Nitrate Dynamics and Source within Nested Watersheds of an Agricultural Stream, Nebraska, USA

Authors

Galen Richards, University of Nebraska - LincolnFollow

First Advisor

Troy E. Gilmore

Second Advisor

Aaron R. Mittelstet

Date of this Version

8-2020

Document Type

Article

Citation

Richards, G.I. 2020. Nitrate Dynamics and Source within Nested Watersheds of an Agricultural Stream, Nebraska, USA. M.S. Thesis. University of Nebraska-Lincoln.

Comments

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: Natural Resource Sciences, Under the Supervision of Troy E. Gilmore and Aaron R. Mittelstet. Lincoln, Nebraska: August, 2020

Copyright 2020 Galen I. Richards

Abstract

Nitrate concentrations have been increasing throughout the globe, primarily due to heightened agricultural activity. Nitrate concentrations in Bazile Creek, located in Northeastern Nebraska, have been steadily rising since the early 2000s. Groundwater nitrate concentrations within the Bazile Creek watershed are high (> 10 mg/L nitrate-N), and there is strong connectivity between groundwater and surface water systems. This study aimed to better understand temporal nitrate concentration variability within the watershed through sampling tributaries and the main channel during baseflow conditions. Nitrate source was also investigated though the use of dual δ¹⁵N and δ¹⁸O nitrate isotopes, with samples collected seasonally and after three summer rain events. Surface water nitrate and nitrate isotope results were then analyzed along with sub-watershed land use and soils characteristics in order to draw conclusions on input pathways. Average surface water nitrate concentrations between sites ranged from 2.7 mg/L to 15.3 mg/L, and nitrate concentrations were found to be significantly different between sites. On the main channel, nitrate concentrations were greatest in the winter. Ammonium from fertilizer was found to be the nitrate source, both seasonally and after rain events. Denitrification was detected, and was greatest in the spring. Analysis of sub-watershed land use revealed a positive correlation between nitrate concentration and watershed area as cultivated cropland. Soils were predominantly well or excessively drained throughout the study area. The results of this study suggest that ammonium from fertilizer applied to fields near Bazile Creek leached into the water table, eventually entering streams as groundwater discharge.

Advisors: Troy E. Gilmore & Aaron R. Mittelstet