Ground-water quality in the Toppenish Creek Basin, near Yakima, Washington, is generally good with respect to U.S. Environmental Protection Agency drinking water standards. Of 487 wells sampled during one phase of the study, only 2 produced water with nitrite-plus-nitrate concentrations greater than the U.S. Environmental Protection Agency standard of 10 milligrams per liter as nitrogen. Ground-water samples with elevated nitriteplus-nitrate concentrations (greater than 5 milligrams per liter as nitrogen) were obtained from wells located in the eastern and southern parts of the basin and in areas underlain by the Touchet Beds, in the central part of the basin. In another phase of the study, in which 60 wells were sampled, atrazine, an herbicide used on asparagus and corn, was detected in water samples from 4 wells, and 2 insecticides, diazinon and disulfoton, were detected in separate samples from 2 wells.
Bacteria, indicators of the sanitary water quality, were detected in samples from 64 wells, suggesting that ground water may be contaminated in some areas. However, other sources of bacteria may include leaks in the plumbing of the well or residence, or improper well construction.
Small seasonal variations in nitrite-plus-nitrate concentrations in ground water appeared to be related to fertilizer use in the basin, indicating that the potential exists for more serious contamination of ground water. Groundwater quality is affected by agricultural activities in some parts of the basin. However, widespread degradation in ground-water quality was not detected.
