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i The radius of pumping influence for Well RW-1A appears to extend a minimum of 168 feet from the well as evidenced by the drawdown observed in Well MW-4 The radius of influence for Well MW-2 was not determined as no discernible drawdown was observed in the nearest observation wells The closest observation well was located 63 feet from MW-2, thus the radius of pumping influence for MW-2 is less than 63 feet It should be noted that the radius of influence observed in these two aquifer tests represents a 6 hour period of pumping It is likely that the radius of influence would increased with prolonged pumping A review of the boring log for Well RW-1A reveals a layer of coarse gravel from 11 to 15 fbg This is the 2-inch drain rock that was used to backfill the former tank excavation It is likely that the relatively high flow rate produced by RW-IA is a result of the large volume of water stored in this gravel layer It is also likely that continued pumping of this well at 15 gpm would dewater the gravel layer and the sustainable flow rate from RW-IA would stabilize in the range of the 1 5 gpm flow rate of MW-2 once the native aquifer became the only source of ground water This lower flow rate will likely keep the gravel layer dewatered and continue to influence ground water flow across the site The results of the ground water samples collected before, and at the end of the pump testing from RW-IA indicate that ground water extraction is an effective method of removing MTBE from the subsurface MTBE concentrations decreased from the beginning to the end of pump testing, however • concentrations at the end of testing still displayed an effective mass removal rate SHALLOW WELi- INSTALLATION Part of the scope of work for the recent phase of investigation included installation of three shallow ground water monitoring wells south of the site along the southern side of Country Club Boulevard Wells MW-9, MW-10 and MW-11 were installed on November 20, 1997 at the locations shown on Plate 2 Encroachment permits from the City of Stockton and Caltrans were obtained prior to installation of these wells as they are located on public property The procedures for drilling and well installation are detailed below Field Procedures Three exploratory soil borings (MW-9, MW-10, and MW-11) were drilled and sampled on November 20, 1997 The borings were drilled using hollow stem auger drilling equipment Borings MW-9, MW-10, and MW-11 were converted to ground water monitoring wells (Wells MW-9, MW-10, and MW-1l, respectively) The locations and designations of each well are shown on Plate 2 A Cambria geologist supervised the drilling and described encountered soils using the Unified Soil Classification System and Munsell Color chart Encountered lithology is described on the • exploratory boring logs presented in Appendix B 240-0783 10