RESPONSE OF REDOX CONDITIONS TO AIR SPARGING IN A GEOCHEMICALLY HETEROGENEOUS GROUNDWATER SYSTEM

The shallow groundwater aquifer at Fort McCoy, WI, was contaminated from leaking petroleum tanks. Elevated BTEX (benzene, toluene, ethylbenzene, and xylene) concentrations (2-10mg/L) are measured in the shallow groundwater that flows through an originally aerobic upland into an anaerobic wetland, discharging into Tarr Creek. The study area is geochemically heterogeneous both spatially and with depth. Terminal electron accepting processes along the contaminated flow path range from iron and sulfate reduction to methanogenesis, and along the uncontaminated path from oxygen and nitrate reduction to iron reduction along deeper paths (Schreiber and Bahr, 1999). An air sparging system, activated 11/8/01, injects air 3-13ft below the water table to remove VOCs and to possibly aid in microbial degradation of BTEX. The spatial variability in groundwater chemistry in the area affected by the BTEX-plume as well as the large existing array of multi-level samplers provide a unique opportunity to conduct a detailed chemical analysis as the air sparging induced oxygen plume migrates through the system. This allows assessment of the effects of subsurface aeration on redox reactions involving dissolved iron and oxygen, the efficiency of BTEX-remediation, and the microbial contaminant degradation. Weekly sparging perturbations and a winter block without sparging provide potential chemical signatures in the groundwater and allow for analysis based on sequence and timing involved in the migration of oxygen-iron reactions from the source area to the discharge point. The specific reactions of interest involve iron II oxidation during sparging followed by possible renewed iron reduction when sparging ceases and available oxygen is consumed by microbial oxidation of BTEX and natural organic matter. Preliminary results show that constant dissolved iron concentrations persist in the wetland area 8 months following the month of initial sparging, suggesting the rapid consumption of the oxygen plume along the flow path. Additionally, sampling results along the contaminated transect, after the initial sparging event, indicate depletion of introduced oxygen electron acceptor followed by the rapid recovery of dissolved iron levels and renewal of microbial oxidation of BTEX.