Date of Award

5-2021

Document Type

Thesis open access

Department

Geology

First Advisor

Brady A. Ziegler

Second Advisor

Kurt Knesel

Abstract

Using a reactive transport model, I simulated the mobilization and sequestration of geogenic trace metals, nickel (Ni2+) and cobalt (Co2+), in a crude-oil-contaminated aquifer. These trace metals can pose threats to human and ecological health and are not commonly regulated or measured at oil-spill sites, making it important to characterize the geochemical mechanisms that release and attenuate potentially toxic trace metals. In the groundwater-contaminant plume, crude-oil is biodegraded coupled to iron (Fe(III)) reduction and methanogenesis. Previously collected field data1 showed concentrations of Ni2+, and Co2+ near the crude-oil source were elevated in groundwater and depleted from aquifer sediments compared to background concentrations. Roughly 80 meters downgradient, in the active Fe(III)-reducing zone, groundwater concentrations of Ni2+ and Co2+ decrease, relative to near the crude-oil body, and concentrations in sediment increase above background levels. Using a reactive transport model, I show that Ni2+ and Co2+ originally sorbed to Fe(III) are released from sediments near the oil body due to microbially mediated Fe(III)-reduction to aqueous Fe2+. Biodegradation in the active Fe(III)-reducing zone, dissolves Fe2+ and produces bicarbonate, causing groundwater supersaturation with respect to siderite (FeCO3), allowing FeCO3 to precipitate. I developed a surface complexation model for Ni2+ and Co2+ on FeCO3, to incorporate into our reactive transport model framework. Our modeling results showed that FeCO3 generates negative surface charge in the pH range measured in the contaminant plume (6.3-7.3), allowing FeCO3 to sorb Ni2+ and Co2+ and remove them from groundwater. Our modeling results were consistent with field observations. Previous sampling has shown that arsenic (As), which also is mobilized due to Fe(III) reduction, does not accumulate in Fe-reducing sediments like Ni2+ and Co2+. The negative surface charge on FeCO3 favors sorption of cations (Ni2+ and Co2+) but not the (oxy)anions of As. Our model effectively delineated mechanisms that could release and attenuate trace metals at oil-spill sites, which can aid in more comprehensive predictions of threats to human and ecological health in aquifers contaminated by crude-oil.

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