Year of Award

2021

Document Type

Thesis

Degree Type

Master of Science (MS)

Degree Name

Systems Ecology

Department or School/College

College of Forestry and Conservation

Committee Chair

Maury Valett

Commitee Members

Payton Gardner, Matthew Church

Keywords

Nitrogen, Stream, Groundwater, Biogeochemistry, Hydrology

Publisher

University of Montana

Subject Categories

Biogeochemistry | Hydrology | Systems Biology | Terrestrial and Aquatic Ecology

Abstract

Human activities have doubled the rate of nitrogen inputs onto the landscape resulting in elevated nitrogen concentrations in our streams. Anthropogenically applied nitrogen is largely transported to stream networks via groundwater movement. Groundwater discharge occurs in distinct points along a stream but whose influences can often persist far beyond that area due to insufficient biogeochemical removal of imported nitrogen potentially causing alterations in community structure and precipitating large algae blooms. To understand the factors governing nitrogen abundance in a historical polluted stream, I used a mass-balance approach to quantify groundwater-surface water interaction and the magnitude of groundwater nitrogen input and stream nitrogen removal. I quantified hydrologic and biogeochemical properties of surface and groundwater environments to inform the mass-balance approach and to determine sources of nitrogen to the aquifer. Results identified localized sections of groundwater discharge that were on average 221% of upstream flow. These groundwater inputs were strong controls on the nitrogen budget of the stream and elevated the dissolved inorganic nitrogen concentration and load by 325% and 448% on average, respectively. In-stream nitrification of ammonium-rich groundwater contributed to elevated nitrification rates of 236.21 ± 27.44 mg N m-2 d-1. In addition, relatively abundant nitrate concentrations (>0.1 mg N L-1) resulted in high nitrate uptake rates of -358.14 ± 219.78 mg N m-2 d-1. An adjacent wetland that covers 1.26 square kilometers was a net sink of nitrogen but had no significant influence on nitrogen transport to the stream due to low substrate permeability. Concentrations of nitrate were correlated to chloride concentrations (r=0.80, p <0.001) suggesting the source of groundwater nitrogen originated from municipal effluent that is discharged into the aquifer adjacent to the study reach.

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