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04/27/95 THU 16:19 FAX 510 246 8082 B.V. WASTE SCI. 1&002 <br /> ADDENDUM 1 TO THE REMEDIAL ACTION PLAN <br /> EAST RAY MUNICYPAL UTH= DISTRICT <br /> STOCKTON SERVICE CENTER <br /> The extent of the petroleum hydrocarbon plume at the East Bay Municipal <br /> Utility District (District) Stockton Service Center has expanded since the Remedial <br /> Action Plan (RAP) and Report of Waste Discharge (RWD) were submitted to the <br /> Central Valley Regional Water Quality Control Board (CVRWQCB) in September <br /> 1994. A monitoring well location map is provided on Figure 1. Quarterly monitoring <br /> results from the fourth quarter 1994'and the first quarter 1995 indicate that the <br /> petroleum hydrocarbon plume has reached monitoring well MW-9. The proposed <br /> groundwater extraction and injection system has been revised to ensure capture of <br /> all the groundwater within the plume. The revised extraction/inje+cdoo system design <br /> has been derived through aquifer testing and groundwater modeling. The aquifer <br /> test and the model are described in the following text. <br /> A step drawdown test was implemented at the site on March 15, 1995. The <br /> purpose of this test was to obtain representative aquifer characteristics for input into <br /> the groundwater extraction/injection model. The step drawdown test was selected <br /> in place of a pump test because the step drawdown test has been demonstrated to <br /> achieve a realistic range of aquifer characteristics while producing a small quantity <br /> of water. The step drawdown test was performed by pumping at four different rates <br /> from MW-8 and simultaneously monitoring water levels in MW-1,MW-8, and MW-9. <br /> The drawdown and recovery data for the monitoring wells were plotted and analyzed <br /> using the AQTESOLV computer software package to determine aquifer parameters. <br /> Using the Theis Method, transmissivity values ranging from 15 square feet per day <br /> (ft'/day) to 35 ftp/day and effective porosity values averaging 0.25 were obtained. <br /> A conservative transmissivity value of 15 fe/day,which results in a smaller radius of <br /> influence for the pumping well, and effective porosity value of 4,25 were used in the <br /> model to evaluate worst case conditions at the site. The ambient groundwater flow <br /> direction and gradient were determined from fourth quarter 1994 groundwater <br /> monitoring measurements. The documented ambient groundwater flow direction is <br /> towards the southwest at this site. <br /> 9SR2 Emi.Bary Municipal Utility Disftid <br /> 4M9.61111perm1t.wp5 j Stock=Sere Cen=-Ramndialitm <br />