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T&T Trucking Subsurface Investigation Results and Site Conceptual Model ' No 4_g 4 2 2 Groundwater Quality ' Regionally, the majority of the groundwater in the Eastern San Joaquin basin is characterized by calcium-magnesium bicarbonate or calcium-sodium bicarbonate types Total dissolved solids ' (TDS) range from 30 to 1,632 milligrams per liter (mg/L) and averages about 310 mg/L The average specific conductance of groundwater ranges within the Eastern San Joaquin basin is 685 micro-mohs per centimeter (µmhos/cm) Elevated saline groundwater has been intruding from the Delta eastward toward Stockton Nitrate concentrations are also elevated in areas southeast of Lodi and Stockton as wells as other County areas (DWR, 2003) General chemistry of the site groundwater has not been characterized, however, electrical conductance measurements collected during quarterly purging events range from 800 to 1300 and average approximately 1,000 µmhos/cm which is consistent with the DWR data for the region ' 4 2 3 Water Levels ' Water levels have been measured on a quarterly basis from January 2000 to the present Depth to groundwater beneath the Site currently ranges from seventy to 71-£t bgs A hydrograph of the water level elevations for all the wells from August 1998 to December 2003 are presented in ' Figure 4-4 As shown on Figure 4-4,water levels in all four monitoring wells behave similarly during the annual hydrologic cycle The highest groundwater levels generally occur during the spring,responding to precipitation The lowest groundwater levels occur during the fall and early winter The correlation of water levels between wells indicates a continuous water table condition and wells are screened across the same water table ' As shown on Figure 4-4 groundwater levels appear to be decreasing over time This trend is consistent with regional water levels due to increased groundwater pumping for irrigation and ' potable water supply Overall, the hydraulic gradient is primarily flat with periods of very shallow gradients and flow directions to almost every compass direction The hydraulic gradient across the Site ranges from ' flat to 0 005-ft/ft to the south-southwest, and there have been two sampling events where a groundwater divide was calculated (September 1998 and January 2000) with the steeper gradients occurring during the wetter months 43 Nature and Extent of Petroleum Hydrocarbons in Soil and Groundwater This section provides a description and summary of individual constituents of concern detected in ' CPT HydropunchO samples Constituents include BTEX,TPH-g,TPH-d, and volatile organic compounds (VOCs) for the fuel oxygenates MtBE, Di-Isopropyl ether (DIPE), ethyl-t-butyl-ether (ETBE), tert-amyl-methyl-ether (TAME), and tert-butanol Analytical results for CPT ' groundwater samples are presented in Table 3-2 • P\23006\23954 i'1 tru6Ang\C11'1 Summary Report\F[NAI 7&T 1 rucking DOC