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. The following tables summarizes the maximum sorbed-phase hydrocarbon concentrations encountered during the assessment activities at the site EPA Regions 9 Maximum Sample PRGs Concentrati Sample Depth Sample Constituent (mg/kg) ons Location (feet) , Date Benzene 130 m a g j Southeast Wall 19 11/05/92 " 1 30 22 SB-13D 465 05/24/99 Toluene 520 1,420 Southeast Wall 19 11/05/92 Xylenes j 210 2,757 Southeast Wall19 11/05/92 TPH --- 17,600 Southeast Wall 19 11/05/92 A complete summary of soil analytical data is provided in Table 1 C. SITE HYDROGEOLOGY—WATER TABLE INFORMATION Interpretation of the monitoring well gauging data indicates that groundwater has a hydraulic gradient which typically flows in a south to southwest direction across the site The groundwater contours generated from the November 2001 measurements indicated an approximate gradient of 0 0012 ft/ft to the southwest, which is consistent with typical site data The groundwater elevations and contours from the November 2001 event are presented on Figure 7 A review of the groundwater level data at the two cluster well points(MW-3,MW-3A, MW-313 and MW-13, MW-13A, MW-1313) indicates that the shallow aquifer appears to be at least partially isolated from the deeper aquifer Water level data from monitoring well MW-13B, screened in the deep aquifer, shows a consistently lower groundwater elevation than in wells MW-13 and MW-13A There appears to be cyclical water level fluctuations at the site due to weather and the proximity of the site to the Stockton Diverting Canal A historical summary of the groundwater measurement and elevation data is provided in Table 2 D. HYDROCARBON CONCENTRATIONS — GROUNDWATER Groundwater at the site has been sampled and analyzed for gasoline range total petroleum hydrocarbons (TPH-G), benzene, toluene, ethylbenzene, and xylenes, and gasoline oxygenates metliyl-tertiary-butyl ether (MTBE), tertiary-butanol (TBA), di-isopropyl ether (DIPE), ethyl- tertiary-butyl ether(ETBE), tertiary-amyl-methyl ether(TAME), 1,2-dichloroethane (1,2-DCA), and ethylene dibromide (EDB) A review of the groundwater analytical data indicated that the dissolved- phase hydrocarbon constituent concentrations of benzene, toluene, ethylbenzene, xylenes, and TPH-G have generally decreased or stabilized in all monitoring wells except MW-13 The fluctuations in MW-13 appear to be the result of continued leaching from the soil impacts in the former tank pit 1Q-2