SITE INFORMATION AND CORRESPONDENCE FILE 2

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Former Chevron Service Station 9-0557 Page 5 139 South Center Street May 6, 2010 Stockton, California Observation well MW-9A, although only seven feet from MW-9S, was screened much deeper, so there may have been a lag time in response to extraction from MW-9S as vertical permeability is often much lower than horizontal permeability. The EHD is aware that the TPE test was performed and initially interpreted by others, but requests that HFA reevaluate the test design and data and provide the EHD their interpretation of the potential cost effectiveness of this method to address the residual contamination. Ozone Injection Test The ozone injection test may not have had an effective design to properly evaluate the effect of injection of the oxidant. Neither the test report nor the FS present an estimate of the dissolved and sorbed contaminant masses, thereby making it difficult to ascertain whether or not the 0.004 pounds of ozone injected per hour (0.096 pounds per day) is likely to be an effective injection rate that could significantly affect the contaminant mass. Also, there is no estimate or measurements of biological oxygen demand (BOD) or chemical oxygen demand (COD) that will compete with the contaminants for the injected oxidant. Without this data, estimating the proper `dose' of ozone to create an observable effect would be difficult. Ozone was injected into the subsurface through four injection points set at twenty-eight to thirty feet bsg and four injection points set at forty-three to forty-five feet bsg. Monitoring wells MW-1, MW-2, MW-9, MW-9A, MW-9S, MW-10 and MW-11 were used as observation wells. Observation wells MW-9 and MW-9A were screened from fifty-two to fifty-seven feet bsg, deeper than the ozone injection wells, and would have limited potential for exhibiting effects of ozone, usually injected as a gas, which would most likely have migrated vertically and laterally until dissolved. The screen interval of observation well MW-1, thirty-five to sixty feet bsg, overlapped the screen interval of injection well OS-1D, forty-three to forty-five feet bsg, but was located nearly twenty-two feet obliquely down-gradient from OS-1D. With the contaminant mass and competing reactions (BOD and COD) unknowns noted above and the low ozone injection rate, response in MW-1 could be attributed to two very different causes: 1) the ozone injection was simply ineffective; or 2) the ozone injection was reasonably effective, but the response was overwhelmed (buffered) by the contaminant mass and/or competing reactions and could not be detected twenty-two feet obliquely down-gradient. The same issues may prevail for observation well MW-2 (approximately twenty feet down-gradient of OS-4D). Observation wells MW-11 (approximately ten feet from OS-1 S and OS-1 D) and MW- 9S (approximately ten feet from OS-2S and OS-2D) may offer the best potential to detect the effects of ozone injection; but again, the interaction of the contaminants, ozone and natural system have not been quantified. Observation well MW-10 was the closest, laterally, to an injection point, but the same evaluation issues obtain. This reevaluation of the ozone injection pilot test design and results leads the EHD to determine that the results are not conclusive regarding the efficacy of ozone injection for site remediation and requests that Chevron and/or HFA also reevaluate the pilot test.