Investigating the source of natural arsenic contamination in Vermont’s groundwater

Senior Nora LaCasse is spending her Field Work Term this year working on an independent study project that investigates the cause of natural arsenic contamination in Vermont’s groundwater.

Parts of the Taconic Mountains of Vermont have naturally elevated levels of arsenic in groundwater, which is a significant public health concern. The source of the arsenic is pyrite minerals within the the Taconic slate rock. When the pyrite is oxidized (rusts), the arsenic is liberated, and may either be absorbed onto the surface of iron oxide or clay minerals, or may go into solution with the groundwater. A study performed by Middlebury College students and the Vermont Geological survey outlined areas of highest concern. This and other studies indicate that the amount of dissolved oxygen in groundwater plays a role in determining whether arsenic ions are absorbed to mineral surfaces or released into solution.

The first step of Nora’s investigation is to perform laboratory experiments on the ability of the minerals in the Taconic slates to absorb arsenic from solution under differing oxygen concentrations. She crushed samples of Taconic slate in order to increase the available surface area for absorption, then added the rock powder to a arsenic solutions of known concentration. These are being stirred under both oxygen-rich and oxygen-depleted conditions and under a range of imposed pH conditions. After stirring for several days, the solutions will be re-analyzed to determine if the arsenic concentration has decreased. Later phases of the study will test arsenic-rich slate samples to determine under what conditions arsenic is de-sorbed and released to solution.

Nora with her experiment setup

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