Date of Award

5-2020

Document Type

Thesis open access

Department

Geology

First Advisor

Brady A. Ziegler

Second Advisor

Glenn Kroeger

Abstract

Reduction of ferric (Fe(III)) hydroxides is an important electron accepting process within a crude oil-contaminated aquifer near Bemidji, MN, USA. Fe(III) hydroxides commonly sorb trace elements in aquifer sediments; when they are reductively dissolved, trace elements can be mobilized into groundwater. We present new analyses of spatial distribution in groundwater and in sediment that document the mobilization of barium (Ba), strontium (Sr), cobalt (Co), and nickel (Ni) from sediment into groundwater. In the most reducing zone of the aquifer, Ba, Sr, Co, and Ni concentrations are elevated in groundwater and depleted in sediments. Downgradient from this zone, supersaturation of groundwater with respect to Fe2+ and HCO3- leads to the precipitation of siderite, which sorbs Ba, Co, and Ni due to negative surface charge generated at the pH conditions in the Bemidji aquifer. At the transition zone between Fe(III) reducing conditions and (sub)oxic groundwater, hydrous ferric oxides precipitate, promoting sorption of Ba, Co, Ni, and Sr.

Results from mass balance modeling show that ≥99.5% of mass for each trace element is associated with sediment, both before and 40 years after the oil spill. However, mobilization into groundwater is associated with a 2x increase in Sr mass, 3x increase in Ba mass, 14x increase in Ni mass, and 33x increase in Co mass relative to the mass contained in uncontaminated groundwater. This dissolved mass was sufficient to exceed health advisory concentrations for Ba and Ni in drinking water. Our results highlight the importance of monitoring inorganic contaminants, and particularly trace elements, at organic contaminant sites.

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