Pesticide concentrations in streams differ according to land use. Pesticide concentrations in streams are highest in areas with permeable, well-drained soils, all other factors being equal. Agricultural tile drains play a major role in transporting pesticides to streams in areas with poorly drained soils. Nitrate concentrations in stream water typically are between these extremes. Median concentrations of nitrate at monitoring sites in streams generally ranged from 2 to 6 milligrams per liter-higher than those at most other USGS monitoring sites in the United States.
Soil drainage is a major factor controlling nitrate concentrations in streams.
Manure from farm animals also increases nitrate concentrations in some streams. Nitrate concentrations in stream water rarely exceeded the Federal drinking-water standard of 10 milligrams per liter. Surficial sand and gravel aquifers underlying cropland had high nitrate concentrations. Samples from 17 percent of shallow monitoring wells in this setting exceeded the Federal drinking-water standard. In many parts of the basin, nitrate concentrations in ground water were low.