A MODELING STUDY OF THE MOBILIZATION AND SEQUESTRATION OF TRACE METALS IN A CRUDE-OIL-CONTAMINATED AQUIFER

Through a reactive transport model, we successfully simulated the mobilization and sequestration of geogenic trace metals, barium (Ba²⁺), 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 better understand the geochemical mechanisms that release and attenuate potentially toxic trace metals. In the groundwater-contaminant plume, petroleum hydrocarbons are biodegraded coupled to iron (Fe(III)) reduction and methanogenesis. Field data showed concentrations of Ba²⁺, 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 Ba²⁺, Ni²⁺, and Co²⁺ decrease relative to the area near the source, and concentrations in sediment increase above background levels. Using a reactive transport model, we show that Ba²⁺, 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⁺. In the active Fe(III)-reducing zone, Fe²⁺ and bicarbonate are supersaturated with respect to siderite (FeCO₃) due to biodegradation processes, allowing FeCO₃ to precipitate. Using literature values, we developed a surface complexation model for FeCO₃, which we incorporated 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 Ba²⁺, Ni²⁺, and Co²⁺ and remove them from groundwater. Our modeling results were consistent with field observations. Previous modeling of arsenic (As) at this site did not indicate sorption onto the precipitated FeCO₃. The negative surface charge on FeCO₃ favors sorption of cations (Ba²⁺, 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.