GEOSTATISTICAL BASED MONITORING OF SOIL WATER NO₃⁻-N: A POTENTIAL NONPOINT SOURCE OF GROUND WATER CONTAMINATION

Abstract

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 50 mg L⁻¹ 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.

Keywords:

• Alfalfa
• Crop rotation
• Geostatistics
• Irrigation
• Nonpoint source
• NO₃ ⁻-N
• Paired field
• Soil water
• Statistics

ACKNOWLEDGMENTS

Funding for this monitoring study was provided by the Idaho Division of Environmental Quality (DEQ) and the USEPA Section 319 National Monitoring Program. We would like to express our appreciation to the DEQ, USEPA, USDA Snake River Water Quality Demonstration Project, the Natural Resources Conservation Service, the Agricultural Research Service, and the Idaho Department of Agriculture for their assistance and support during the project. We also would like to thank Lamar Forgeon (farmer) for access to his field and for his help during the monitoring study.