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Table 2. Groundwater Chemical Data, con't. Well Date TPHG Benzene Toluene Ethylbenzene Xylene Fuel Ox NoSampled ----------------------------- ug/I ----------------------------- Blank 4/29/98 ND ND ND ND ND ND ND - None detected NA - Not Analyzed ug/l - micrograms per liter Fuel Ox - Fuel Oxygenates by Method 8260 from Dec , 1997 forward - Quantified as MTBE @ - matrix interference reported 1,2 Dichloroethane ND in all wells I i Discussion Groundwater movement is northerly at a low flow gradient and the flow direction is consistent with historically observed trends Contaminants were observed in the Wells MW-2, MW-3, MW-6 and MW-9 Concentrations declined in Wells MW-9 and MW-6, and rose slightly in MW-1, MW-2 and MW-3 This slight concentration rise is attributed to a slight rise in water levels allowing a minor increase in desorbtion from the area in the Immediate vicinity of the wells MTBE and oxygenates were observed In MW-2, however the laboratory reports a matrix interference with the samples This is apparently due to the degraded condition of the gasoline Fluctuations of observed residual contaminant concentrations are attributed to aquifer recharge and rising water levels which allow more desorption of residuals In our opinion, a highly degraded fuel and fluctuating concentrations of contaminants continue an overall declining trend The continuing occurrence of contaminants in MW-6 is currently attributed to an offsite source (presumably somewhere south across East Eleventh Street) since MW-6 is upgradient of the known on-site contaminant source This off-site plume continues to migrate onto the site at 595 11th Street and has been observed since August, 1997 This trend strongly indicates that an off-site upgradient source is present and contributing similar contaminants to the site Rational for Site Closure Introduction The following information and discussion has been prepared at the request of PHS EHD for use in a site closure summary for the Mr Ed's Muffler site The Information is presented in the same order as that outlined in the RWQCB "Requests for Closure" Appendix B of the Tri Regional Board Staff dated 1 March, 1994 The source of the contaminants has been removed and the existing area geography and dissolved Plume is located in an area shown on Figures 2, 3 and 4 The plume dimension has not changed significantly since 1996, and the data Indicate that the plume has reached a "steady-state" condition where there is no net motion of the dissolved contaminants and the volatile fraction of the gasoline fuel is highly degraded The estimated contaminant mass is calculated below and is Interpreted to be declining, except for a plume migrating onto the site from an upgradient source Page 7