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Logistic Model of Nitrate in Streams of the Upper-Midwestern United States

U.S. Geological Survey, P.O. Box 25046, MS-415, Lakewood, CO 80225;

William A. Battaglin

U.S. Geological Survey, P.O. Box 25046, MS-406, Lakewood, CO 80225.

^* Corresponding author (mueller{at}usgs.gov).

ABSTRACT

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) was analyzed in water samples collected from streams throughout 10 Midwestern states during synoptic surveys in 1989, 1990, and 1994. Data from the period immediately following crop planting at 124 sites were analyzed using logistic regression to relate discrete categories of NO₃-N concentrations to characteristics of the basins upstream from the sites. The NO₃-N data were divided into three categories representing probable background concentrations (<3 mg L^–1), elevated concentrations (3–10 mg L^–1), and concentrations that exceeded the U.S. Environmental Protection Agency maximum contaminant level (MCL) for drinking water (>10 mg L^–1). Nitrate-N concentrations were positively correlated to streamflow, upstream area planted in corn (Zea mays L.), and upstream N-fertilizer application rates. Elevated NO₃-N concentrations were associated with poorly drained soils and were weakly correlated with population density. Nitrate-N and streamflow data collected during 1989 and 1990 were used to calibrate 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 throughout the study area. The potential for elevated NO₃-N concentrations was predicted to be greatest for streams in Illinois, Indiana, Iowa, and western Ohio.