ASSOCIATION OF ARSENIC WITH IRON MINERALS IN A DISSOLVED HYDROCARBON PLUME

Arsenic (As) strongly associates with iron (Fe) minerals in aquifer sediments due to strong adsorption affinities. While this is generally a stable relationship in oxic conditions, an influx of organic carbon can spur microbial activity that depletes groundwater of oxygen and causes the onset of Fe-reducing conditions. During Fe-reduction, solid phase Fe(III) is microbially reduced to Fe²⁺, and As associated with the once solid Fe(III) may also be released to solution. This process of As mobilization under Fe-reducing conditions is occurring in a dissolved hydrocarbon plume created by a 1979 crude oil pipeline spill near Bemidji, MN, USA. In the plume, concentrations of dissolved As show a strong co-occurrence with aqueous Fe²⁺. However, where the dissolved Fe/As plume migrates through less reduced sediments farther downgradient in a “transition zone”, both Fe²⁺ and dissolved As are below detection in groundwater. Results from sediment chemical extractions show elevated concentrations of solid phase As in the Fe-reducing and transition zone sediments relative to background conditions upgradient from the contaminated site. Arsenic associated with poorly crystalline and amorphous Fe oxides ranged from 493-789 μg/kg and 356-605 μg/kg in the Fe-reducing and transition zones, respectively, and is statistically significantly elevated relative to background sediments ranging from 166-403 μg/kg. In our current study, we are using microscopic and spectroscopic methods to evaluate the association of solid phase As with Fe minerals in the Fe-reducing and transition zones. Specifically, this research aims to identify the mechanism of association between As and Fe in aquifer sediments (e.g. adsorption vs. co-precipitation) and the specific Fe minerals with which As associates. We hypothesize that the mechanism sequestering As may change between the Fe-reducing and transition zones. Fe mineralogy in aquifer sediments can act as a main control for As sequestration because the strength of the As-Fe association is dependent on the Fe mineral. Our goals are to identify: 1) the mechanism of association between As and Fe; and 2) the Fe minerals sequestering As. These two aspects are vital to understanding the overall fate of As in the aquifer as the dissolved hydrocarbon plume evolves over time and changes aquifer geochemistry.