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Memorandum - 2 - 27 March 1992 vapor extraction system does not address the 14 feet of soil contamination left behind as ground water dropped from a depth of 44 feet to 58 feet between 1985 and 1992. All soil contamination must be addressed. If additional vapor extraction wells are installed, long screen lengths may be unnecessary because soil is not contaminated near the surface. 5. The previous consultant, PES Environmental , performed a limited aquifer test on the extraction well , EW-1 . However, the test was only conducted over 186 hours and calculations were not provided. By letter, dated 28 August 1991, San Joaquin County staff indicated, with my concurrence, that if ground water extraction and reinjection were proposed for disposal of treated ground water, more extensive aquifer testing was necessary. The well proposed for extraction must be used for the aquifer test and subsequent calculations must be shown for aquifer properties including transmissivity, capture radius, hydraulic conductivity, and hydraulic gradient. 6. GAS proposed to extract ground water from MW-4. MW-4 did not contain enough ground water to collect a sample for analysis in December 1991 . It does not appear that MW-4 contains an adequate supply of ground water for operation as an extraction well . Either Geweke should extract ground water from EW-1 or construct a new extraction well near MW-4. 7. GAS did not specify the upgradient well into which they intend to reinject treated ground water. Several of the upgradient wells, including MW-1, MW-2, MW-5, and MW-6, are inappropriate for reinjection because they contain or formerly contained contaminated ground water. Flushing additional contaminants from soil and ground water away from the ground water and soil vapor extraction systems must be avoided. 8. Discharge by reinjection requires a permit from the Regional Board. Board staff have recently prepared general Waste Discharge Requirements (WDRs) for discharges of treated ground water which was contaminated with petroleum products. The general WDRs include discharges to waste water treatment ponds, infiltration basins, spray disposal , reinjection and other disposal methods not involving discharge to surface waters. The effluent limitations specified in the WDRs are the laboratory detection limits. 9. On page 5 of the FRP, it is stated that the ground water treatment system "is to achieve the State Drinking Water Standard" prior to discharge back into the aquifer. The Drinking Water Standard for benzene is 1 ppb. However, the effluent limitation in the WDRs for benzene is 0.5 ppb. All discharges must meet the effluent limits specified in the WDRs.