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rW %TCATC Associates Inc. 1117 Lone Palm Avenue, Suite 201 Modesto,California 95351 v (:vYj209-579-2221 AS SO t I AT E S t N c. Fax:209-579-2225 declining trend in petroleum hydrocarbon constituents detected in samples collected from this well is evident. Graphs depicting TPHg, benzene, and MTBE concentrations over time in samples collected from MW1 are presented as Figures 7,8,and 9,respectively. • MW2 is located south of the former UST location in an apparent cross-gradient to down-gradient position relative to the former location of the UST. TPHg, BTEX, and MTBE have frequently been detected in samples collected from this well. TPHk and TPHd have been detected to a lesser �•• extent. A general declining trend in concentrations of TPHg, BTEX, and MTBE was observed until vapor extraction was introduced in the well in 2004 and 2005 resulting in a temporary increase in concentrations. TPHg, TPHk, and TPHd were last detected in samples collected from this well in January of 2009, October of 2007,and January of 2008 at concentrations of 200 pg/L, 2,700 µg/L, and 1,800 ltg/L, respectively. Benzene, toluene, ethylbenzene, and total xylenes were last detected in samples collected from this well in July of 2008, January of 2009, July of LO 2008, and January of 2009 at concentrations of 9.0 µg/L, 3.6 pg/L, 4.6 µg/L, and 35 µg/L, respectively. MTBE is typically present in samples collected from this well at concentrations below the primary MCL of 13 µg/L. A generally declining trend in petroleum hydrocarbon constituents detected in samples collected from this well is evident. Graphs depicting TPHg, 4. benzene, and MTBE concentrations over time in samples collected from MW2 are presented as Figures 10, 11,and 12,respectively. • MW3 is located east-southeast of the former UST location in an apparent down-gradient to cross- gradient location relative to the former location of the UST. Very low concentrations of TPHk, TPHd,toluene,and total xylenes were initially detected in the groundwater sample collected from this well when monitoring and sampling was initiated in December of 1999. Total xylenes were L" detected once more in the groundwater sample collected in April of 2000. Very low concentrations of MTBE were detected in groundwater samples collected from this well in April of 2005 and February of 2010. l� • MW4 is located north-northeast of the former UST location in an apparent cross-gradient to up- gradient position relative to the former location of the UST. Petroleum hydrocarbons have never Lo been detected in groundwater samples collected from MW4,with the exception of a sole reported detection of TPHd in the groundwater sample collected on June 22, 2000. * • MW5 is located south-southwest of the former UST location and MW6 and MW7 are located Lo north and south of the former UST location, respectively. Petroleum hydrocarbons have never been detected in groundwater samples collected from these wells. 1, ATC calculated the volume and mass of MTBE in groundwater based on the distribution of detected concentrations during the first quarter 2010 sampling event. The calculated mass of MTBE was estimated to be approximately 0.002 pounds. Mass calculations for MTBE in groundwater are contained in LN Appendix B. An isoconcentration map depicting the estimated lateral extent of MTBE in groundwater during the first quarter 2010 sampling event is provided as Figure 13. 1.. S:Environmental1625771Reports12010 closure summary.doc 7 1..