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' although only low concentrations of contaminants have been detected, it is unlikely that the g Y PHSIEHD will allow the contaminated soil to remain in place Therefore, remediation alternatives should be considered Further excavation to remove the residual contamination would require re-excavating the ' backfill material and special methods and equipment to reach depths in excess of 40 feet In addition, space for stockpiling the removed soil is limited Therefore, excavation is not ' recommended In-situ remediation altei natives include vapor extraction and bioremediation Both methods would requii e the installation of one or more vadose-zone wells Either method is suitable ' for remediation of silty soil, but prior experience indicates that vapor extraction is more effective in remediating highly-contaminated sites than in reducing low concentrations of ' hydrocarbons within the smear zone to levels that are acceptable to PHS/EHD and the Central Valley Regional Water Quality Control Board Therefore, bioremediation may be a better alternative at this site ' 5.2 Groundwater Contamination ' Groundwater contamination at the site appears to be limited to petroleum hydrocarbons within the diesel range Diesel contamination was reported in all three monitoring wells, with the gi eatest concentration in MW-2, approximately 50 feet down-gradient of the former tank pit Since contamination was present in all the wells, further groundwater investigation is warranted, both up-giadient and down.-gradient of the tank cavity We recommend the installation of three additional monitoring wells to further investigate the limits of ' contamination in groundwater The diesel contamination reported in the webs was unusual in composition The AAL ' laboratory report for monitoring wells MW-1 and MW-2 noted that"although the sample(s) contained hydrocarbons within the reportable range for diesel/motor oil, the chromatogram did not resemble either fresh or aged diesel/motor oil' (see Appendix C, page 3) This may ' indicate an additional source of contamination Conversations with the Client and local residents have revealed that UST's may have been on the property to the northeast of the site at 2811 Farmington Road (the location of MW-2) No other information regarding the ' presence or removal of UST's from 2811 Farmington Road has been made available to GeoAudit An historical investigation of the 2811 Farmington Road property should be ' considered Another possible off-site source of contamination is the USA Gasoline Station located ' immediately southeast of the site This property is known to have groundwater contamination associated with the operation of the gasoline station Inquiry into the extent of contamination at the USA station should also be considered 1 12 P.aGc G.ill2 L69S16525 1