Title

Geostatistical Assessment and Hydrogeologic Implications of Agricultural Best Management Practices to Reduce Nonpoint Source Nitrate Pollution Southern Minidoka County, Idaho

Publication Date

12-1999

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Geology

Department

Geosciences

Major Advisor

James L. Osiensky

Abstract

The results of geostatistical analyses were used to assess the effects of a crop-rotation Best Management Practice (BMP) and an irrigation BMP on shallow ground water nitrate contamination at two demonstration sites, independently. The demonstration sites were located at two private farm fields situated just south of United States Interstate 84 within southern Minidoka County, Idaho. A United States Environmental Protection Agency suggested paired watershed approach was used to help evaluate each BMP. To implement the paired watershed approach, each demonstration field was split into a control half and treatment half and two periods of study were evaluated, a four-year calibration period and a two-year treatment period. Analyses of the BMP treatment period indicated that the BMPs reduced shallow ground water nitrate contamination at both demonstration fields.

At the crop rotation BMP demonstration field, a grain rotation (treatment half) was evaluated against a traditional beans rotation (control half) after the killing of alfalfa prior to BMP implementation. Time series comparisons based on nitrate samples obtained from monitoring wells indicated that mean ground water nitrate concentrations ranged from 3 mg/L to 22 mg/L less in the treatment half of the field during BMP implementation. Spatial mapping results using sequential Gaussian simulations (SGS) supported these findings. SGS results, which were based on nitrate data collected from a network of 35 soil water samplers and 35 ground water point samplers, suggested hydrogeologic conditions were, in part, a controlling factor in observed BMP effects. Within sandy subsoils of the western two-thirds of the demonstration field, ground water nitrate increases were 16mg/L to 34 mg/L less in the treatment half as compared to the control half of the field. These concentration differences occurred 2-months after peak growing season irrigation amounts. Differences in ground water nitrate concentrations in the eastern third of the field within clay-rich subsoils were negligible.

At the irrigation BMP demonstration field, a 12-hour irrigation set sprinkler rotation (treatment half) was compared against a traditional 24-hour irrigation set sprinkler rotation (control half) to evaluate leaching of nitrate from a uniformly applied total application of 230 lb/acre granular ammonium nitrate fertilizer for a growing season of potatoes. Ground water nitrate data collected solely from 12 monitoring wells were used in this evaluation. As compared to the crop rotation BMP findings, differences in net ground water nitrate changes over the BMP implementation period between control and treatment halves of the field were minimal, in part, due to slow leaching caused by the distinctly clay rich subsoils at the field. Month-to-month net ground water nitrate changes observed from monitoring well sample data typically were less than 5 mg/L. Trend surface analysis results indicated a trend in monthly net changes from increasing nitrate concentrations in the treatment half of the field to decreasing concentrations in the control half of the field prior to BMP implementation. This trend reversed over the period of BMP implementation, indicating reductions in nitrate leaching to the ground water as a result of the 12-hour irrigation set sprinkler rotation.

This study showed that geostatistical analysis methods are a useful tool to evaluate BMPs effects on ground water nitrate concentrations. Geostatistically derived spatial maps of ground water nitrate concentration distributions appeared to be consistent with monthly sample data collected from each demonstration site. In addition, subtraction of geostatistically computed monthly estimates proved essential to define net ground water nitrate changes. Results of these geostatistical analyses suggested that the BMPs significantly impacted leaching of nitrate to the shallow ground water below each demonstration site.

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