RATES OF ARSENIC MOBILIZATION DURING FE-REDUCTION IN WETLAND SEDIMENTS CONTAMINATED WITH BTEX: INFLUENCES OF ETHANOL AND NITRATE AS CO-CONTAMINANTS

Biodegradation of hydrocarbons in groundwater can create undesired secondary water-quality effects. Identifying these secondary effects is crucial to understanding overall water quality at contaminated sites. We investigated the mobilization of naturally occurring arsenic (As) and other trace metals from wetland sediments caused by biodegradation of petroleum hydrocarbons coupled with reduction of Fe(III) minerals. Arsenic has strong adsorption affinities for Fe(III) minerals. However, an influx of organic carbon can reduce solid Fe(III) to aqueous Fe²⁺, and adsorbed As can be simultaneously released to groundwater. In-situ push-pull tests were conducted at a wetland-aquifer interface for two months to evaluate rates of Fe-reduction and As mobilization when petroleum hydrocarbons such as BTEX (benzene, toluene, ethylbenzene, and xylenes) contaminate the subsurface. This study also evaluated effects of ethanol and/or nitrate as co-contaminants on the rates of Fe-reduction and As mobilization. When BTEX alone was added, our results show simultaneous onset and similar rates of Fe-reduction and As mobilization (k = 0.0032 hr^-1 and k = 0.0041 hr^-1, respectively). The addition of ethanol created an earlier onset of Fe-reduction and As mobilization than when BTEX alone was added; dissolved As concentrations increased from <1 μg/L in background waters to a maximum of 99.2 μg/L. When ethanol was added, rates of Fe-reduction and As mobilization occurred in multiple stages with different rates. Fe-reduction occurred in two stages (k₁ = 0.0113 hr^-1, k₂ = 0.0017 hr^-1). Arsenic mobilization occurred in three stages (k₁ = 0.0841 hr^-1, k₂ = 0.0072 hr^-1, k₃ = 0.00084 hr^-1) with the first stage being independent of Fe-reduction. After the initial stage, Fe-reduction and As mobilization occurred simultaneously and at similar rates. The addition of nitrate inhibited Fe-reduction and As mobilization for the duration of the push-pull tests. During this experiment, biodegradation was coupled with denitrification rather than Fe-reduction. Rates of denitrification were an order of magnitude faster with the addition of ethanol and BTEX compared to the addition of BTEX alone, suggesting that ethanol causes a faster consumption of nitrate and an earlier onset of Fe-reducing conditions necessary to mobilize As from sediment.