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Surface Water and Well Survey C A M B R I A and CPT Investigation Report BP Oil Site No 11193 • Stockton, California December 16, 1999 Site Hydrogeology The soil beneath the site consists of silt, clayey silt and/or silty clay to 5-10 feet bgs which is underlain by silty sand/sand to the total depth explored of 35 feet bgs Beneath the northern portion of the site, a 5-foot thick clayey silt/sandy silt interval has been encountered within the silty sand/sand interval Water levels in site monitoring wells have risen 8 to 10 feet since 1992 On January 19, 1999, groundwater was encountered at 11 to 14 feet bgs and the groundwater flow direction ranged from northeasterly to southwesterly This broad range of groundwater flow directions has been shown using wells screening similar intervals and soil types during previous groundwater monitoring events Rose diagrams constructed using one average groundwater flow direction for each monitoring event would oversimplify site conditions and may not be representative of the aggregate groundwater flow direction at the site Slug tests performed on wells MW-1, MW-2 and MW-5 indicate that the hydraulic conductivity ranges from 8 68 x 10-5 to 4 48 x 10"4 centimeters per second (cm/sec) with an average of • 2 53 x 10-4 cm/sec Based on these values, the 0 007 ft/ft hydraulic gradient reported for the first quarter 1999 monitoring event, an effective porosity of 0 3, and using the appropriate unit conversions, a groundwater flow velocity of 2 1 to 11 feet per year is calculated SURFACE WATER AND WATER WELL SURVEY Surface Water Bodies Based on a review of the topographic maps produced by the United States Geological Survey for the site vicinity, an unnamed, levy-restricted slough is the only surface water body located within a 2,000 feet of the site (see Figure 1) More recent maps for the area indicate that a man-made lake is located in a residential development approximately 1,500 feet southwest of the site Water Well Survey The California Department of Water Resources (DWR) provided available Well Driller's Logs for water wells within 2,000 feet of the subject site (see Appendix B) The information provided typically includes well use, location, total depth and installation date, as well as the owner name and address The available information is summarized in Table I 5