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I ,I i I I Project No 58080-06-02 May 5, 1995 ' range petroleum hydrocarbons in soil beneath the site and a discussion of the possible sources of groundwater impacts ' Data indicates the existence of a plume of residual benzene, toluene, ethylbenzene and total xylenes (BTEX) and petroleum hydrocarbons in vadose zone soil beneath the site The remaining BTEX plume exists beneath approximately 30%of the site and the remaining petroleum hydrocarbon plume ' exists beneath approximately 55% of the site Based on an average combined BTEX concentration of 50 mg/kg in soil, it is estimated that approximately 168 pounds of BTEX mass exists beneath the site Assuming an average petroleum hydrocarbon concentration in soil of 1,600 mg/kg, it is estimated that approximately 9,000 pounds of petroleum hydrocarbons remain in soil beneath the site The estimated vertical limits of BTEX and petroleum hydrocarbon migration range from approximately 25 to 45 feet below ground surface (bgs) and the depth to groundwater beneath the site between June 1992 and November 1994 ranged from approximately 54 to 57 feet bgs ' Petroleum hydrocarbons, VOCs and/or MTBE are present in onsite groundwater monitoring wells and in upgradient and crossgradient offsite wells at similar concentrations Monitoring well MW-5 is located upgradient and north of the subject site adjacent to an existing independent Exxon service station In November 1994, detectable MTBE concentrations in onsite wells ranged from 76 to 270 ug/L while the concentration of MTBE in MW-5 was 610 ug/L Onsite well MW-2, which is the furthest downgradient onsite well,and is located closest to the deepest onsite impacted soil contained ' only trace concentrations (2 ug/L or less) of VOCs, with no detectable petroleum hydrocarbons or MTBE ' The above evaluations of groundwater quality data combined with the vadose zone petroleum hydrocarbon plume configuration suggest that hydrocarbon releases from former USTs operated at the site have not impacted the underlying groundwater The aerial extent of dissolved petroleum ' hydrocarbons and VOCs in groundwater beneath and beyond the site as well as the detection of MTBE in onsite and offsite groundwater suggest the source of groundwater impacts are from offsite locations MTBE is a motor vehicle fuel additive introduced into Northern California only after the ' removal of the USTs at the subject site 1 A vapor extraction pilot test performed by Geocon indicates that soil vapor extraction is a feasible ' technology for the remediation of BTEX and petroleum hydrocarbon impacted soil beneath the site The use of other soil remediation technologies, as discussed in this PAR, are either not technically feasible or are cost prohibitive Test data indicates that an effective radius of influence greater than ' 50 feet can be achieved using the existing onsite vapor extraction wells and that the vacuum distribution for a properly engineered soil VES will encompass the portions of the plume targeted for remediation -2-