Date of Completion

4-25-2019

Embargo Period

4-24-2022

Keywords

groundwater, nitrogen, legacy, water quality, biogeochemistry, land use, modflow

Major Advisor

Ashley Helton

Co-Major Advisor

Chadwich Rittenhouse

Associate Advisor

Glenn Warner

Associate Advisor

Martin Briggs

Associate Advisor

J. Jeff Starn

Field of Study

Natural Resources: Land, Water, and Air

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

Human activity and land use patterns are altering solute concentrations in waterbodies worldwide, frequently with effects that are delayed and/or displaced from their cause. Understanding the resulting water quality trends allows managers to tailor interventions and preemptively respond to likely future conditions. My research addressed two broad questions:

1) How and why does surface water quality vary spatially?

Many U.S. states classify waterbodies according to groups of designated uses, suggesting that water quality and classifications are correlated. The relationship between classification and water quality, however, is untested. I analyzed existing data for the State of Connecticut to identify differences in water quality between waterbody classes and as a function of land cover (Chapter 1). My results suggest that land cover is a better proxy for water quality than classification.

2) What is the role of groundwater in delivering nitrogen to streams and rivers, and removing nitrogen through denitrification?

Reactive nitrogen applied to land surfaces percolates with precipitation and moves along groundwater flowpaths before discharging to surface waters. Patterns of groundwater discharge are difficult to measure and predict due to spatial heterogeneity. I explored the feasibility of using regional groundwater models and thermal infrared imagery to identify areas of groundwater discharge at the regional-scale. I implemented a suite of groundwater models using common assumptions to quantify precision in modeled flowpath characteristics (Chapter 2). Then, I used thermal infrared imagery and extensive field surveys to compare modeled and observed patterns (Chapter 3). I found substantial variation in modeled flowpath characteristics among models, but that thermal infrared imagery was a useful tool for evaluating modeled patterns.

Groundwater discharges can deliver large nitrogen loads to streams and rivers, but nitrogen can also be removed by microbial processes along groundwater flowpaths. I analyzed discharging groundwater from spatially extensive surveys to quantify patterns of nitrogen loading to surface waters and removal within the groundwater system. Then, I used land cover, soil, and flowpath characteristics to quantify patterns of nitrogen loading and removal at the watershed scale (Chapter 4). I found that removal dominates in forested areas, and loading dominates where development and agriculture overlay coarse sediments.

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