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Shell Service Station page 2 3011 W. Ben Holt Dr., Stockton. The proposal (work plan) included within this report proposes to place one(1)deep monitoring well northeast of"SB-11, 12, & 13" and existing MW-2. This monitoring well is proposed with a ten foot(10')screened interval from 77'to 87' bgs. EHD has discussed this most recent phase of investigation with the Central Valley Regional Water Quality Control Board (CVRWQCB)staff. We agree that your proposal to place a short- screened interval (5' to 10') monitoring well is this area is appropriate. With the highest detected concentration of MtBE (120 ug/1)from"SB-12"at 77'bgs, it would appear that this monitoring well design would allow for repeatable confirmation groundwater samples at this depth of concern. Cone penetrometer testing (CPT)in this area identified MtBE in strata other than the 77'to 87' bgs interval where the proposed monitoring well screen will be set. EHD and the CVRWQCB concluded that confirmation of the impact in these additional areas, by installation of additional short-screened, deep monitoring wells are needed. Monitoring wells with short(<10')screened intervals should be placed at areas where previous soil borings and CPTs have identified MtBE contamination in excess of concentrations that would require remediation (MCL). Conclusions for plume delineation, extent of contamination, or need for remediation must be supported by data collected from repeated sampling events that show a trend in the level of contamination. The work plan in not approved as written. An addendum is needed that is to address the concerns presented above. Please submit a revised work plan or addendum in a timely manner. You may contact Michael Infurna at(209)468-3454 should you have any questions. Donna Heron, REHS, Director Enviro ental Health Divisio chael J. Infurna Jr Senior HS Mar�Iagon., RR LOP/Site Mitigatio Unit IV Supervisor MI/ C: CVRWQCB—Marty Hartzell, Sacramento. C: Cambria—Joe Neely, PO Box 259, Sonoma, CA.,95476. c: SW RCB-CUF—Mark Owens, Sacramento.