DIFFERENTIAL ATTENUATION OF TRACE ELEMENTS BY IRON CARBONATES AND IRON HYDROXIDES IN A PETROLEUM-CONTAMINATED PLUME

Biodegradation of organic carbon can cause secondary geochemical processes that negatively affect groundwater quality. We investigated trace element mobilization in a petroleum-contaminated aquifer, where hydrocarbon biodegradation is coupled with the reduction of ferric (Fe(III)) hydroxides in sediment. During Fe-reduction, Fe(III) in the hydroxide is reduced to Fe²⁺, which dissolves into groundwater; geogenic trace elements sorbed to the dissolving hydroxide are thus released into groundwater. We observed the mobilization of strontium (Sr), barium (Ba), nickel (Ni), and cobalt (Co) into groundwater due to Fe-reduction, resulting in increases in dissolved masses by factors of 2x, 3x, 14x, and 33x, respectively. Ba exceeded the WHO drinking water health advisory level (700 μg/L) and Ni exceeded the EPA standard (70 μg/L). In previous studies, mobilization of arsenic (As) oxyanions exceeding the EPA and WHO standard (10 μg/L) was also observed.

Groundwater-sediment interactions attenuate mobilized trace elements in in two geochemically distinct zones, which both have sediment concentrations elevated above background levels. Within the Fe-reducing zone (~70-110 m downgradient from the oil source), groundwater is supersaturated with respect to FeCO₃. We hypothesize that FeCO₃ precipitates in this zone and generates negatively charged surface sites, which sorb Sr, Ba, Ni, and Co cations. In a downgradient transition zone (~110-130 m downgradient from the oil source), the anoxic plume mixes with regionally oxic groundwater, causing oxidation of dissolved Fe²⁺ and precipitation of Fe(III) hydroxides, which can generate both positive and negative surface sites to sorb both cations (Sr, Ba, Ni, and Co) and anions (As oxyanions).

This study demonstrates different geochemical processes that attenuate mobilized trace element cations and anions. It also highlights the importance of monitoring secondary groundwater contaminants at sites impacted by biodegradable organic carbon and adds to a more comprehensive knowledge of the processes affecting groundwater quality during bioremediation or monitored natural attenuation.