LOGISTIC MODEL OF NITRATE IN STREAMS OF THE UPPER-MIDWESTERN UNITED-STATES

Nitrate in surface water can have adverse effects on aquatic Life and, in drinking-water supplies, can be a risk to human health. As part of a regional study, nitrate as N (NO3-N) vies analyzed in water samples collected from streams throughout 10 iMidwestern states during synopt ic surveys in 1989, 1990, and 1994. Data from the period immediately f ollowing crop planting at 124 sites were analyzed using logistic regre ssion to relate discrete categories of NO3-N concentrations to charact eristics of the basins upstream from the sites. The NO3-N data were di vided into three categories representing probable background concentra tions (<3 mg L-1), elevated concentrations (3-10 mg L-1), and concentr ations that exceeded the U.S. Environmental Protection Agency maximum contaminant level (MCL) for drinking water (>10 mg L-1). Nitrate-N con centrations were positively correlated to streamflow, upstream area pl anted in corn (Zea mays L.), and upstream N-fertilizer application rat es. Elevated NO3-N concentrations were associated with poorly drained soils and were weakly correlated with population density, Nitrate-N an d streamflow data collected during 1989 and 1990 were used to calibrat e the model, and data collected during 1994 were used for verification , The model correctly estimated NO3-N concentration categories for 79% of the samples in the calibration data set and 60% of the samples in the verification data set. The model was used to indicate where NO3-N concentrations might be elevated or exceed the NO3-N MCL in streams th roughout the study area, The potential for elevated NO3-N concentratio ns was predicted to be greatest for streams in Illinois, Indiana, Iowa , and western Ohio.