ARSENIC MOBILIZED INTO GROUNDWATER FOLLOWING PETROLEUM SPILLS CAN CONTAMINATE LARGER AQUIFER VOLUMES THAN THE HYDROCARBONS ORIGINALLY SPILLED (Invited Presentation)

Naturally occurring arsenic (As) was mobilized from sediments into groundwater in a petroleum-contaminated aquifer near Bemidji, MN. These field observations were used to calibrate a reactive transport model to simulate As cycling at the site during a 400-year period. Using surface complexation modeling, As mobilization and attenuation were simulated as sorption/desorption reactions from ferrihydrite (Fe(OH)­₃) mineral surfaces during biodegradation of petroleum hydrocarbons coupled with iron (Fe)-reduction. The model predicts that once mobilized, As concentrations in groundwater will exceed the 10 µg/L Maximum Contaminant Level (MCL) for 387 years. Arsenic and Fe were kept in solution due to biodegradation of non-volatile dissolved organic carbon (NVDOC), which consumes dissolved oxygen introduced via recharge. After NVDOC attenuates, the aquifer becomes more oxygenated from recharge, allowing As to resorb to Fe(OH)₃ in sediment. Over the 400-year simulation, the model predicts that As mass in the aquifer is redistributed due to sorption and desorption. The original background As in sediment is depleted in the region extending from the oil source to 300 m downgradient. Beyond 300 m, dissolved As is attenuated in sediment via sorption to Fe(OH)₃, resulting in sorbed As concentrations substantially higher than the original background. To assess the long-term water quality risks at this site, we compared the volume of groundwater exceeding the MCLs for As and benzene, the primary regulatory driver at petroleum sites, over 400 years. Results show that a far greater groundwater volume is contaminated by As than by benzene over the model simulation. For example, one century after the spill, the groundwater volume exceeding the As MCL is nearly 6x greater than the volume exceeding the MCL for benzene. This suggests that As may pose a greater long-term threat to water quality than the original contaminant of concern, benzene, in this petroleum-contaminated aquifer.