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------- ---- ------ Report: ----Report: Groundwater-quality Monitoring—June 23-28,2005, 7500 West Eleventh Street, Tracy, CA. Page 7 Mks That assessment has been confirmed by data obtained by analyses of groundwater samples recovered from Monitoring Well MW-13, which was installed in the median of West Eleventh Street in April 2004. The results show that concentrations of petroleum- and diesel-range hydrocarbons are significantly lower than those of similar analytes in Monitoring Well MW-7, which is further down-gradient from the release of hydrocarbons to the subsurface. The floating product that accumulated in Monitoring Well MW-7 over the period April 2002 to October 2003 was eliminated temporarily by a program of purging of LNAPL from that well in late 2003 and early 2004. However, given the small 2-in. casing diameter of Monitoring Well 1\4W-7, SJC was concerned that the intermittent rounds of LNAPL purging from that well would not be successful in removing floating product from more than a limited area of the subsurface within close proximity to that well and that such any limited effect would be temporary. This assessment was supported by the steady increase in concentrations of fuel hydrocarbons in samples of groundwater recovered from MW-7 in the period following January 2004, when the quality of } groundwater in that well was first analyzed following completion of the purging program. d Unfortunately, despite quarterly purging of Monitoring Well MW-7 since that time, minor thicknesses of floating product, up to 0.02 ft., have periodically reappeared in that s well(see Table 3). 1.5.2 Secondary Plume A secondary plume of diesel and gasoline, as shown on Figure 3, emanates from an area to the rear and slightly to the east of the Casa Mendoza restaurant on the 7500 West Eleventh Street property. Although no physical evidence of an underground or above- ground storage tank has been found there, it appears that, at some time in the past, there was a source of fuel hydrocarbons located in the vicinity of Monitoring Wells MW-12 and MW-12A (see Figure 3 for locations). When the 6,000 linear feet of underground piping that was found beneath the Navarra Site was removed in December 1998, a disconnected pipeline was discovered that ran from the general area of those monitoring l wells toward the West Eleventh Street frontage of the property where the underground storage tanks that were removed at that time were located. '.3 As shown on Figure 2, underground fuel tanks were formerly located on the property at 24195 Chrisman Road, which adjoins the 7500 West Eleventh Street site to the south. They had contained diesel and were removed in December 1998. Although no additional investigation of the 24195 Chrisman Road site has been performed, SJC believes that because samples recovered from the bottom of the tank pit at the time of removal contained only low concentrations of diesel (Dietz Irrigation 1999b), it is likely that any `f contribution made by diesel migrating from the 24195 Chrisman Road site to the subject property would have had little, if any, deleterious effect on the environmental condition of the subsurface beneath the subject property. i SJC