Attenuation of Barium, Strontium, Cobalt, and Nickel Plumes Formed during Microbial Iron Reduction in a Crude-Oil-Contaminated Aquifer

We assessed the spatialdistribution of 35 elements in aquifersediments and groundwater of a crude-oil-contaminated aquifer andshow evidence of the dissolution of barium (Ba), strontium (Sr), cobalt(Co), and nickel (Ni) during hydrocarbon oxidation coupled to historicmicrobial Fe(III)-reduction near the oil. Trace element plumes occurin the crude-oil-contaminated aquifer, where 50% Co, 47% Ni, 24% Ba,and 15% Sr have been mobilized from the sediment near the oil intogroundwater, resulting in dissolved masses >33, 18, three, andtwotimes greater than estimated dissolved masses prior to contamination,respectively. Ba2+ and Ni2+ concentrations exceededthe World Health Organization's drinking-water guidelines of700 and 20 mu g/L, respectively. Sediments attenuate trace elementplumes in two geochemically distinct zones, resulting in <0.01%total trace element masses dissolved in groundwater, despite the substantialmobilization near the oil body. Geochemical modeling of the modernFe(III)-reducing zone suggests trace elements are likely attenuatedvia coprecipitation with/without sorption on iron carbonate precipitates.In the suboxic transition zone at the leading edge of the plume, Fe(III)-hydroxidessorb Ba2+, Sr2+, Co2+, and Ni2+. This study emphasizes that slow but persistent biogeochemicalactivity can substantially alter aquifer chemistry over decadal timeframes,a phenomenon we term biogeochemical gradualism.