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Use of Electrical Conductivity Logging to Characterize the Geological Context of Releases at UST Sites
Citation:
WILSON, J. T., K. JEWELL, T. M. LANKFORD, C. J. ADAIR, A. P. Robinson, AND R. Pasupathy. Use of Electrical Conductivity Logging to Characterize the Geological Context of Releases at UST Sites . Presented at the EPA Region 3 States LUST Technical Workshop, Shepherdstown, WV, September 08 - 10, 2009.
Impact/Purpose:
to conduct a survey of electrical conductivity, and evaluate lithology from the logs to produce a site conceptual model for ground water flow
Description:
Risk is the combination of hazard and exposure. Risk characterization at UST release sites has traditionally emphasized hazard (presence of residual fuel) with little attention to exposure. Exposure characterization often limited to a one-dimensional model such as the RBCA equations, or a two dimensional model such as BIOSCREEN. At many UST release sites, the fuel is spilled into geological material with low hydraulic conductivity. Exposure through extraction of ground water for use as drinking water may be minimal, even though the hazard exceeds concentration based standards. This situation is particularly common in flood plain landscapes. The experimental approach to characterize exposure at a LUST site was to conduct a survey of electrical conductivity, and evaluate lithology from the logs to produce a site conceptual model for ground water flow. Then ground water samples were collected from temporary wells, and the vertical distribution of benzene in ground water was compared to the capacity to move ground water and produce a plume. The approach to characterize hazard was to collect core samples at various depths, extract the cores and determine concentrations of TPH and Benzene in the sediment (mg/kg), then determine weight loss of core sediment on drying. This information was used to calculate concentration of benzene in pore water. Finally the calculated distribution of benzene in water was compared to the site conceptual model for flow of ground water. At one of the study sites, based on the distribution of TPH and the lithology, there is little chance that fuel contamination can enter the deep aquifer at this site. The measured concentration of contaminants in temporary wells at the bottom of the clay layer was below the MCLs. At another site, in an interval below the water table in material that can transmit ground water laterally the concentration of TPH was high (up to 13,000 mg/kg) and the capacity of the residual gasoline to contaminate ground water was high (12 mg/L benzene). Risk is the combination of hazard and exposure. Risk characterization at UST release sites has traditionally emphasized hazard (presence of residual fuel) with little attention to exposure. Exposure characterization often limited to a one-dimensional model such as the RBCA equations, or a two dimensional model such as BIOSCREEN. At many UST release sites, the fuel is spilled into geological material with low hydraulic conductivity. Exposure through extraction of ground water for use as drinking water may be minimal, even though the hazard exceeds concentration based standards. This situation is particularly common in flood plain landscapes. The experimental approach to characterize exposure at a LUST site was to conduct a survey of electrical conductivity, and evaluate lithology from the logs to produce a site conceptual model for ground water flow. Then ground water samples were collected from temporary wells, and the vertical distribution of benzene in ground water was compared to the capacity to move ground water and produce a plume. The approach to characterize hazard was to collect core samples at various depths, extract the cores and determine concentrations of TPH and Benzene in the sediment (mg/kg), then determine weight loss of core sediment on drying. This information was used to calculate concentration of benzene in pore water. Finally the calculated distribution of benzene in water was compared to the site conceptual model for flow of ground water. At one of the study sites, based on the distribution of TPH and the lithology, there is little chance that fuel contamination can enter the deep aquifer at this site. The measured concentration of contaminants in temporary wells at the bottom of the clay layer was below the MCLs. At another site, in an interval below the water table in material that can transmit ground water laterally the concentration of TPH was high (up to 13,000 mg/kg) and the capacity of the residual gasoline to contaminate ground water was high (12 mg/L benzene).