A large field-scale bioremediation pilot test (BPT) was conducted to demonstrate the ability of an in situ sulfate delivery system to treat groundwater impacted by a mixed plume of chlorinated and non-chlorinated solvents at a former industrial landfill site in New Hampshire. The primary chemicals of concern (CoCs) included cis- 1,2-dichloroethene (cDCE), vinyl chloride (VC), 1,2-dichloroethane (1,2-DCA), 1,1- dichloroethane (1,1-DCA), methylene chloride (MC), benzene, toluene, acetone, methyl ethyl ketone (MEK) and methyl isobutyl ketone (MIBK). Total VOC concentrations in the groundwater entering the system consistently exceeded 10 mg/L during the pilot test. Given the high degree of geologic heterogeneity at the Site, a gravel delivery trench (GDT) was installed in the subsurface, to a depth of 22 to 24.7 m (top of bedrock), to deliver and uniformly mix the added sulfate with the CoC-impacted groundwater. The GDT was installed using innovative construction techniques to minimize costs and excavation volumes. The sulfate delivery system was successfully installed and operated for a period of 13 months. At the end of the pilot test, the average declines of the CoCs due to biodegradation in the short-residence-time pilot test monitoring area (PTA) (I.e., concentrations corrected using bromide data to account for concentration decreases related to dilution) were as follows: cDCE, 68%; VC, 39%; 1,2-DCA, 21%; 1,1-DCA, 23%; MC, 75%; toluene 9%; benzene, 36%; acetone, 44%; MEK, 49%; and MIBK, 66%. Half-lives were calculated using these changes in concentration and breakthrough data from bromide tracer testing. Average half-lives (in days) for the CoCs were as follows: cDCE, 3; VC, 5; 1,2-DCA, 20; 1,1-DCA, 18; MC, 2; toluene, 36; benzene, 9; acetone, 4; MEK, 5; and MIBK, 4. Concentrations of CoCs at the compliance boundary were calculated using average half-lives and potential combinations of influent concentrations and groundwater velocity. Results of these calculations indicate that biodegradation alone is sufficient to achieve compliance with Ambient Groundwater Quality Standards (AGQS) at the downgradient compliance boundary for all conditions and CoCs except for VC and toluene under the condition of the maximum historic (5-year) influent concentration in conjunction with the maximum groundwater velocity. No significant operational issues (e.g., biofouling, precipitation or system downtime) occurred over 13 months of sulfate addition, and system performance exceeded the initial design expectations. The results of the BPT demonstrate that an in situ bioremediation strategy employing sulfate addition can be successfully applied to treat a mixed contaminant plume in heterogeneous hydrogeologic conditions.
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