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uY �l . Additional Investigation to Define the Extent of Groundwater Contamination Quik Stop Market No 121 Page 7 screened within a separate aquifer(B-aquifer) This is based on the fact that the three deep wells are under hydrostatic pressure The three deep wells are screened from 65 to 75 feet and the water levels were measured slightly below the other seven shallow site wells, which are screened at shallower depths In addition,the conductivity levels are at much lower readings than the shallow aquifer The boring logs from monitoring wells MW-9 and MW-10 correspond very will with CPT-1 log, which shows two clay,clayey silt and sandy silt horizons between approximately 43 and 49 feet and 54 and 66 feet Some localized "hard pan" material was noted in these soil horizons and appear to act as confining layers between the two aquifers Further the clay horizons appears to be an impediment to vertical contaminant migration as confirmed by the reported clean soil samples collected from depths of 50 , 60 and 75 feet, and the water samples from MW-9 and MW-10 are free of detectable petroleum hydrocarbons A groundwater gradient map for the B-aquifer was constructed and indicates a gradient of 0 015 feet per foot to the southeast(Figure 2) Groundwater in the vicinity of monitoring well MW-6 continues to show relatively low levels of detectable MTBE at the subject site It is possible the well seal on MW-6 may be weak, allowing MTBE in the upper shallow aquifer to migrate to the lower B aquifer It should be noted the concentration of MTBE in MW-6 has remained at approximately 30 ug/l over the last three sample events No detectable petroleum hydrocarbons were detected at perimeter well MW-11, located on the northeast side of the site As previously discussed, CCI completed a 24-hour pump test on recovery well RW-1 Due to the low water table at the site and limited depth of RW-1, the upper A aquifer was unable to be stressed to its maxium limit and limited information on the aquifer characteristics was obtained from the test CCI conducted the pump test to measure the yield of water from well RW-i for an anticipated groundwater extraction system at the subject site The water Ievel declined in the pumping well throughout the test Groundwater measurements collected in near by observation wells MW-3 and MW-4 were inconclusive The water levels in these two wells,while initially dropping,began to rise after a few hours into the test This may be due to barometric pressure changes The low pumping rate in RW-1 had no measurable impact on these two wells or the perimeter shallow wells In addition, the water level in deep well MW-6 was measured and found to nse and fall over one foot throughout the test It is CCI's opinion that this well was being influenced from pumping activity not associated with the site The drawdown and recovery data from RW-1 were used to calculate the transvisivity (T) and hydraulic conductivity (K) for RW-1 An estimate of hydraulic conductivity (K) from this data is 0 0064 ft/min and transvisivity (T) of 0 118 ft2/min Based on this data, CCI concludes and recommends the following 1 CCI is currently working on obtaining an encroachment permit from the City of Manteca to install one additional shallow groundwater well on the northwest cornier of W Louise Avenue and Union Road It is anticipated this well will be installed in the month of September so