Geostatistical based monitoring of soil water NO3--N: A potential nonpointsource of ground water contamination

Soil water NO3--N concentrations were monitored for an alfalfa-oat-bean rot ation and an alfalfa-bean-bean rotation in the Idaho Snake River Plain as p art of the USEPA Section 319 National Monitoring Program. This monitoring s tudy was conducted to evaluate potential beneficial impacts of a USDA recom mended crop rotation on subsurface NO3--N concentrations at a 4.9 hectare ( ha) farm test field. Soil water monitoring was conducted in cooperation wit h the USDA Snake River Plain Water Quality Demonstration Project. Geostatis tical and statistical analyses of NO3--N data collected from a network of s oil water solution samplers (lysimeters) coupled with hydrogeological chara cterization of the field indicated that alfalfa followed by oats reduced so il water NO3--N concentrations at least temporarily compared to alfalfa fol lowed by beans which is the traditional practice in the area. Soil water NO 3--N sample data showed a unimodal distribution, through the first two mont hs of the split field rotations, that changed to a distinct bimodal distrib ution three months into the rotations. Development of the bimodal distribut ion of soil water NO3--N appeared to correspond directly to the rotational split of the field. The median soil water NO3--N value calculated from the sample data was approximately 50mgL(-1) greater in the field half planted i n beans as compared to the field half planted in oats. Geostatistical spati al mapping results using sequential Gaussian simulations (SGS) supported th ese findings. SGS results suggested that elevated concentrations of NO3--N in the soil water were related to both stratigraphic factors as well as the rotational split.