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SMITH <br />ENVIRONMENTAL TECHNOLOGIES CORPORATION <br />October 15, 1997 <br />Ms. Mary Meays <br />San Joaquin County <br />Public Health Services EHD <br />PO Box 388 <br />Stockton, CA 95201-0388 <br />Subject: Addendum to Problem Assessment Report - Interim Remedial Plan for <br />26056/26080 Thornton Road, Thornton, California, USTCF Claim #000835 <br />Dear Ms. Meays: <br />Smith Technology Corporation (ATC Associates Inc.) has prepared this correspondence to serve <br />as an addendum to Smith Technology's Problem Assessment Report - Interim Remedial Plan <br />(Plan) dated September 4, 1997. The Plan recommended that a pilot scale test of soil vapor <br />extraction and air sparge be implemented to determine the effectiveness of these methods as a <br />remedial option. <br />To implement the pilot scale test the Plan proposed to install three vapor extraction wells, one air <br />sparge well, and a sentinel monitoring well. It was proposed that the sentential monitoring well <br />be nested to evaluate the vertical migration of contaminants. San Joaquin County Public Health <br />Services, Environmental Health Division (PHS/EHD) correspondence dated September 22, 1997 <br />concurred with the Plan's recommendations. <br />Based on further evaluation of Thornton Municipal Supply Well 42 (Supply Well) data, in <br />addition to the close proximity of the contaminant plume from the Supply Well (estimated 30 <br />feet), it appears warranted that the proposed deeper monitoring well casing be placed in separate <br />bore hole and not nested with a shallower monitoring well casing. A conductor casing will be <br />employed in the construction of the deeper well monitoring well to prevent the deterioration of <br />water-bearing formations which may be penetrated. The use of a conductor casing will require <br />the shallow monitoring well would be completed in a separate borehole drilled to a depth of <br />approximately 30 feet bgs. This shallower well would be placed ("clustered") in clOse proximity to <br />the deeper monitoring well. <br />1260 Graphics Drive • Modesto, CA 95351-1502 • (209) 579-2221 • Fax (209) 579-2225 <br />; <br />Engineering • Consulting • Remediation • Construction