Shallow ground-water quality was studied in the Columbia metropolitan area as part of the U.S. Geological Survey National Water-Quality Assessment Program. The study was designed to examine the recent effects of human activities on shallow ground water in an urban setting. Thirty shallow monitoring wells were installed in selected residential and commercial areas that were constructed between 1960 and 1990. Ground-water samples were collected during the summer and fall of 1996 and analyzed for major ions, nutrients, pesticides, volatile organic compounds, chlorofluorocarbons, and dissolved gasses.

Significant findings are as follows:

• Nitrate nitrogen was detected at 26 ground-water sites. The median concentration was 1.0 milligram per liter and all concentrations were below the U.S. Environmental Protection Agency maximum contaminant level of 10 milligrams per liter for drinking water.

• Pesticides were detected at 22 ground-water sites. All pesticide concentrations were below existing U.S. Environmental Protection Agency maximum contaminant levels. Atrazine, deethyl atrazine, simazine, and dieldrin were the most commonly detected pesticides and pesticide metabolites in samples from the monitoring wells. Atrazine and simazine have large ground-water leaching potentials which make them more likely to be detected in ground water.

• Volatile organic compounds were detected at 27 ground-water sites, and 2 sites had 15 different volatile organic compounds. Methyl tert-butyl ether and trichloroethylene exceeded the U.S. Environmental Protection Agency maximum contaminant levels once each at 2 different ground-water sites. Chloroform was detected at 21 ground-water sites. Other volatile organic compounds occurring in more than 5 wells include chloromethane, dichlorobromomethane, benzene, 1,1,1-trichloroethane, iodomethane, trichloroethylene, tetrachloroethylene, 1,1-dichloroethane, dichloromethane, 4-isopropyl-1-methylbenzene, methyl tert-butyl ether, and acetone.

Results indicate that shallow ground water is affected by human activities. However, concentrations of contaminants at most ground-water sites in the study area do not currently present a human health risk as the majority of the population in the metropolitan area uses surface water for its drinking water supply. There could be some risk to aquatic biota from ground water containing elevated concentrations of contaminants that discharges to streams.

--- April 1999

To order SANT NAWQA publications, please email or call:

Celeste A. Journey
SANT NAWQA Project Chief
cjourney@usgs.gov
(803) 750-6141