ARSENIC CONTAMINANT PLUME DYNAMICS IN A CRUDE-OIL CONTAMINATED AQUIFER UNDERGOING NATURAL ATTENUATION

At a crude-oil contaminated aquifer near Bemidji, Minnesota, anaerobic biodegradation of hydrocarbons coupled to microbial iron (Fe) reduction has resulted in a contaminant plume with dissolved iron concentrations in excess of 100 mg/L near the subsurface oil body. Our previous results have shown elevated concentrations of arsenic (As) in groundwater from monitoring wells near the crude oil source and within the high iron zone. We hypothesized that natural attenuation of hydrocarbons coupled to microbial iron reduction resulted in release of trace metals, including As,from naturally occurring ferric (Fe (III)) oxyhydroxides. Water samples collected from monitoring wells in 2009 and 2010 from the anaerobic plume showed concentrations of As greater than 200 μg/L, which is well above the 10 μg/L drinking water limit. The crude-oil source has low levels of As, 35 µg/L, too low to explain the relatively high groundwater As concentrations. In 2013 we sampled groundwater along a transect starting closer to the oil source zone where the redox system is dominated by methanogenesis, through the Fe(III) reducing zone, and ending at the leading edge of the plume where mixing of the dissolved Fe plume with dissolved oxygen results in precipitation of Fe (III) oxyhydroxides. Groundwater was sampled from 30 wells and analyzed for dissolved oxygen, dissolved Fe²⁺ and total Fe, total As, As(III) and As(V), in addition to BTEX, DOC and other geochemical parameters. Sediments were sampled from cores within the different redox zones and analyzed for Fe, total As, As(III), and As(V). We expect that As concentrations in groundwater will be elevated in the Fe-reducing zone but that they will decrease to below detection limits at the leading edge of the plume due to adsorption to Fe(III) oxyhydroxides. Furthermore, we expect that As concentrations within the active Fe-reducing zone will increase over time as Fe-reduction reactions progress, resulting in a downgradient migration of the dissolved As plume.