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Mr. Russell Chapin <br /> 1766 W. Monte Diablo Avenue <br /> Page 3 of <br /> MW-1 <br /> Date Remedial Activit TPH-g Benzene <br /> 12/90 Assessment 200,000 31,000 <br /> 8/93 -9/93 Free Product <br /> Removal <br /> 7/95 27,000 7,200 <br /> 3/96 23,000 6,900 <br /> 6/96 Assessment 66,000 14,000 <br /> 1/97 85,000 20,000 <br /> 4/97 99,000 19,000 <br /> 8/97 6,800 100 <br /> 12/97 2,800 170 <br /> 3/98 1,100 2 <br /> 6/98 SVE 1,100 24 <br /> 9/98 1,200 67 <br /> 12/98 1,300 43 <br /> 3/99 680 4.9 <br /> 6/99 410 5.9 <br /> 10/99 1,500 280 <br /> 12/99 3,900 1,100 <br /> 4/00 3,000 570 <br /> 7/00 4,600 850 <br /> 11/00 Monitoring 5,700 1,700 <br /> 5/01 4,100 920 <br /> 9/01 4,000 1,200 <br /> 1/02 4,600 720 <br /> 4/03 660 <0.5 <br /> 8/03 GWE 480 2.2 <br /> 11/03 270 <0.5 <br /> 4/04 320 <0.5 <br /> This data shows that active remediation has been the dominant mechanism for TPH-g <br /> and benzene concentration reductions. It seems also suggestive that discontinuation of <br /> the groundwater extraction process may cause a rebound of contaminant concentrations <br /> in MW-1. The portion of the contaminant concentration reduction attributable to natural <br /> attenuation is not easily determined. <br /> Data from the other key well, MW-3, does not respond as directly to the remedial <br /> actions, which may be an effect of the lag time involved for a monitoring point some 125 <br /> feet down-gradient of MW-1. Run 2 of ATC's MT3D model predicted TPH-g <br /> concentration in MW-3 at 927 ppb in 2001 and at 715 ppb in 2004 — the average <br /> concentration in 2001 was actually 4700 ppb and the only monitoring event reported in <br /> 2004 had a concentration of 2100 ppb. The actual monitoring data shows that Run 2 of <br /> the MT3D model is not valid for the site. <br />