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.

Automated summary

We evaluated the distribution of 35 elements in aquifer sediments and groundwater contaminated with crude oil, and found that barium, strontium, cobalt, and nickel were dissolved during hydrocarbon oxidation coupled to Fe(III)-reduction by microorganisms. These dissolved trace elements formed plumes in the contaminated aquifer, with mobilization rates exceeding the pre-contamination estimates. Concentrations of Ba2+ and Ni2+ in the groundwater exceeded the drinking-water guidelines. However, sediments acted as a barrier and attenuated the plumes, resulting in minimal dissolved trace element masses in the groundwater.