USE OF SUB-RIVER DIFFUSIVE SAMPLERS TO DETERMINE THE EXTENT OF CONTAMINATED GROUNDWATER DISCHARGING TO SURFACE WATER

Groundwater contamination was discovered in 1978 at the W. R. Grace facility located in Acton, Massachusetts. The groundwater contamination, consisting mainly of 1,1-dichloroethene (VDC), vinyl chloride and benzene, originated from a series of wastewater lagoons and an industrial landfill located above a groundwater divide between Fort Pond Brook to the north and northeast, and the Assabet River to the south and southeast. Contaminated groundwater migrated downward through the unconsolidated deposits, which reach thicknesses of 180 feet, and into fractured bedrock. The contaminated groundwater migrated to the north and northeast toward Fort Pond Brook and to the south and southeast toward the Assabet River.

Groundwater samples were collected from beneath the Assabet River and Fort Pond Brook to evaluate the discharge of contaminated groundwater. Sampling was done over a two year period, with the second years sampling plan design based, in part, on the results of the first year. Diffusive groundwater samplers were installed at 26 transects along the river and 22 transects along the brook. Samples were collected at two to four locations across each transect. The sampling results were used to reliably delineate the upstream and downstream boundaries of the groundwater contaminant discharge zones to the Assabet River and Fort Pond Brook. The results indicated that contaminated groundwater discharges across approximately half of the width of the river with the highest concentrations approximately one third of the way across. The sampling results were also helpful in selecting sediment and surface water sampling locations for the ecological risk assessment. Transect results identified a previously unsuspected area of groundwater contamination and were used to design a land-based drilling/sampling program to characterize the extent and locate the likely source of the groundwater contamination. The details and results of this sub-riverbed sampling will be described.