The source of the FS-28 groundwater plume is unknown. AFCEE is continuing its investigations that may identify the source of the FS-28 plume. Since chlorinated solvents were discovered with the FS-28 plume, north of Route 151, it was determined that the FS-28 remedial investigation should be folded into the much larger regional effort, which is referred to as the Southwest Operable Unit (SWOU). SWOU-related activities, such as the water level survey which measured water levels in over 800 monitoring wells, ponds and streams, will assist in the work being done to find the FS-28 source area. This survey information will be used to determine groundwater flow directions and aid in modeling efforts to attempt to identify the plume source. One hundred and five existing monitoring wells were sampled for volatile organic compounds (VOCs) and ethylene dibromide (EDB), along with drilling and sampling 60 new monitoring wells.

Primary Contaminants

The FS-28 plume contains EDB, a probable carcinogen, at levels in groundwater up to 16 parts per billion (ppb). The safe drinking water standard is 0.02 ppb. EDB is a compound that was added to leaded gasoline, including aviation gasoline (AVGAS), to inhibit the buildup of lead in engines.

Affected Area

Based on available data, the FS-28 plume is approximately 9,000 feet long and varies in thickness up to 100 feet and width up to 3,000 feet. It flows under the western side of Coonamessett Pond and the Town of Falmouth Coonamessett Municipal Well. From there it flows south where a large portion of the plume rises from a depth of 150-200 below ground surface to the point where the Broad River meets the Coonamessett River. The deeper portion of the plume continues to move southward, just north of Thomas B. Landers Road in the Town of Falmouth.

Surface water detections in the Broad and Coonamessett Rivers have been found since EDB was first detected in October 1996. EDB concentrations decrease when the nearby cranberry bogs are flooded during the winter months. Detections of EDB in Coonamessett River surface water have been found as far south as Route 28. Surface water detections have historically been at concentrations of less than 0.3 parts per billion (ppb). There was a one-time sample event with a result of 0.7 ppb that has not been repeated in subsequent sampling.

Plume Cleanup

As part of a Time Critical Removal Action, one groundwater extraction well (69EW001) was installed in an area of higher EDB concentrations, just before the plume upwells to the river system. It became operational in September 1997. The extraction well is drawing out groundwater at a rate of 600 gallons per minute (gpm). The extracted water is treated using granular activated carbon and then returned to the river, meeting the Massachusetts Surface Water Quality Standards.

Ten irrigation wells have been installed for cranberry growers along the river system to replace their previous use of surface water. Growers were compensated for their 1997 crop. Biweekly testing of private wells, conversion to town water, and air quality testing have been conducted to ensure the protection of public health.

Additional work began in January 1998 to determine the source of the FS-28 plume and to investigate other constituents of concern as part of the SWOU study.

Next Steps

Final cleanup options will be evaluated in the SWOU Feasibility Study due in December 1998.