To date, BNL's Environmental Restoration Division has identified six distinct chemical groundwater plumes originating within the Lab site . Three treatment systems have been constructed to prevent the migration of these contaminants and remove them from the aquifer. Construction of a fourth is under way. Although these plumes do not impact private water wells, more than 1,300 property owners south and east of the Lab have been offered hookups to public water as a precautionary measure.
Plumes form when chemicals enter the ground through an accidental or intentional release. After entering the top levels of the soil, the chemicals are pushed deeper into the ground by precipitation, eventually reaching the top levels of the water table. Once it reaches this point, the contamination migrates with groundwater at a slow rate, diluting, spreading and moving deeper within the aquifer. BNL's plumes generally travel south within the aquifer at a rate of about one foot per day.
At BNL, each plume has been classified by the geographical and administrative area of the Lab or "Operable Unit" in which it is located. The following is an overview of each plume:
The Operable Unit I plumes are comprised primarily of solvents known as "volatile organic compounds," or VOCs, including tetrachloroethene (PCE), trichloroethene (TCE) and carbon tetrachloride. The plumes are located at depths between 180 and 210 feet below land surface, at concentrations ranging from 5 to 1,800 parts per billion (ppb). Area residents have been offered public water. Two groundwater extraction wells were installed at BNL's site boundary last year, part of a pump-and-treat system currently treating this plume.
The Operable Unit III plume also contains PCE, TCE, carbon tetrachloride and another solvent known as trichloroethane (TCA). It is located at depths between 180 and 300 feet below land surface, and contaminant levels from 5 to 5,000 ppb have been detected in monitoring wells. In June, a pump-and-treat system at the site boundary began operation to prevent further migration off the BNL site. An investigation of the full extent of groundwater contamination in this area is still proceeding (see story, page 3). Area residents have been offered public water.
The Operable Unit IV plume consists of petroleum breakdown products like benzene, toluene and xylene. It is fully contained within the BNL site, and located at depths up to 80 feet below land surface. Contaminant levels ranging from 5 to 6,000 ppb have been detected in monitoring wells. A treatment system is under construction to remediate area groundwater.
The Operable Unit V plume contains TCE at levels ranging from 5 to 32 ppb. It is located 190 to 230 feet below land surface. Area residents have been offered public water.
The Operable Unit VI plume consists of a pesticide known as ethylene dibromide (EDB), and is found between 90 and 130 feet below the land surface. Concentrations of EDB from .05 to 3.5 ppb have been detected. Area residents have been offered public water. The selected remedy for this area, currently under review by regulators, involves natural attenuation of the EDB through dilution and decay along with additional monitoring.