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ENVIRONMENTAL_ HEALTH DEPARTMENT opau ..�e SAN JOAQUIN COUNTY Unit Supervisors Car!Borgman,R.E.H.S. Donna K.Heran,R.&H_S. 304 Fast Weber Avenue,Third Floor Mike Huggins,R.E.H_S.,R.D.I. f ;< Director 95202-270$ Al Qlsen,R.�E.H.S, StoCktori, California. Douglas W.Wilson,R.E.H.S. ..�4 Program Manager. Te ephoi'e: (209).4.6$-342 . Mirgaret Lagorio,R.E.H.S. FORd Laurie A.Cot.I R.E:H.S. Robert McClellon,R.E.H.S. Pax: (209)464-01�$ Mark Barcellos,R.E.H.S. Program Manager' NOV 17 2003 PAUL SUPPLE ATLANTIC RICHFIELD COMPANY P O BOX 6549 MO ,AGA CA 94570 RE: ARCO Service Station No.2185 3212 North California Street Stockton,California 94570 San Joaquin County Environmental Health Department(EHD)has reviewed Corrective Action Plan (CAP), dated 19 September 2003,prepared by URS Corporation Americas(URS)on your behalf. The recommended remedial actions in the above referenced CAP, for remediation of petroleum hydrocarbons and oxygenates present in soil and groundwater beneath the site by a combination soil vapor extraction(SVE)and an air sparging system is approved as an interim measure. However,EHD cannot approve the CAP-at this time as EHD believes that the cross sections submitted in the CAP do not adequately.model the.hydrogeology of the site,and the vertical delineation of contaminants is still incomplete in the underground storage tank(UST) area. Therefore, additional site characterization is needed. Upon receipt of the final site characterization materials,the CAP may be resubmitted for approval consideration_ EHD suggests using Cone Penetrometer Testing(CPT)to refine the hydrogeologic model. CPT may better characterize the fine grain and coarse grain intervals,and groundwater samples taken during a well-designed CPT investigation may help delineate and/or characterize the plume of impacted groundwater at the UST site. Information obtained from such a study may demonstrate the need,or lack of need,for additional groundwater monitoring wells and aid selection of advantageous monitoring well locations. EHD believes that there are three sand-rich intervals beneath this site,identified as Sand 1 at 25 to 35 feet below surface grade(bsg); Sand 2 at 45 to 60 feet bsg;and Sand 3 at 65 to 75 feet bsg (all depths approximate); and presents this view of the current state of the site: • The three shallow vapor wells encounter Sand 1 in the vadose zone. This may account for the significant radius of influence found during the SVE test study. + Apprdximately ten feet of silt lie above and below Sand 1 so it may be necessary to install additional vapor wells to specifically target these silt zones. • Sand 2 and Sand 3 may be interconnected;however,EHD believes that these sand intervals are separated and monitoring wells screened across both Sand 2 and Sand 3 may be providing a vertical conduit for contaminants. • Insitu air sparging(IAS)may not be a practical remediation method considering the site geology and current water table. IAS is generally more effective for addressing dissolved hydrocarbons than MTBE. If implemented,EHD recommends considering placing the spargers in the silt layers and injecting the air at a slow rate"for the interim remediation.