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rA > <br /> Supplemental Investigation Report <br /> BP Oil Site No 11193 <br /> C A M B R I A Stockton, California <br /> . October 13, 2000 <br /> Site Hydrogeology The soils encountered beneath the site consist of several feet of fill and up to <br /> 5 feet of clay which are underlain by silt and sand mixtures to the total depth explored of 30 feet bgs <br /> CPT electric logs were more detailed but generally consistent with adjacent boring logs based on soil <br /> samples collected at 5-foot intervals For example, the CPT electric logs indicate the presence of <br /> several clay or silt intervals that were not described in adjacent borings <br /> Soils encountered in the previously mentioned Caltrans well consisted of interbedded sands and clays <br /> to approximately 180 feet bgs, which was underlain by a 20-foot thick sandstone unit, then <br /> interbedded sands and clays to approximately 347 feet bgs, and a siltstone to a total explored depth <br /> of 350 feet bgs A clay unit was noted between 43 and 80 feet bgs (see Appendix A) <br /> Water levels in site monitoring wells have risen 8 to 10 feet since 1992 In May 2000, groundwater <br /> was encountered at 9 to 11 feet bgs and the groundwater flow direction ranged from northeasterly to <br /> southeasterly This broad range of groundwater flow directions has been shown using wells screening <br /> similar intervals and soil types during previous groundwater monitoring events Rose diagrams <br /> constructed using one average groundwater flow direction for each monitoring event would <br /> • oversimplify site conditions and may not be representative of the aggregate groundwater flow direction <br /> at the site <br /> Slug tests performed on wells MW-1, MW-2 and MW-5 indicate that the hydraulic conductivity <br /> ranges from 8 7 x 105 to 4 5 x 104 centimeters per second (cm/sec) with an average of <br /> 2 5 x 104 cm/sec Based on this average value, a typical hydraulic gradient of 0 007, an effective <br /> porosity of 0 3, and using the appropriate unit conversions, a groundwater flow velocity of 6 0 feet <br /> per year is calculated using the relationship shown below <br /> v = kiln <br /> where v =groundwater flow velocity (feet per year) <br /> k=hydraulic conductivity(cm/sec) <br /> i=hydraulic gradient(feet/feet) <br /> n =effective porosity(unitless) <br /> using the conversion factors <br /> 1 foot= 30 48 cm <br /> 1 year=365 days/yr * 24 hrs/day * 60 min/hr* 60 sec/min <br /> 5 <br />