Geostatistical based monitoring of soil water NO3- -N: a potential nonpoint source of ground water contamination.

Soil water NO3- -N concentrations were monitored for an alfalfa-oat-bean rotation and an alfalfa-bean-bean rotation in the Idaho Snake River Plain as part of the USEPA Section 319 National Monitoring Program. This monitoring study was conducted to evaluate potential beneficial impacts of a USDA recommended crop rotation on subsurface NO3- -N concentrations at a 4.9 hectare (ha) farm test field. Soil water monitoring was conducted in cooperation with the USDA Snake River Plain Water Quality Demonstration Project. Geostatistical and statistical analyses of NO3- -N data collected from a network of soil water solution samplers (lysimeters) coupled with hydrogeological characterization of the field indicated that alfalfa followed by oats reduced soil water NO3- -N concentrations at least temporarily compared to alfalfa followed by beans which is the traditional practice in the area. Soil water NO3- -N sample data showed a unimodal distribution, through the first two months of the split field rotations, that changed to a distinct bimodal distribution three months into the rotations. Development of the bimodal distribution 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(-3) greater in the field half planted in beans as compared to the field half planted in oats. Geostatistical spatial mapping results using sequential Gaussian simulations (SGS) supported these 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.