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Groundwater Monitoring Data and Analytical Results Tosco (Unocal) Service Station#5886 2701 West March lane Stockton, California E.XPI,ANATION& 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 rust = Relative to mean sea level MTBE = Methyl tertiary butyl ether TPH(D) = Total Petroleum Hydrocarbons as Diesel TPH(G) = Total Petroleum Hydrocarbons as Gasoline * 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 rust 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 calculated by using the previously surveyed elevations for wells MW-1, MW-2 and MW-3 (MW-1 = -0 84 feet msl, MW-2=-0 30 feet msl, and MW-3= -0 14 feet msl)and the resurveyed elevation of well MWA (=-I 35 feet msl) x* Well were re-surveyed on April 27, 1999 by Morrow Surveying TOC elevations are based on City of Stockton Benchmark#226,top of 314" iron pin to monument box at the intersection of the centerlme of Feather River and the S R O W line of March Lane, (Elevation = -0 92 feet) i Laboratory report indicates unidentified hydrocarbons unidentified hydrocarbons >C 14 Z Laboratory report indicates unidentified hydrocarbons unidentified hydrocarbons >C15 3 Laboratory report indicates unidentified hydrocarbons 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 with silica gel cleanup 7 MTBE by EPA Method 8260 8 Detection limit raised Refer to analytical reports 9 Laboratory report indicates unidentified hydrocarbons C9-C24 10 Laboratory report indicates unidentified hydrocarbons C9-C24 and Discrete peak @ C24 I 11 laboratory report indicates unidentified hydrocarbons C6-C12 12 Laboratory report indicates unidentified hydrocarbons >C16 t3 MTBE by EPA Method 8260 required reanalysis past hold time due to failing internal standard on first run First analysts was within hold time Note Wells designated as "S" are screened m the shallow zone, wells designated as "i" are screened in the intermediate zone, and wells designated as "D" are screened in the deep zone 5886 As1#180137 8 As of 08/17/99