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1 ' 23 April 2003 i AGE-NC Project No 95-0130 ' Page 5 of 6 ' • The only detection ofdissolved TBA was in the sample collected from well MW-3,however,TBA was previously detected at wells MW-1,MW-2,MW-3,MW-5,VW-1C,VW-2A,VW-2B and VW-3A suggesting a significant TBA-Impact to ground water at the site ' • EDB was ony detected in water samples collected from VW-1 C and VW-2A during the December 2002 samples,however,EDB has previously been detected at wells MW-7B and ' VW-2B in concentrations sigmficantly above the DHS's Maximum Contaminant Level(MCL)In drinking water • The high concentrations of petroleum hydrocarbons detected in samples collected from wells ' MW-6(A-C),MW-7(A and B),VW-1 C,VW-2A and VW-3A,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) r5.0. RECOMMENDATIONS Based upon the results of the fourth quarter 2002 investigation, AGE recommends • Continued quarterly ground water monitoring of wells VW-I (B and C), VW-2A, VW-3A, MW-6 (A-E) and MW-7 (A-E), the next two quarterly ground water monitoring event are tentatively scheduled for March 2003 and June 2003 • Continued quarterly depth-to-water measurements in all monitoring wells at the site,but reduced Iground water sampling frequency from a quarterlyto a semi-annual program,AGE believes that continued quarterly ground water sampling from these wells at this stage ofthe 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 momtor groundwater impacts at these wells and should be re-evaluated on a semi-annual basis • Conduct future groundwater monitoring events concurrently with the Roek Property site(102 South Wilson Way)and the Arco AM/PM site(130 South Wilson Way),AGE recommends that a well-head elevation surveybe performed by a state-certified surveying engineer so that reported ground water elevations are relative to a City of Stockton benchmark datum • Due to 1) the high concentrations of contaminants (i e MTBE, BTEX, 1,2-DCA, EDB) encountered at the site that are sigruficantly 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 Advanced GeoEnvironmental,Inc