Date of Award

Summer 2016

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Geosciences

Committee Chair

Glenn Shaw

First Advisor

Chris Gammons

Second Advisor

Gary Icopini

Third Advisor

Raja Nagisetty

Abstract

Human impacts to water resources occur in a variety of instances and are often associated with, but not limited to, nor always a result of, industrial processes. Manufacturing of Cr compounds for use as a corrosion inhibitor require the transformation of Cr into a toxic state, requiring extreme safety and caution during processing and handling.

Historical processing of chromite ore at the Mouat Industries site led to Cr-contaminated groundwater within the city limits of Columbus, MT. After the site was listed on the Environmental Protection Agency’s National Priority List in 1986, soil remediation was conducted at the site in 1994. Contaminated soils were excavated, mixed with an acid/ferrous sulfate solution (reducing hexavalent Cr (CrVI) to trivalent Cr (CrIII)), neutralized with lime (CaO), secured into blocks with cement, and the blocks were buried in an on-site in a repository.

Portions of the contaminated soils that were below the clean-up threshold were not treated, but were used to backfill the excavated area. A synthetic liner was placed above the repository, and the cover is clean foreign soil and gravel or grass. Aqueous Cr(VI), a carcinogen, remains present at variable concentrations, locally above the maximum contaminant level (MCL) within and rarely down-gradient of the site.

Groundwater monitoring and sampling has been ongoing since 1996; however concerns remain about Cr mobility and transport off-site. As part of the field investigation, water quality samples were collected from 22 wells in June and 15 wells in September, 2015; groundwater levels were monitored in 26 wells; well measuring point elevation and surface water elevations were surveyed. As part of the laboratory investigations, leaching and sorption tests were performed, and a groundwater flow model (GMS-MODFLOW) was constructed using field and laboratory data to simulate groundwater flow and contaminant transport. Leaching studies show that Cr is available to be leached from the repository at similar concentrations to groundwater; sorption batch test results suggest that Cr(VI) is not adsorbed onto aquifer soils; and Cr(VI)/Cr(III) speciation data provide spatial variation and confirmation of the existence of Cr(VI) at high concentrations in the repository and low concentrations in the point of compliance wells.

The model results indicate a range of transport time between 5 and 13 months for Cr(VI) to reach POC wells at the MCL of 100μg/L.

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