Hydrogeologic Investigation of VOC Contamination in Bedrock Using Mass Flux Analysis – A Case History

Courtesy of US EPA and NGWA proceedings of the 2004 U.S. EPA/NGWA Fractured Rock Conference: State of the Science and Measuring Success in Remediation, September 13-15, 2004, Portland, Maine

A fractured bedrock system was investigated using a combination of hydrogeologic methods to minimize the number and depth of bedrock wells installed during the investigation. These methods were used to determine the vertical and horizontal extent of volatile organic compounds (VOCs) in groundwater. Mass-flux analysis proved to be extremely useful in determining the vertical extent of VOC contributions to the groundwater-flow system.

The Site is located near a groundwater divide within the wellhead protection areas of two public drinking-water supplies. VOCs (primarily tetrachloroethene, chlorobenzene, dichlorobenzene isomers, trichlorobenzene isomers, and trichloroethene), were released directly to the fractured bedrock through a catch-basin structure installed within the shallow bedrock. No evidence of DNAPLs was found, but a strong downward hydraulic gradient observed in relatively uniformly fractured shallow bedrock in the source area resulted in VOC migration deep into the bedrock system. Downgradient of the source area, groundwater flow direction is strongly influenced by a vertical fractured/weathered zone within the bedrock.

A fracture-trace analysis was used to identify potential fracture zones downgradient of the Site to determine potential migration pathways and assist in locating monitoring wells. An electrical resistivity survey was conducted to ground truth results of the fracture-trace analysis. Nested well clusters were then installed to evaluate horizontal and vertical flow patterns. Vertical profiles of hydraulic conductivity and VOC concentrations were developed using packer testing and groundwater sampling of discrete intervals within each borehole. Pumping tests were also conducted to evaluate interconnections within the bedrock and Site-wide hydraulic properties of the bedrock-fracture system.

Mass-flux analyses were conducted using the discrete hydraulic conductivity and VOC-concentration data to determine the vertical extent of VOC contributions from the relatively low permeability bedrock encountered with increasing depth at the site. The mass-flux approach provided a practical and cost-effective alternative to installing and sampling additional deep bedrock wells in relatively low-permeability bedrock.

Results of the discrete VOC sampling and packer pressure testing provided hydraulic conductivity values and VOC concentrations of 10-foot test intervals within well boreholes. Mass flux was calculated using the hydraulic conductivity and VOC concentrations to quantify the relative contributions of VOCs from each test interval. Total mass flux for an entire well bore was calculated by adding the individual mass flux values for each interval. The ratio of the mass flux calculation of each test interval to the mass flux calculation of the entire well bore was used to quantify the relative percent of the total mass flux contributed by the corresponding bedrock test interval. Profiles of mass flux calculations for each well bore demonstrated a significant decrease in mass flux with depth. These data were used to limit the number and depth of additional deep bedrock well clusters installed during the investigation.

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