ARCHIVED REPORTS XR0012491 - Laserfiche WebLink

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i Technical Rationale In its October 14, 1997 correspondence, SJCPHS has requested a three dimensional definition and investigation of soil and ground water contamination by fuel hydrocarbon constituents This request is made based on SJCPHS's conclusion that vertical and lateral definition is not complete at this site SJCPHS notes in the October 14, 1997 letter that ground water levels in wells MW-1, MW-5, MW-7, MW-8, MW-9, and MW-10 presently exceed screened intervals, and concludes that rises in ground water elevation have resulted in the decreased contaminant levels observed in the wells Well MW-5 is the last monitoring point on-site between the Shell site and six contaminated sites located 250 feet to the east Concentrations in this well have dropped since 1991, when separate phase product was present, to current non-detectable levels The most significant decrease in petroleum hydrocarbon concentrations occurred between the October 1992 and February 1993 sampling events TPH-g decreased from 16,000 ppb to 170 ppb, and benzene decreased from 3300 ppb to 7 4 ppb The depths to water during these two events were 32 79 feet and 32 18 feet respectively, a fluctuation of only 0 61 feet The period of time covered by these two sampling events corresponds to the startup and operation of a soil vapor extraction system While the screens for this well are currently submerged, six consecutive sampling events between August 1993 and November 1994 were non-detect for TPH-G and BTEX Depth to ground water during these events ranged from 25 15 and 31 90 feet below grade (fbg), which is within the screened interval for this well Six additional sampling events since November 1994 and prior to water levels rising over the screened interval of this well were ND or contained only low levels of petroleum hydrocarbons Given these data, it is feasible that decreases in concentrations observed in Well MW-5 and other site wells are a result of remediation performed by soil vapor extraction, rather than the result of rising water levels masking contamination which SJCPHS believes to be present at depths of greater than 30 fbg Rates of removal for soil vapor extraction decreased substantially beginning in January 1993 As noted in the SJCPHS letter, removal rates decreased from 16 95 lbs/hour in September 1992 to 1 74 lbs /hour in January 1993 SJCPHS attributes this decrease to a rise in water levels above 30 feet below grade However, depth to water for the wells used for soil vapor extraction (MW-2, MW-3, MW-7, MW-9) during the February 1993 sampling event, at which time this decrease in removal rate had already occurred, ranged from 32 18 to 35 76 feet, all at depths greater than 30 fbg and showing little change from depths to water during system startup These data suggest the decrease in removal rates results from removal of available hydrocarbons from the vadose zone prior to rising water levels The following tasks are proposed to evaluate the vertical and lateral extent of petroleum hydrocarbons in ground water These data will also be used to evaluate the effectiveness of soil vapor extraction in remediating soil and ground water Proposed Scope of Work Task I Permits Appropriate permits for drilling permits will be obtained from SJCPHS 375 2