National Water Quality Assessment Program: Eastern Iowa Basins

Nutrient concentrations in the Iowa River alluvial aquifer, Iowa County, Iowa,1996

by Tobias, J,

Program abstract of the 109th Session of the Iowa Academy of Science meeting, April 27, 1997, Dubuque, Iowa

The Iowa River alluvial aquifer in the northwest corner of Iowa County, Iowa underlies an area of extensive agricultural activity, where nitrogen-based fertilizers are extensively used. The U.S. Geological Survey sampled 23 wells at 13 sites over a three-week period during the summer of 1996 as part of the National Water Quality Assessment program. Nitrite plus nitrate (referred to as nitrate due to fractional nitrite values) was found above the detec- tion limit in 21 of the 23 wells. Phosphorus was found above the detection limit in 20 of the 23 wells. Three wells had nitrate con- centrations above the U.S. Environmental Protection Agency's (USEPA) Maximum Contaminant Level (MCL) for drinking water of 10.0 milligrams per liter (mg/L). Groups of wells completed at different depths in five locations, defined as well nests, show two distinct vertical distributions of nitrate: decreasing concentration with increasing depth and increasing then decreasing concentration with increasing depth. The maximum variation in concentration for nitrate within a single well nest is 16.3 mg/L. The phosphorus con- centration varied less than 0.12 milligrams per liter vertically in all five well nests. No areal distribution pattern was found for nitrate or phosphorus.

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