U.S. Geological Survey U.S. Geological Survey
Stephen R. Hinkle
U.S. Geological Survey, 10615 S.E. Cherry Blossom Drive, Portland, Oregon 97216
Water samples were collected from 75 randomly distributed, domestic wells in the Willamette River Basin, Oregon, during summer 1993 to begin assessing factors affecting the regional distribution of nitrite-plus-nitrate in shallow ground water. All wells were less than 25 meters deep and were completed in unconsolidated geologic materials. Concentrations of nitrite-plus-nitrate ranged from less than the analytical detection limit (0.05 mg/L as N) to 26 mg/L as N. Ground water underlying areas dominated by irrigated agriculture contained generally higher nitrite-plus-nitrate concentrations than did ground water underlying areas dominated by non-irrigated agriculture. Although differences in concentrations of nitrite-plus-nitrate in ground water are expected to be a function of land-use practices on soils overlying aquifers, the differences also may be a function of hydrogeologic factors. For instance, sampled wells associated with irrigated agriculture have a shallower median depth than those associated with non-irrigated agriculture. Redox conditions in ground water can affect the fate of nitrite-plus-nitrate through control of denitrification processes. In this regard, samples that contained low concentrations of dissolved oxygen generally contained low concentrations of nitrite-plus-nitrate.