QUANTIFYING BIOREMEDIATION EFFICIENCY OF CHLOROBENZENES IN REACTIVE BARRIERS AT THE WETLAND-SURFACE WATER INTERFACE USING MASS DISCHARGE AND SEDIMENT MASS

Performance metrics for remediation near the groundwater surface-water interface include the reduction in dissolved contaminant concentrations in groundwater discharge and the reduction in contaminant mass in the sediments. Achieving and documenting remediation where upward groundwater flow rates are relatively high and often variable can be challenging. During a pilot test of 25-centimeter-thick reactive barriers constructed at the wetland surface at the Standard Chlorine of Delaware Superfund site, we used mass discharge of dissolved chlorobenzenes and chloride coupled with sediment mass measurements over time to determine bioremediation efficiency of the chlorobenzenes. Bioaugmented granular activated carbon (GAC) and chitin were mixed into the sediment to construct two reactive barrier plots. Using peepers, piezometers, and sediment cores, samples were collected 4 times over 18 months in the reactive barrier plots and adjacent control areas.

Attenuation rates of dissolved chlorobenzenes along upward flowpaths in the reactive barriers were consistently high, with half-lives ranging from 0.97 to 1.8 hours during the monitoring period. The flux of dissolved chlorobenzenes discharging at the wetland surface in the reactive barrier plot adjacent to the tidal creek was reduced by two orders of magnitude compared to the surface flux in the control area. Chlorobenzene mass in the reactive barrier sediment was 44 to 74 percent lower than the mass in the control area sediment within 12 days of installation. After 12 days, chlorobenzene mass in the reactive barrier sediment did not change substantially, although groundwater influx continued to contribute 5 to 12 grams of chlorobenzenes per day during the monitoring period (1900 pore volumes). Total mass removal rates of chlorobenzenes in the reactive barriers, calculated from groundwater mass discharge and sediment mass, remained within a factor of 4 of the influx mass discharge. Thus, biodegradation was approximately equal to contaminant influx rates, minimizing sorption to the GAC and an increase in the reactive barrier contaminant mass. Degradation rates estimated from increases in mass discharge of chloride in the reactive zones agreed with the chlorobenzene mass removal rates within a factor of 0.5 to 5, confirming efficient bioremediation.