A modeling study of the mobilization and sequestration of trace metals in a crude-oil-contaminated aquifer

Using a reactive transport model, I simulated the mobilization and sequestration of geogenic trace metals, nickel (Ni) and cobalt (Co), 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 data showed concentrations of Ni, and Co 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 Ni and Co decrease, relative to near the crude-oil body, and concentrations in sediment increase above background levels. Using a reactive transport model, I show that Ni and Co originally sorbed to Fe(III) are released from sediments near the oil body due to microbially mediated Fe(III)-reduction to aqueous Fe. Biodegradation in the active Fe(III)-reducing zone, dissolves Fe and produces bicarbonate, causing groundwater supersaturation with respect to siderite (FeCO3), allowing FeCO3 to precipitate. I developed a surface complexation model for Ni and Co 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 Ni and Co 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 Ni and Co. The negative surface charge on FeCO3 favors sorption of cations (Ni and Co) 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.