Date of Award

6-2020

Document Type

Open Access

Degree Name

Bachelor of Science

Department

Geology

First Advisor

Mason Stahl

Keywords

groundwater; stable isotopes; hydrology; water resources; isoscape

Abstract

Groundwater is one of the world’s most important natural resources. The use of stable water isotopes (𝛿2H and 𝛿18O) as natural tracers through the water cycle has provided a unique observational technique for characterizing hydrological processes and establishing connections between water distribution systems and their respective environmental sources. Groundwater contains information about the timing and efficiency of recharge, allowing for the use of isotopes to understand the physical hydrology and climatic influences on such processes in places with groundwater isotope measurements. We estimate the seasonal recharge proportion and efficiency at thousands of locations across the U.S., and interpret the climatic and environmental influences responsible for our findings. Results along coastal California suggest fog drip contributes to groundwater recharge and necessitates additional research in areas where this process may be an important source of recharge to aquifers. To combat pre-existing limitations of the lack of groundwater data across all locations in the United States, a predictive model for groundwater isotopes was developed across the contiguous U.S. using a random forest model based on environmental parameters. We find evident spatial coherence in the model predictions, generally mirroring the signal of isotopes of precipitation, and highlight the potential for its application across hydrology and ecology.

In addition, to demonstrate the applicability and versatility of groundwater isotopes, we investigated the local municipal water supply in Schenectady, New York, to understand the source and timing of aquifer recharge. The Schenectady municipal well-field is sited less than a kilometer from the Mohawk River, making the interaction between surface water and groundwater highly complex and seasonally dependent. Schenectady tap water, which is drawn from local groundwater, and Mohawk River were collected at regular intervals and analyzed in the Union College Stable Isotope Laboratory for stable isotopes of hydrogen and oxygen. The seasonal signal of isotopes can be approximated by sine waves, and the phase and amplitude of these signals can be used to calculate the average linear velocity (3.53 m/day) of the water moving into the aquifer and fraction of young water (57% < 2.7 months) in the local groundwater. Our results highlight the connection between the Mohawk River and the aquifer in the vicinity of the Schenectady well-field, and motivates further research to characterize the potential for vulnerabilities. Thus, this study not only provides an isoscape to detail the spatial distribution of isotopes regionally, but also demonstrates how we can leverage our understanding of isotopes for insight into the chemical and physical hydrology in a local water system.

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