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 i s being reported in the literature with increasing frequency. Some occ urrences of high NO3--N concentrations have been attributed to irrigat ion and fertilization practices. A private well water testing program in Montana, involving nearly 3400 well owners, found NO3--N concentrat ions > 10 mg L-1 in nearly 6% of all tested wells. Most of the agricul tural land in Montana is nonirrigated and is not subject to high rates of N fertilization. Dryland crop/fallow cereal grain rotations are th e main practices. Well water test results were combined with MAPS, a g eographic information system (GIS), to identify correlations between c ounty average NO3--N concentration in groundwater, well water sample p robability of exceeding 10 mg L-1 NO3--N, geographic, climatic, and ge ologic conditions, and land-use practices. From a list of 67 independe nt variables, county average well water NO3--N concentration and perce ntage 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-u se practices. The closest correlations were with March 1 through June 30 precipitation, distribution of dryland crop production and summer f allow, soil waterholding capacity, and mapping units of the general so il map of Montana. Two-, three-, and four-variable, linear, multiple r egression models indicated that 53 to 61% of the variability in county average well sample NO3--N concentration could be accounted for by th ese independent variables. Results of these analyses support the hypot hesis that summer fallow practices and associated mineralization of or ganic matter may be contributing to regionalized NO3--N contamination of shallow groundwater in Montana.