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i EXECUTIVE SUMMARY This report presents the results of drilling and sampling activities that were completed in the third quarter of 1996 and integrates these results with data that were collected in previous assessment, morutonng, and remediation phases in 1990-1996 in order to fully characterize soil and groundwater contamination resulting from underground storage tank leaks at the G hes Trucking yard in Stockton, California Drilling and sampling activities in the third quarter of 1996 were outlined in a work plan that was originally submitted in February 1996 and approved by San Joaquin County Environmental Health Division in July 1996 The work plan was subsequently amended in October 1996 to include additional drilling and laboratory analyses As a result of the expanded scope of work the scope of this report has been expanded beyond merely presenting the results of the current investigation Thus report summanzes the total investigation and presents the maps, cross sections, and other data that are necessary to fully assess the extent of contamination in accordance with guidelines for the preparation of Problem Assessment Reports as described in Appendix A of the Tn-Regional Board Recommendations for Preliminary Evaluation and Investigation of Underground Tank Sites Diesel and gasoline contamination was detected in sort samples after the removal of one 10,000-gallon diesel UST and one 2,000-gallon gasoline UST in 1989 In 1990, the excavation was enlarged and deepened to a feasible limit of 3 5 feet, but 11 of 15 soul samples collected from the floor and sndewahls of the excavation contained residual hydrocarbons that could not be removed by excavation Between 1,000 and 1,500 cubic yards of excavated soul were stockpiled on site and bioremediated under the supervision of PHS/EHD The stockpile was remednated for approximately two years, after which it . was sampled and spread out on site with the approval of PHS/EHD Two borings, including one groundwater monitoring well, were drilled in 1990 Diesel was detected at high concentrations to a depth of 55 feet, and very low concentrations were reported as deep as 95 feet Diesel and gasoline were detected at very low concentrations in the groundwater Two more wells were installed in 1994, and contamination was detected in both A southward groundwater gradient, toward the southern property boundary, was identified After a period of more than 12 months, during which the groundwater table was several feet above the screened interval in the monitoring wells, preventing the collection of valid groundwater samples, three sort borings and three moritoring wells were drilled in August and October 1996 Soil borings were drilled to a depth of 65 feet, and monitoring wells were drilled to 75 feet and completed with 20 feet of screened casing Using a combination of mobile and stationary laboratories, twenty-one soil samples and eight water samples from these borings and two water samples from nearby domestic wells were analyzed for hydrocarbons` As part of the expanded scope of work,two soul samples were analyzed for bacterial abundance, two were analyzed for Ph, ritrates, and other inorganic nutrients, six were analyzed for total organic carbon, and seven were analyzed for hydraulic conductivity and related hydrologic characteristics The analytical results are presented herein in tabular format as well as in a series of maps and cross sections that depict the lateral and vertical extent of contamination Diesel is the primary contaminant at the site, and forms an elongate plume along the axis of an east- west trending sandy channel deposit that trends directly beneath the former UST's The channel ranges in depth from 35 to 55 feet and in thickness from 10 to 20 feet Diesel concentrations generally exceed