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4 Additional Groundwater Investigation and Remediation Feasibility Evaluation Work Plan Stockton Center Site Authority-University Park January 25,2005 Page 3 On April 14 through 17, 2003, Condor conducted additional groundwater investigation activities at the ' site using CPT and direct push drilling methods to further characterize site geology and to further evaluate the lateral and vertical extents of groundwater contamination. Results of the investigation are summarized in Condor's Additional Groundwater Investigation Report, dated August 19, 2003. The results indicated - that, while present to the maximum depths investigated, the gasoline constituents in groundwater generally attenuate rapidly with depth in the source area, and with the possible exception of 1,2- dichloroethane (1,2-DCA) and diesel range petroleum hydrocarbons, do not appear to be particularly mobile laterally at the site. Results of the investigation also indicated that a large percentage of the soil and groundwater contamination could be addressed through the proposed soil over-excavation remedial alternative. Soil over-excavation was proposed, however, to meet the very rapid property development plan anticipated for the site at that time with the full knowledge that it would be considerably more costly 1 than some other potentially viable remedial alternatives. As the plan for site development was modified and clarified during 2003, very aggressive and timely but less cost-effective site remediation (over-excavation asro osed in the above de p p sorbed March 3, 2003 work plan) was no-longer considered necessary by the.SCSA: Condor, on behalf of the SCSA; proposed evaluation of site source area remediation by pilot testing of air sparging in combination with soil vapor extraction in a work plan dated September 15, 2004. In a letter dated November 8, 2004, Mr. Henderson of the SJCEHD expressed concern with two aspects of the proposed approach and requested submittal of a work plan addendum by January 17, 2005, addressing his concerns. Concurrently, site development plans continued to evolve such that a structure planned for the site vicinity was to be located well away from the site contaminant source area and that a parking lot for the structure would be built over the site source area. These considerations indicated the potential for consideration of less aggressive site remediation techniques. As a result of the above considerations, a meeting was held on December 17, 2004, with all concerned parties in attendance. As a result of the meeting it was agreed that Condor would prepare a work plan for additional groundwater investigation and feasibility evaluation of various remediation options forth-, site. In an email transmission on January 6, 2005, to Mr. Henderson at the SJCEHD, Condor requested'an extension of the work plan submittal due date to February 4, 2005. Mr. Henderson.approved the extension in a return email on January 6, 2005. 3.0 GEOLOGY AND HYDROGEOLOGY k The site is located in the Great Valley Geomorphic Province of California. The Great Valley is a broad structural trough filled with unconsolidated to semiconsolidated sedimentary deposits, bound on the east by the rotated block of the Sierra Nevada and on the west by the complexly folded and faulted Coast Ranges. According to the Department of Water Resources (DWR)t, the site is located within the Eastern San Joaquin Subbasin of the San Joaquin Valley Groundwater Basin. The upper water bearing formations in the Subbasin are the Alluvium and Modesto/Riverbank. Formations (undifferentiated), ranging from e Recent to Pleistocene in age, and comprised dominantly of sand and gravel in the fan areas and clay, silt, and sand in the interfan areas:These units range from a thin veneer to over 150 feet thick near the center of the basin. Underlying these units is Plio-Pleistocene age, discontinuous lenses of stream-laid sand and silt with lesser amounts of clay and gravel. Delta deposits interfinger with these units in the Delta area of the San Joaquin Valley. The Delta deposits are generally much finer grained basinward deposits than their depositional equivalents to the east and west,and have a higher percentage of fine sand and clay. t California Department of Water Resources, Bulletin 118, Groundwater Basins of California,.first published 1975 and updated October 2003. 'i r �a CONDOR