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VATC ( 0 A S S O C I A T E S I N C. December 15, 1997 Ms. Mary Meays San Joaquin County Public Health Services Environmental Health Division P.O. Box 388 Stockton, CA 95201-0388 Subject: Workplan Addendum to Implement a Feasibility Study and to Complete the Groundwater Investigation at 26080 Thornton Road, Thornton, CA, Site Code 1864 Ms. Meays: This correspondence has been prepared by ATC Associates Inc. as a follow-up letter summarizing the project meeting held on December 8, 1997 at the above referenced site. The meeting was held at the request of local agencies including the San Joaquin County Utility Maintenance District (SJCUMD), San Joaquin County Public Works Department (SJCPWD), and San Joaquin County Public Health Services Environmental Health Division (PHS/EHD), to discuss ATC's Workplan to Implement a Feasibility Study and to Complete the Groundwater Investigation (Workplan) dated October 30, 1997. In brief, the workplan proposed to advance a total of six soil borings and complete the borings as three vapor wells, two monitoring wells, and one air sparge well. One of these borings was to be completed as a deep monitoring well to address vertical definition of the site's dissolved groundwater plume and it's potential impact to Thornton Municipal Well No. 2 (the Supply Well). During the meeting SJCUMD and SJCPW officials expressed concerns with respect to advancing the proposed deep monitoring well in such close proximity of the contaminant plume to the Supply Well and the potential of contaminants being introduced into a deeper water bearing zone. The boring log for the Supply Well indicates that the annular seal extends from the surface to a depth of 70 feet below ground surface. The proposed deep monitoring well was estimated to be set at a depth of approximately 98 feet bgs based on the lithology described on the Supply Well's well log. It was discussed during the meeting that well log included descriptions of soils encountered during drilling activities based primarily on soil cuttings and drilling fluid observations and no cored soil samples were actually collected. This is apparent since reverse circulation drilling methodology was employed in the installation of the Supply Well.