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I )%TC ASSOCIATES I NC , I Summary Report of Subsurface Investigation Diamond Walnut Growers 1050 South Diamond Street Stockton, California h 1.0 INTRODUCTION ATC Associates Inc. (ATC) has performed a subsurface investigation on behalf of Diamond amond Walnut Growers (DWG) at the facility located at 1050 South Diamond Street in Stockton, California (Figure 1). This work was done at the direction of California Regional Water Quality C Control Board (RWQCB) in their correspondence dated October 18 2001. The purpose of the additional subsurface investigation was to evaluate the horizontal extent of groundwater impacted with hexavalent chromium at the site. The subsurface investigation was conducted in general accordance with the workplan titled: Workplan for Additional Subsurface Investigation at Diamond Walnut Growers, 1050 South Diamond Street, Stockton, California, dated July 2001 and subsequent addendum dated September 2001. The work was authorized by Mr. Mike Lescisin of DWG. The findings of the subsurface investigative activities are summarized in the following report. 1.1 Obiective . The purpose of the additional subsurface investigation was to evaluate $ the lateral extent of groundwater impacted with hexavalent chromium at the site. w 1.22 Site Location The DWG plant is located northeast of the intersection of Charter Way and South Diamond Street in Stockton, California (Figure 1). The site boundaries along with pertinent features are shown on Figure 2. 1.3 Site Background The DWG plant is located northeast of the intersection of Charter Way and South Diamond Street in Stockton, California (Figure 1). According to DWG, the facility has been operating as a walnut processing plant since the 1950s. Five underground concrete vaults were located north and east of the main processing building and formerly contained refrigerants used in the plant cooling systems (Figure 2). Vaults 1, 5, and 6, installed between 1974 and 1976, were concrete and historically contained propylene glycol. Vaults 1 and 5 had 1,200-gallon capacities and Vault 6 had a 500-gallon capacity. These vaults were Iocated along the eastern side of the property adjacent to large walnut storage bins (Figure 2). Vaults 7 and 8, which were constructed with poured concrete and lined with fiberglass and asphalt, w;lwordpro1625831rcports\Subsurf report.doc i