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PUBLIC hEALTH .SERVfCES �kl <br /> SAN JOAQUIN COUNTY <br /> ENVIRONMENTAL HEALTH DMSION <br /> Daren Furst,M.D., M.P.H., Health Officer <br /> 304 East Weber Avenue, Third Floor Scockron,CA 95202 <br /> 209/46TiEl' <br /> M D <br /> C (Dry <br /> DARIN L�-ROUSE1 <br /> 18 1999 <br /> EXXON COMPANY USA FEB 17 7999 <br /> 2300 CLAYTON ROAD SUITE 1250 <br /> CONCORD CA 94524-4032 _._..- <br /> RE: Exxon #73708 SITE CODE: 1072 <br /> 2705 Country Club Blvd. <br /> Stockton, CA., 95204 <br /> San Joaquin County Public Health Services, Environmental Health Division (PHS-EHD) has <br /> reviewed the "Work Plan to Over-Drill and Reinstall Monitoring Wells"submitted by Delta <br /> Environmental Consultants (DEC) on January 29, 1999 and offers the following comments. <br /> If replacing the four monitoring wells is only for providing properly over-lapping screened <br /> intervals in the groundwater wells, then destroying the existing wells is not approved. Since <br /> proper over-bore of monitoring wells requires larger boring equipment than original placement <br /> equipment, the use of 12" augers is not recommended. Since there has been no TPH-G or <br /> BTEX detected in MW-1, 2, or 3 since November 24, 1997 and never in MW-7, PHS-EHD can <br /> not justify the need to destroy the existing monitoring wells for the purpose of replacing them <br /> with new wells in larger, reamed-out bore holes. <br /> Instead of replacing all four monitoring wells, PHS-EHD will approve the additional placement of <br /> two new monitoring wells. PHS-EHD recommends that an additional monitoring well be added <br /> next to MW-3 and MW-2 ("clustered"). These additional monitoring wells should be constructed <br /> of 2" PVC casing with a maximum-screened interval of 20 feet. The existing MW-3 and MW-2 <br /> should be left in place undisturbed and sampled for petroleum if and when the depth to <br /> groundwater(DTW!) falls within the top and bottom of the screened interval. <br /> Currently, vapor extraction wells VEW 1 and VEW 3 have their screened interval overlapping <br /> the current DTW at the site. Theae wells have been constructed so that the filter pack and slot <br /> size would allow the filtered groundwater to enter the casing and provide for repeatable <br /> groundwater sample collection. These vapor wells were placed in their current location due to <br /> the soil contamination that was detected in previous investigations. Currently they provide for <br /> 'up-gradient' groundwater data and can substitute for MW 1, thereby avoiding it's destruction. <br /> The presence of groundwater now, inside these vapor wells, provides a 'worst-case" scenario <br /> for the status of the impact to the groundwater. These vapor extraction wells can be used as <br /> monitoring wells as long as groundwater remains in the screened interval. They are to be <br /> sampled quarterly along with the other monitoring wells. <br /> As noted in PMS-EHD correspondence dated February 27, 1998, the vertical extent of soil <br /> contamination has not been verified. San Joaquin County Flood Control maps indicate that the <br /> deepest groundwater at the site was more than 20 feet below ground surface (' bgs) in Fall <br /> 1978. <br /> A Division of San Joaquin County Health Care Services <br />