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13 December 2002 . AGE-NC Project No 95-0130 Page 5 of 6 • The high concentrations of petroleum hydrocarbons were detected in samples collected from Iwells MW-6 (A, B and C), MW-7B, VW-1 (B and C), VW-2 (A and B) and VW-3 (A and B),Indicating that these wells are within or near the dissolved petroleum hydrocarbon"core area" of the plume (Figures 4 and 5) • The lateral and vertical extent of the dissolved petroleum hydrocarbon plume is not known toward the northwest, north, northeast, east and southeast of the area bound by wells MW-1 through VW-3 (Figures 4 and 5) I5.0. RECOMMENDATIONS IBased upon the results of the third quarter 2002 investigation, AGE recommends • Continued quarterly ground water monitoring of wells VW-1 (B and C),VW-2A, VW-3A, I MW-6 (A, B, C, D and E) and MW-7 (A, B, C, D and E), the next quarterly ground water monitoring event is tentatively scheduled for December 2002 • Continued quarterly depth-to-water measurements in wells MW-1 through MW-5,but reduce the ground water sampling frequency from quarterly to semi-annual, AGE believes that continued quarterly groundwater sampling from these wells at this stage of the investigation is not cost effective and will not provide enough useful Information to supplement the ongoing Investigation Semi-annual sampling events should be adequate to monitor ground water impacts at these wells and should be re-evaluated on a semi-annual basis I • Conduct future quarterly ground water monitoring events concurrently with the Rock Property site (102 South Wilson Way)and the Arco AM/PM site(130 South Wilson Way), AGE recommends that a well-head elevation survey be performed by a state-certified surveying engineer so that reported ground water elevations are relative to a City of Stockton benchmark datum I • Due to 1) high concentrations of contaminants (i e MTBE, BTEX, 1,2-DCA, EDB) encountered at the site that are significantly above the DHS's MCLs,2)the high mobility of MTBE dissolved in ground water and 3) a recent trend of increasing MTBE concentrations eastward towards 102 South Wilson Way(Rock Property),AGE recommends that a ground water interim remediation work plan be implemented(I c ground pump-and-treat)at the site to Inhibit further off-site migration of the fuel-impacted ground water plume until a full scale Iremediation system can be Installed A Feasibility Studies and Monitoring Well Installation Work Plan was submitted to the EHD on I 1 March 2002 and approved by the EHD by letter on 17 April 2002,however,the UST Cleanup Fund only approved a portion of the work plan including Installation of one on-site ground water IAdvanced GeoEnvironmental,Inc