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iT 1 Groundwater Monitoring Data and Analytical Results Tosco (Unocal) Service Station#5886 2701 West March Lane Stockton, California EXPLANA PIONS. Groundwater monitoring data and laboratory analytical results prior to March 25, 1998, were compiled from reports prepared by MPDS Services, Inc TOC = Top of Casing elevation B = Benzene ppb = Parts per billion DTW = Depth to Water T = Toluene -- = Not Measured/Not Analyzed (ft ) = Feet E = Ethylbenzene ND = Not Detected GWE = Groundwater Elevation X = Xylenes msl = Relative to mean sea level MTBE = Meihyl tertiary butyl ether TPH(D) = Total Petroleum Hydrocarbons as Diesel TPH(G) = Total Petroleum Hydrocarbons as Gasoline � I * The TOC elevations were surveyed relative to Mean Sea Level (msl) These well casing elevations were used prior to the May 2, 1997 monitoring event The TOC elevations of the well casings were resurveyed(except for wells MW-2 and MW-3) relative to msl on April 28, 1997 The well casing elevations are used beginning with the May 2, 1997 The new TOC elevations for wells MW-2 and MW-3 were calculmtd by using the previously surveyed elevations for wells MW-1, MW-2 and MW-3 (MW-1 = -0 84 tett msl, MW-2=-0 30 faet nnsl and MW-3= -0 14 feet nnsl)and the resurveyed elevation of well MW-1 (=-I 35 feet msl) ' Unidentified hydrocarbons >C14 2 Unidentified hydrocarbons >C15 3 Diesel and unidentified hydrocarbons >C20 4 Laboratory report indicates the hydrocarbons detected did not appear to be diesel 5 Laboratory report indicates the hydrocarbons detected appeared to be a diesel non-diesel mixture 6 TPH(D)analysts wuh silica get cleanup 7 MTBE by EPA Method 82.60 s Detection limit raised Refer to analytical results Note Wells designated as S are screened in the shallow zone, and wells designated as D are screened in the deep zone I I � 5R86 x1sl#180137 5 As of 06/17/98