the U.S. Environmental Protection Agency (USEPA) maximum contaminant level (MCL) for drinking water of 10 milligrams per liter (mg/L). The median nitrate concentration was low at 1.0 mg/L. The presence of pesticides, detected in 20 percent of the samples, and volatile organic compounds (VOCs), detected above their laboratory reporting level in 40 percent of the samples, indicates that while concentrations of contaminants are low and do not pose an immediate health risk, contaminants used at the land surface were able to migrate to ground water. Radon was detected in all samples from the study-unit survey, ranging from 100 to 860 picocuries per liter (pCi/L).
In the study of agricultural raspberry production, or agricultural land-use study, concentrations of nitrate exceeded the USEPA MCL for drinking water in 64 percent of the samples, with a median value of 12.9 mg/L. These nitrate concentrations were significantly greater than the nitrate concentrations in samples from the study-unit survey. Sources of nitrate to ground water in the agricultural land-use study area were reported by earlier studies to be primarily animal manure and inorganic fertilizers.
Pesticide compounds were widely detected in the agricultural land-use study, being found in 77 percent of the samples. The most commonly detected compounds, oxamyl, atrazine, and simazine, were known or inferred to be used in Washington or in British Columbia on raspberries and corn, which are also grown in the agricultural land-use study area. All pesticide concentrations were well below any health advisories or drinking-water guidelines.
Volatile organic compounds were detected in 82 percent of the samples from wells near raspberry production in agricultural areas. Fumigant-related compounds, mainly 1,2-dichloropropane, comprised the majority of the VOC detections. Two concentrations of 1,2-dichloropropane and one concentration of 1,2-dibromo-3-chloropropane exceeded their respective USEPA MCLs. Four samples had concentrations of 1,2-dichloropropane that exceeded the health advisory level for 10-6 (1 in a million) cancer risk. Concentrations of 1,2-dichloropropane in fumigant formulation have decreased markedly over the past few decades. As a result, much of the 1,2-dichloropropane detected may be due to historical uses of fumigants and may not be derived from current practices.
In the urban residential study, or urban land-use study, nitrate concentrations in ground-water samples were significantly greater than those from the study-unit survey when comparing samples from wells of similar depths. Differences in chemistry between sewered and unsewered areas were analyzed to determine whether septic systems were a source of nitrate to ground water in the urban area. Although analysis of chloride concentrations and isotopic ratios of nitrate nitrogen from unsewered and sewered urban areas indicated septic effluent was mixing with ground water, median nitrate concentrations in both areas were similar. These results likely reflect the fact that while septic systems are a source of nitrate where they are used, nitrate in the urban land-use study area is likely derived from multiple sources, which vary in intensity in the study area.
Pesticides were detected in 14 percent of the samples from the urban land-use study and were likely from nonresidential applications of pesticides. Volatile organic compounds were detected more often and in higher concentrations in the urban land-use study than in samples from the regional study. Forty-eight percent of the samples from the urban land-use study had detections of VOCs above laboratory reporting levels, consisting mostly of solvents and fuel-related compounds.
USGS WRIR 00-4100