We assessed the spatial
distribution of 35 elements in aquifer
sediments and groundwater of a crude-oil-contaminated aquifer and
show evidence of the dissolution of barium (Ba), strontium (Sr), cobalt
(Co), and nickel (Ni) during hydrocarbon oxidation coupled to historic
microbial Fe(III)-reduction near the oil. Trace element plumes occur
in the crude-oil-contaminated aquifer, where 50% Co, 47% Ni, 24% Ba,
and 15% Sr have been mobilized from the sediment near the oil into
groundwater, resulting in dissolved masses >33, 18, three, and
two
times greater than estimated dissolved masses prior to contamination,
respectively. Ba2+ and Ni2+ concentrations exceeded
the World Health Organization’s drinking-water guidelines of
700 and 20 μg/L, respectively. Sediments attenuate trace element
plumes in two geochemically distinct zones, resulting in <0.01%
total trace element masses dissolved in groundwater, despite the substantial
mobilization near the oil body. Geochemical modeling of the modern
Fe(III)-reducing zone suggests trace elements are likely attenuated
via coprecipitation with/without sorption on iron carbonate precipitates.
In the suboxic transition zone at the leading edge of the plume, Fe(III)-hydroxides
sorb Ba2+, Sr2+, Co2+, and Ni2+. This study emphasizes that slow but persistent biogeochemical
activity can substantially alter aquifer chemistry over decadal timeframes,
a phenomenon we term biogeochemical gradualism.
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