Date of Award:

5-2015

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Civil and Environmental Engineering

Committee Chair(s)

Joan E. McLean

Committee

Joan E. McLean

Committee

R. Ryan Dupont

Committee

Paul Grossl

Abstract

Significant amounts of Arsenic (As) are seen in groundwater used for drinking water in the United States, especially in the Southwest (SW). Long-term exposure to As concentrations over the drinking water limit of 10 μg/L can cause skin and lung cancer, along with several other adverse health effects. Previous studies have evaluated the hydro-biogeochemical processes leading to the exposure of millions of people in West Bengal, Bangladesh, and Southeast Asia to unsafe levels of As in drinking water. There are some similarities between Southeast Asia and the SW U.S., but only a few studies analyzing As have been performed in the SW U.S. Levels of As above the drinking water limit were measured in groundwater wells throughout Cache Valley, UT. Field research performed by our research team found that As is naturally occurring. The processes controlling As release from the soil to groundwater in this area have not been extensively studied.

Modeling was used to determine which As mineral forms exist in the soil and lab experiments were conducted to see if changing groundwater levels affect As release. Solids prediction modeling results found that As(III), the most toxic form of As, forms an As-S mineral, orpiment. However, when considering sorption mechanisms as well, modeling results indicated most of the As(V) sorbs onto hydrous ferric oxides (HFO). Although much less As(V) was sorbed to CaCO3, the percentage associated with calcite was 1.7 to 3.3% and 6 to 59% in the surface and water table zones for NP 9 and NP 13, respectively.

The lab experiments took soil from groundwater depth and looked at As release under different conditions; 1) exposed to air, 2) oxygen free and exposed to nitrogen gas, and 3) poisoned to eliminate microbial activity. Despite the condition, large amounts of As were released into the water from the soil. The samples not exposed to air released both As(III) and As(V), but the samples exposed to air and the poisoned samples only released As(V). Since As was released under every condition, but only As(III) was seen in the samples not exposed to air, As release was deduced to be chemically controlled. However, in order to release As(III), microbes were needed. Chemical extractions were conducted to determine the amount of As and iron (Fe) associated with carbonate minerals. Desorption of As(V) from carbonate minerals and the reduction of As(V) to As(III) played a significant role in explaining As(III) in solution.

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