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Beacon Station No 641 September 28, 1998 Stockton, California _ Remedial Testing Report similar to the water-level data measured at the end of the pump test on May 29, 1998 and depicted on the Groundwater Pumping Contour Map (Figure 6) The results of the aquifer test and capture zone calculations suggest that the sandy water- bearing materials and the semi-confined aquifer conditions present beneath the site may be laterally extensive and capable of a higher sustained pumping rate The 4-inch diameter pump was placed at a depth of approximately 58 feet within well MW-5 Pumping of well MW-5 at approximately 9 gpm created a relatively narrow cone of depression in the immediate vicinity of the pumping well, with a more gradual, gently sloping cone of depression farther from the pumping well, as shown on Figure 6 Summary of Remedial Testing The VET performed indicates that soil vapor extraction will effectively remove hydrocarbons from the sandy soils beneath the site In addition, it appears that air sparging to conjunction with vapor extraction would be effective in remediating hydrocarbons in the groundwater Increasing the pumping rate from monitoring well MW-5 should increase the effectiveness of the cone of depression observed during the pump test at the site Assuming an increased . pumping rate of 15 gpm results in a downgradient stagnation point of approximately 2 5 feet northwest of pumping well MW-5, with a maximum width of the capture zone estimated to be approximately 15 5 feet toward the southeast fiom pumping well MW-5 It may be necessary to pump groundwater from more than one well (other than well MW-5) to effectively capture and control the migration of gasoline-impacted groundwater beneath the site area Quarterly groundwater sampling in March 1998 indicated that detectable concentrations of benzene (between 0 67 ppb and 2,100 ppb) were present in water samples collected from all five monitoring wells at the site Quarterly groundwater sampling in June 1998 indicated that detectable concentrations of benzene were present in water samples collected from monitoring wells MW-2 (20 ppb), MW-4 (6 ppb) and MW-5 (510 ppb) Removal rates based on the average concentrations of 3,000 ppb of TPHg and 500 ppb of benzene indicate that approximately 0 54-pound of TPHg and 0 09-pound of benzene would be removed per day from the groundwater beneath the site by pumping well MW-5 at a rate of 15 gpm (or 21,600 gallons per day) Therefore, groundwater pumping may only be effective in controlling migration of the hydrocarbons from a limited area, and not cost-effective for effluent treatment of hydrocarbons in the groundwater beneath the site 1641-21 rpt 7 HORIZON ENVIRONMENTAL INC.