U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings
of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993,
Water-Resources Investigations Report 94-4015
  
Relation of Nitrate Concentrations in Surface Water to Land
Use in the Upper-Midwestern United States, 1989-90
by
David K. Mueller (U.S. Geological Survey, Lakewood, CO), Barbara
C. Ruddy (U.S. Geological Survey, Lakewood, CO), and William A. Battaglin
(U.S. Geological Survey, Lakewood, CO)
Abstract
As part of a study on contamination from agricultural chemicals, nitrate
data were collected during several synoptic surveys at a large number of
surface-water sites in 10 midwestern states during 1989-90. These data were
analyzed using logistic regression to relate discrete categories of nitrate
concentrations to land use in the drainage basins upstream from the sampling
sites. The nitrate data were divided into three categories representing
background concentrations, elevated concentrations, and concentrations that
exceeded the U.S. Environmental Protection Agency maximum contaminant level
for drinking water. Land-use data were derived from spatial-digital data
available from several sources in national data bases. The explanatory variables
selected for the best-fit model were percentile of streamflow at the time
of sampling, acreage of the basin in corn, acreage in soybeans, density
of cattle, and population density. All these variables have qualitative
relations to nitrate sources, mobilization, or transport. Classification
of nitrate categories from this model was 80 percent accurate in comparison
to observed categories. The accuracy of the model was better for classification
into categories that represented lower concentrations; however, in- correct
classifications were not biased either high or low. Results from this study
indicate that land-use data can be useful in analyses of water-quality conditions
in large regions and that logistic regression is a valuable technique for
use in such analyses.
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