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. Fourth Quarter 1996 VES Report - Unocal Station No 6981 February 7, 1997 will continue to monitor VES operational parameters every two weeks Influent and effluent vapor stream samples will be collected monthly and analyzed as required in the operating permit The First Quarter 1997 Status Report will be submitted to the appropriate regulatory agencies by April 30, 1997 GSI will continue to evaluate the VES in an attempt to solve the flameout problem Response to San Joaquin County Letters In a letter dated September 18, 1996, the SJCPHS requested a "comprehensive remediation annual report" be prepared Between December 19, 1995 and December 20, 1996, the VES operated approximately 164 days, or 44% This low operational rate was apparently due to a flame-out problem within the unit The flameout problem became especially acute beginning in June 1996 However, this flame-out problem has the effect of pulsing the system, which appears to increase the hydrocarbon extraction rates This effect is shown by the TPHg and benzene concentration versus time plots on Figures 3 and 4, respectively. As currently configured (extracting from wells VW-2, VW-4, and VW-5), it is not possible to prepare an accurate estimate of the radius of influence for the VES. First, the pulsed operation of the system, an effect of the flameout problem, is likely to have a limiting effect on the lateral extent of the system's lateral influence Second, wells VW-1, VW-3, and VW-6 were the only vadose zone monitoring available for monitoring during the third and fourth quarters of 1996 These three wells are all approximately the same distance and same direction from the extraction points Directional variation in observation well orientation could be an important factor in radius of influence calculation, given the apparent discontinuous and lenticular nature of the coarse-grained material beneath the site. The system continues to effectively remove hydrocarbon-laden vapors from the subsurface, and GSI will continue to operate the system in 1997 extracting from wells VW-2, VW-4, and VW-5, which have the highest concentrations This is consistent with historical soil chemical analytical data, which indicate the highest soil hydrocarbon concentrations were identified at the north ends of the dispenser islands As the influent vapor TPHg concentrations in December 1996 were 1,600 parts per million vapor, and the cumulative mass extraction rates (Figures 3 and 4) are not asymptotic, it would be premature to drill soil bonngs to evaluate residual hydrocarbon concentrations in soil To date, the VES has extracted approximately 11,205 pound of TPHg and 167 pounds of benzene. In SJCPHS's letter dated January 3, 1997, SJCPHS again requested wells MW-12 and MW-13, abandoned to facilitate intersection reconstruction, be replaced As stated in the Unocal Corporation letter of October 10, 1996, "it would not be prudent to install the necessary groundwater monitoring wells until after the construction activities have been 485520-1 2