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C A M B R I A <br /> extraction points spaced fairly close together, which still might or might not achieve hydraulic <br /> control. --- ? <br /> Furthermore, a well survey conducted by Cambria in 1999 identified one domestic well located <br /> slightly less than 1/2 mile southwest of the site. The well was installed in 1988 and is screened <br /> from 150 to 310 feet below grade(fbg)and sealed from the surface to 50 fbg. Impact of this well <br /> appears unlikely due to its distance and direction relative to the MTBE plume migration rate and <br /> path. No 'other water-production wells were identified within a half-mile radius of the site. No <br /> surface water bodies were identified within a half-mile radius of the site. The Mokelumne River <br /> © and industrial waste ponds are located just over 1/2 mile north of the site. Impact of surface <br /> waters due to site activities is unlikely due to groundwater depth and distance from the subject <br /> site to surface water. An immediate or imminent threat to a sensitive receptor, warranting <br /> hydraulic control through continuous groundwater extraction,has not been identified. <br /> Since issuing Cambria's December 13, 2002 Aquifer Pump Test Report, the first quarter 2003 <br /> quarterly monitoring data has been received. The following table presents pre- and post-aquifer <br /> test groundwater concentrations: <br /> MTBE Concentration g ) <br /> Well ID September 19,2002 October 30,2002 February 19,2003 <br /> MW-1 500 98 130 <br /> MW-2 3,600 730 18,000 <br /> MW-3 460 360 140 <br /> P-1 120,000 150,000 200 <br /> P-2 23.000 2,600 540 <br /> MW-4 71,000 89,000 49,000 <br /> MW-5 <2 <0.5 0.81 <br /> MW-6 <2 <0.5 <0.5 <br /> MW-7 1,300 310 5,500 <br /> This recent groundwater data shows a decreasing MTBE concentration trend in the vicinity of <br /> the October 2002 groundwater extraction activities. Specifically, the MTBE concentration in the <br /> extraction -41 has decreased 31% since September 19, 2002. A more significant <br /> decrease is observed in the adjacent piezometers. Since September 19, 2002, MTBE <br /> concentrations in piezometers P-1 and P-2 decreased 99.8% and 97.7%, respectively. This recent <br /> data further suggests that the October 2002 groundwater extraction event was effective in <br /> depleting MTBE concentrations in this vicinity. Cambria maintains that additional groundwater <br /> extraction events from well MW-4 may decrease MTBE concentrations to acceptable levels. A <br /> 0955 2 <br />