PHYSIOGRAPHIC AND LAND-USE CHARACTERISTICS ASSOCIATED WITH NITRATE-NITROGEN IN MONTANA GROUNDWATER

Occurrence of NO3--N in drinking water at concentrations > 10 mg L-1 is being reported in the literature with increasing frequency. Some occurrences of high NO3--N concentrations have been attributed to irrigation and fertilization practices. A private well water testing program in Montana, involving nearly 3400 well owners, found NO3--N concentrations > 10 mg L-1 in nearly 6% of all tested wells. Most of the agricultural land in Montana is nonirrigated and is not subject to high rates of N fertilization. Dryland crop/fallow cereal grain rotations are the main practices. Well water test results were combined with MAPS, a geographic information system (GIS), to identify correlations between county average NO3--N concentration in groundwater, well water sample probability of exceeding 10 mg L-1 NO3--N, geographic, climatic, and geologic conditions, and land-use practices. From a list of 67 independent variables, county average well water NO3--N concentration and percentage of tested wells in each county with NO3--N concentration > 10 mg L-1 were correlated (P < 0.10) with 16 independent variables, most of which were associated with precipitation, soil properties, and land-use practices. The closest correlations were with March 1 through June 30 precipitation, distribution of dryland crop production and summer fallow, soil waterholding capacity, and mapping units of the general soil map of Montana. Two-, three-, and four-variable, linear, multiple regression models indicated that 53 to 61% of the variability in county average well sample NO3--N concentration could be accounted for by these independent variables. Results of these analyses support the hypothesis that summer fallow practices and associated mineralization of organic matter may be contributing to regionalized NO3--N contamination of shallow groundwater in Montana.