Hydrogeologic Investigation of Ground Water Nitrate for Analysis of Implemented Best Management Practices, Eastern Snake River Plain, Idaho

Publication Date

12-1999

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Geology

Department

Geosciences

Supervisory Committee Chair

James L. Osiensky

Abstract

Shallow ground water is common in irrigated agricultural areas of southern Idaho. Many rural residents utilize this shallow ground water for domestic water supplies. The shallow nature of the ground water makes it vulnerable to contamination from agricultural practices.

The overall goal of the Eastern Snake River Plain National Monitoring Project is to demonstrate to farmers that USDA recommended best management practices (BMPs) will maintain current crop yields while reducing existing and/or potential ground water contamination. The specific monitoring objective is to evaluate if implementation of selected BMPs results in statistically significant changes in ground water nitrate levels.

Two demonstration fields, each designed for paired field study, are located within the most vulnerable areas for groundwater contamination. The Moncur Field will implement an irrigation BMP (12 versus 24 hour irrigation sets), focusing on nutrient leaching. The Forgeon Field will implement a nutrient management BMP through crop rotation, focusing on soil nitrogen uptake by the crops. Monitoring wells, ground water point samplers, and vadose zone water sampling devices (lysimeters) are installed in the fields. The project has been on going since 1992.

Statistical analyses of baseline data for the Moncur Field revealed that populations of nitrate data for each monitoring well have low variances. However, the median nitrate concentrations for each of the 12 wells within the field vary by up to a factor of 10. Soil heterogeneities most likely have a significant influence on the variability in nitrate concentrations between wells.

Measurable differences in soil water nitrate concentrations between the treatment and control fields in the Moncur Field were recorded during the first year of BMP implementation (1996). Median nitrate concentration under the 12 hour irrigation set is much higher than the median nitrate concentration under the 24 hour irrigation set. This suggests more nitrate leaches through the root zone under a 24 hour set.

Calibration phase monitoring in the Forgeon Field showed that significant increases in ground water nitrate concentrations can be correlated to applications of agrichemicals at land surface. Time series plots of ground water nitrate concentrations indicate a relationship exists between planting of certain crops and nitrate concentrations. This analysis also suggests that certain crops have a greater influence on nitrate concentrations at some sampling points than at others. Continued study of this relationship will improve our understanding of the distribution and magnitude of this spatial variability, and the effects of the BMP implementation.

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