Modeling Overdraft-Driven Nitrate Transport in Shallow Wells for Mitigation and Scenario Planning

Authors

Jonathan Cronk, University of Nebraska-LincolnFollow

First Advisor

Francisco Muñoz-Arriola

Second Advisor

Sorab Panday

Date of this Version

11-2023

Citation

A thesis presented to the faculty of the Graduate College at the University of Nebraska in partial fulfillment of requirements for the degree of Master of Science

Major: Agricultural and Biological Systems Engineering

Under the supervision of Professors Francisco Muñoz-Arriola and Sorab Panday

Lincoln, Nebraska, November 2023

Comments

Copyright 2023, Jonathan Cronk

Abstract

In Nebraska, average nitrate concentrations in groundwater have doubled since 1974, making water quality management more important than ever. As droughts, heat waves, and floods become more common climate events, understanding their impacts will be necessary to make informed management decisions. Emerging literature describes that drought correlates to an increase in the concentration of nitrate-N at domestic and irrigation wells, however the relative contributions of the mechanisms thought to be responsible is currently unknown.

This research assessed the impact of recharge and pumping rate changes as two mechanisms affecting nitrate-N concentration during drought, assessed the relationship between well depth and nitrate-N concentration, and performed a sensitivity analysis on leaching rate, pumping rate, and half-life. Utilizing a dataset from 2000-2015 in a 25.3km²area south of the Platte River at Kearney, NE, we applied MODFLOW-USG Transport as a hydrogeological numerical model. A statistically significant relationship (P

The sensitivity analysis results suggest that leaching rate is the most significant component of changes to nitrate concentration. Decreases in pumping rates are seen to cause a minor reduction in nitrate concentration for deeper wells, and minor increase for shallow wells. When altering well construction, uniformly deeper wells result in a marginal increase in nitrate concentration at all depths, and vice versa. This may offer a strategic avenue for agricultural producers when considering irrigation well construction. Finally, an assessment of the time lag associated with changes in nitrate-N leaching rates was conducted, which found that the rate of denitrification is a critical factor to the timescale for management changes to be seen. Overall, our findings underscore the necessity for strategic conservation strategies that consider extreme weather events, dynamic rates of contaminant leaching, and proper time for results to be seen in order to protect groundwater quality amid changing climatic conditions.

Advisors: Francisco Muñoz-Arriola and Sorab Panday