Development of a Conceptual Site Model for Comingled Chlorobenzene Plumes in Weathered Bedrock

The presence of a relatively high hydraulic conductivity (K) weathered bedrock zone at the overburden/bedrock interface created challenges in the development of a Conceptual Site Model (CSM) for the fate and transport of chlorobenzene at a former industrial Site in Southeastern Massachusetts. The CSM formulation required a multi-line of evidence approach including a detailed evaluation of hydraulic gradients and chlorobenzene distribution across multiple units, the use of borehole geophysics to assess vertical gradients prior to bedrock well installation, and characterization of the weathered zone K and thickness across the Site. The bedrock chlorobenzene plume appears to discharge to weathered bedrock and shallow overburden in an area with artesian conditions in bedrock. The plume later becomes comingled with the overburden and weathered bedrock plumes beneath an encapsulated lagoon. Downward gradients beneath the lagoon drive groundwater flow into the weathered bedrock in a narrow area where its K is much higher than overburden. Downgradient of this area the plume appears to be confined to the weathered bedrock. A tracer was injected over a 12-hour period into a 100-ft deep bedrock well near the bedrock chlorobenzene source, and tracer concentrations detected in downgradient wells exceeded the background concentrations along the inferred flow path.