ARCHIVED REPORTS XR0001816 - Laserfiche WebLink

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' 7 SUMMARY AND CONCLUSIONS Residual petroleum hydrocarbons are present in the soil beneath the site The highest concentrations of residual petroleum hydrocarbons appear to exist in the northern portion of the dispenser islands and near the northern most waste oil tank from 20 to 40 feet bgs Residual petroleum hydrocarbons have been laterally delineated with borings B-9, B-10, B-11, B-16, B-19, and B-24 and wells VW-12 and VW-13 ' EMCON has estimated, using conservative concentrations, that approximately 345 pounds of TPHG and approximately 405 pounds of TPHD has been removed from the site In addition, EMCON has estimated that approximately 10,390 pounds of TPHG and 8,331 pounds of ' TPHD are present beneath the site The groundwater flow direction has consistently been toward the south to southwest since March 1994 However, during the second quarter 1994, the groundwater flow was toward the northeast Depth to groundwater has fluctuated from 56 to 65 feet bgs and has consistently been below the top of screen of each well From March 1994 through May 1996, dissolved gasoline petroleum hydrocarbons have been detected in the three monitoring wells TPHG concentrations have ranged from ' below laboratory detection limits to 320 ppb during the last seven quarterly monitoring events However, benzene concentrations have been below laboratory detection limits ' during the last seven groundwater sampling events TPHD, TOG and MTBE have not been detected in groundwater samples collected from the monitoring wells Therefore, it is EMCON's opinion that minimal petroleum hydrocarbons have leached from the impacted ' vadose soil to groundwater In addition, it is EMCON's opinion that natural biodegradation will further reduce residual and dissolved petroleum hydrocarbons as evidenced in the decline of dissolved hydrocarbon concentrations In the monitoring wells ' with time, therefore, suggesting that the hydrocarbon plume is relatively stable In addition, no drinking water wells are within 750 feet of the site, thereby, reducing the likelihood of any potential hydrocarbon impact In conclusion, EMCON requests closure for this site for the following reasons ' + Benzene has been below laboratory detection im-uts for the last seven quarters Therefore, hydrocarbons present in soil do not appear to be leaching to groundwater • The groundwater impacts are minimal and the plume is stable WNN TW805\08051420 4BS-97\prz 1 Rev 0 3/12197 20805-142 004 7-1