Date of Award

Summer 2019

Project Type

Thesis

Program or Major

Earth Sciences

Degree Name

Master of Science

First Advisor

Julia G. Bryce

Second Advisor

Matthew J. Davis

Third Advisor

Ryan Gordon

Abstract

Arsenic contamination is a major human health issue both globally and within the United States with roughly 30% of privately sourced drilled wells in New Hampshire containing arsenic exceeding the MCL. A novel shallow well design has been developed by the US Geological Survey to remedy the issues of the traditional dug well design that resulted in it being phased out for drilled wells. By tapping into shallow glacial aquifers, the physiochemical parameters are favorable for the adsorption and removal of arsenic from drinking water. Event-based field time-series and laboratory batch dissolution experiments were carried out on crushed backfill media used in a novel shallow well design to determine the effects of changing physicochemical parameters and water table conditions on the release of trace metals in privately sourced drinking water. During the hours and days following a precipitation event pH decreased between 0.35 and 0.4 and DO increased by about 0.25 mg/L. These changes in physicochemical parameters and the associated changes in water table elevation between low water table conditions and the heavy precipitation event did not show any statistically significant changes in water quality. There was a release of copper in both wells in the hours following the precipitation event but they were not statistically significant from copper concentrations across a year-long sampling period. Copper concentrations were also quickly reabsorbed or removed from the aquifer in the days following the precipitation event. Laboratory batch dissolution experiments indicate that in acidic conditions (pH ~2) arsenic is strongly correlated to changes in iron in solution, whereas, at more alkaline pH (pH >8), pH was the dominant control on arsenic dissolution. In the Concord Granite experiments, uranium concentrations are highest where rubidium-strontium ratios are lowest indicating a linkage between calcium and magnesium-rich minerals and uranium. Uranium release from the glacial till was linked to the dissolution of zinc into solution, likely the result of sphalerite dissolution. A large release of zinc into solution during pH 2 experiments resulted in uranium concentrations over 4 times the maximum contaminant level. Overall, leaching of uranium from outside sourced backfill material appears to be ephemeral release events rather than long-term chronic dissolution events and therefore the currently used backfill sources are adequate to provide safe drinking water.

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