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3 HYDROGEOLOGY Soli strata at this facility, to the maximum explored depth of 90 feet bgs, consists of Interbedded, discontinuous sequences of primarily silt and clay with some fine- grained sand Geologic cross-sections, shown in Figure 4 and Figure 5, illustrate generalized subsurface stratigraphy Geologic data from the CPT soundings (in Appendix 2) provide more detailed strata information Pore pressure dissipation tests performed on May 18, 1999, during the CPT soundings, indicated that the groundwater table was at a depth of about 39 feet to 40 feet bgs Groundwater levels have been steadily rising since a significant drought period occurred in 1992 Groundwater at this site generally does not occur In clean, well-sorted sands Rather, groundwater is more likely found within the finer grained silty and clayey sediments Groundwater has consistently been measured to flow in a northeasterly direction across the site The groundwater gradient was measured in October 1998 to be about 2 8 x 10-3 foot per foot ACG performed a 24-hour well pumping test at this facility on June 11 and 12, 1991 Details of the pumping test are included in ACC's December 19, 1991, Problem Assessment Report Well MW-1 was pumped at an average discharge flow rate of 6 7 gallons per minute For the last 4 hours of the test, drawdown in MW-1 ranged from 10 36 to 11 64 feet Wells MW-2, MW-3, MW-4, MW-6, and MW-8 were observed for drawdown to evaluate the zone of influence of the pumping well All five 1 observation wells were affected by the pumping From the pump test data, ACG estimated the average aquifer transmissivity to be 6,248 gallons per day per foot of aquifer thickness (837 2 square feet per day), and the average storage coefficient to be 0 027 Lateral hydraulic conductivity of the aquifer is derived by dividing transmissivity by the saturated thickness of the aquifer Using a saturated thickness of 50 feet, hydraulic conductivity was calculated to be 16 74 feet per day (ft/d) Average linear flow velocity of the groundwater system is estimated by multiplying lateral hydraulic conductivity and gradient values, and dividing this product by effective porosity Using an approximate effective porosity value of 35 percent (0 35) for sandy clay, the groundwater flow velocity was calculated to be about 0 13 ft/d 1 May 1999 Subsurface Investigation and Groundwater Monitoring Report rhe Earthgrams Company Facility 2651 South Airport Way Stockton,California Prepared for T11E CAR IMPAINS COMPANY Prepared by Philop Sc MUS Corp I \1213821121382CG112599AC0 99REP DOC .lune 22, 1999 9