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i 440 West Charter Way, Stockton, California September 16, 1992 Texaco Environmental Services Page 4 (wells 1 A and 3A were the only wells which did not have enough influence on adjacent wells to determine a radius of influence The greatest recorded induced vacuum of 17 inches of water was observed in VE-26, when vacuum was applied the VE-2A This extremely high induced vacuum is probably attributed to an inadequate bentonite seal between the two screened sections within VE-2 The radius of influence was graphically determined for each measured condition from a log- normal plot of drawdown versus distance (Appendix B) The radius of influence was defined as the distance at which measured vacuum would be 0 1 inch of water Because of the variety of extraction levels and monitoring levels, the radius of influence was determined for each extraction level by only using monitoring data acquired from the same stratigraphic level Interpretation of the results is greatly simplified into a single radius of influence for each individual well The results are summarized in Table 2 Based on the log normal plots of drawdown versus distance, the measured radii of influence for, the VE's ranged between 23 and 54 feet and were highly asymmetric, indicating anisotropic pneumatic conductivity of the subsurface materials The average radius of influence was approximately 40 feet Conservatively, a 20 cfm flow rate will produce approximately a 25 foot radius of influence, overlapping the influence field of the neighboring VE's A 25 foot i radius of influence and the placement of the vapor extraction wells is sufficient to influence the area containing adsorbed hydrocarbons in the subsurface Hence, by drawing vapors from all the extraction wells at a flow rate of 20 cfm, a 160 cfm total flow is required' However, this flow is subject to change based on the operating parameters for the off-gas control device to be used to destroy hydrocarbon emissions If necessary, vapors can be drawn from individual wells by manifoldmg the system together and providing independent valves for each extraction well Table 3 summarizes laboratory analyses yses of hydrocarbon concentrations in vapor collected on June 24, 1992 Copes of the complete laboratory reports and chain-of-custody manifests are presented in Appendix C Laboratory analyses reported concentrations of TPH-G ranging from 4,700 to 30,000 ug/I in the A level wells and from 3,300 to 21,000 Ng/I in the B level VE's Benzene concentrations ranged from 86 to 240 ug/l in the A level and 55 to 500 Ng/1 in the B . level VE's The average concentrations of vapors extracted from all the wells was approximately 1 1,500 Ng/I TPH-G and 200 Ng/l benzene rI GROUNDWATER I '' i�� TECHNOLOGY