Continued study of ground-water contamination at Wurtsmith Air Force Base, Michigan, defined the movement and distribution of volatile organic compounds in the glacial sand and gravel aquifer at known sites of contamination, and has defined new plumes at two other sites.

The Arrow Street purge system, installed in 1982 to remove contaminants from the Building 43 plume, has lowered concentrations of trichloroethylene in ground water in the central part of the most contaminated area from a range of 1,000 to 2,000 micrograms per liter to about 200 micrograms per liter. Trichloroethylene is not escaping off-Base from this area.

In the southern part of the Base a plume containing principally trichloroethylene and dichloroethylene has been delineated along Mission Drive. Maximum concentrations observed were 5,290 micrograms per liter of trichloroethylene and 1,480 micrograms per liter of dichloroethylene. Hydrologically suitable sites for purge wells are identified in the southern part of the plume using a new ground-water flow model of the Base.

A benzene plume near the bulk-fuel storage area, delineated in earlier work, lias shifted to a more northerly direction under influence of the Arrow Street purge system. Sites initially identified for purging the benzene plume have been repositioned because of the change in contaminant movement. JP-4 fuel was found to be accumulating in wells near the bulk-fuel storage area, largely in response to seasonal fluctuations in the water table. It is thought to originate from a spill that occurred several years ago.

A more thorough definition of contaminants in the northern landfill area has permitted a determination of the most hydrologically suitable sites for purge wells. In general, Concentrations found in water do not differ greatly from those observed in 1981.

Since 1981, concentrations of trichloroethylene have decreased significantly in the Alert Apron plume. Near the origin of the plume, the concentration of trichloroethylene has decreased from 1,000 micrograms per liter in 1980 to 50 micrograms per liter in 1984. Water from Van Etten Lake near the termination of the plume had only a trace of trichloroethylene at one site.

Benzene detected in water from well AF2 seems to originate near the former site of buried fuel tanks west of the operational apron. During periods of normal purge pumping along Arrow Street, contaminants are drawn to the purge system. During periods when pumping is low, contaminants are drawn toward water-supply wells AF2, AF4, and AF5.