MS Thesis Defense
Nitrate Dynamics in an Agricultural Watershed, Minnesota, U.S.A.
Nitrate is a common agricultural pollutant with severe ecological consequences. The Cottonwood River Watershed is an intensively managed agricultural setting within the Mississippi River Basin, which exports nitrate that contributes to the hypoxic zone in the Gulf of Mexico. Understanding nitrate sources, pathways, and processes within the Cottonwood River Watershed sheds light on the larger issue of nitrate loading to the Gulf. This study utilizes an end-member mixing analysis (EMMA) approach to identify water and nitrate sources to the Cottonwood River; performs a nitrate mass balance to find magnitudes of in-stream nitrate transformation over a range of discharges and dates, and assesses long-term concentration-discharge relationships in the watershed to elucidate nitrate processes. The three main sources of water that contribute to the Cottonwood River at Lamberton (approximately halfway up the watershed) are tile drainage, shallow groundwater, and Quaternary aquifer groundwater. In-stream nitrate removal is found to be highest at low discharge levels, which occur late in the crop growing season. Concentration-discharge relationships from long-term datasets confirm this finding and demonstrate that nitrate concentrations have become more consistent during the period of record. Nitrate removal within the stream channel is attributed to biogeochemical processes such as denitrification and dissimilatory nitrate reduction to ammonium, which disproportionately decrease in-stream nitrate concentrations at low discharges. Given the low in-stream nitrate removal at high discharges, management of nitrate in agricultural watersheds should strive to decrease peak discharges.