This paper describes microcosm testing and remedial planning for a site that is impacted with halogenated volatile organic compounds (VOCs), trichloroethene (TCE), trichlorobenzene (TCB), and daughter products, as well as aromatic hydrocarbons (mostly toluene) and other organics (isopropanol) from historic releases.
Microcosm studies were conducted on site groundwater in order to evaluate the potential to enhance anaerobic dechlorination as well as aerobic degradation of VOCs. Groundwater samples were collected from four site monitoring wells (within and downgradient of the main source area) and were tested in the laboratory under a variety of conditions.
Anaerobic microcosms constructed with groundwater from well MW-8 (within the major source area) and amended with lactate as electron donor, demonstrated the presence of anaerobic TCB and dichlorobenzene (DCB) degraders able to reductively dechlorinate TCB to DCB, then completely to chlorobenzene (CB) within 13 days (which could subsequently be degraded aerobically). Well MW-8 microcosms also demonstrated the presence of ethenogens capable of fully dechlorinating cis-1,2 dichloroethene (DCE) to ethene. Aerobic Microcosms demonstrated that groundwater from MW?8 also contains aerobic CB degraders and toluene degraders. Although CB could not be anaerobically dechlorinated within the 120 day testing period, when oxygen was added to a MW-8 microcosm containing CB it (CB) was oxidized to non-chlorinated products Within 9 days, and toluene was fully degraded.
In summary, a combination anaerobic and aerobic treatment can fully degrade the VOCs in the groundwater at the site. Aerobic microcosms constructed with groundwater from a slightly downgrdaient well also demonstrated the presence of aerobic CB and toluene degraders. The approved site remedy calls for a sequential combination of anaerobic dechlorination and aerobic oxidation for TCB degradation, aerobic degradation of the toluene and other simple hydrocarbons, and enhanced anaerobic dechlorination for the chlorinated ethenes on the western end of the facility.