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Chevron U S A , Inc February 20, 1992 A02320 2338 Page 4 3 4 Potential Effects of Residual Contaminants on Water Supply Based on the last round of quarterly monitoring and sampling, and historical analytical data for groundwater, the immediate threat posed to currently usable water supplies by impacted groundwater at the site is minimal Groundwater samples collected during the last quarterly sampling event were all below the MDL for TPH-G and BTEX, and therefore well below the action levels set for state- regulated contaminants All perimeter monitoring wells were below the MDL indicating that the dissolved BTEX plume has not migrated laterally beyond the boundaries of the Chevron property Af present, the water table is at a historical low level trends in historical sampling data suggest that the water table levels and concentrations of dissolved BTEX in groundwater are directly related When water levels drop, so do concentrations of BTEX Groundwater beneath the site has an average vr vi— hydraulic gradient of approximately 0 01 to the east-northeast Groundwater beneath the site flows r . at an estimated velocity of 20 feet per year in the east-northeast direction Velocity was estimated by using approximate values for hydraulic conductivity and porosity, derived from the (Ethology of the water bearing zone, and combining these numbers with the average hydraulic gradient 4 0 FEASIBLE ALTERNATIVES FOR CORRECTIVE ACTION 4 1 Overview Based on the results of previous site assessment, three alternatives for corrective action at the site are proposed as follows i ■ Continuation of quarterly groundwater monitoring and sampling ■ Excavation of impacted soil ■ Soil vapor extraction A brief description of the implementation, objectives and cost effectiveness of each alternative are summarized below 4 2 Continuation of Quarterly Monitoring and Sampling Groundwater monitoring and sampling would be conducted at the site on a quarterly basis All monitoring wells with a sufficient water column would be gauged for depth to water to determine the magnitude and direction of groundwater flow beneath the site, and sampled to determine the concentration of dissolved BTEX and TPH-G The objectives of this alternative are to further define the velocity of groundwater flow and to identify any trends in dissolved concentrations of BTEX and zssecnP aPr _ GROUNDWATER TECHNOLOGY