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siltstone%claystones (Tnscl), and it underlies most of the central Site 300 GSA. Ground eater elevations in this unit range from 491 ft. above mean sea level (MSL) to 515 ft. above MSL. Unconfined groundwater flows southward in the Qt-Tnscl hydrologic unit. Where groundwater in this hydrologic unit discharges to the stream channel alluvium in the Corral Hollow streambed, the groundwater flow direction follows the streambed toward the east. The lowest stratigraphic unit, Tnsci, serves as a confining laver between the Qt-Tnscl hydrologic unit and the underlying Neroly lower blue sandstone (Tnbsl) hydrologic unit, as indicated by water chemistry and hydraulic head differences between the two units. Water level elevations in the Qt-Tnsci hydrologic unit are higher than in the underlying Tnbsi hydrologic unit. Although the Tnsci stratigraphic unit has low porosity and primary permeability, our observations of mineral coatings on fractures in cores indicate that it has moderate fracture (secondary) permeability. Neroly Lower Sandstone (Tnbsl) Hydrologic Unit: The Tnbsl hydrologic unit underlies the Qt-Tnscl hydrologic unit throughout the central General Services Area. Groundwater elevations within this unit range from 493 ft. above MSL to 514.7 ft. above MSL. Within the Tnbsi hydrologic unit, groundwater is confined and flows in a southerly direction, oriented subparallel to the bedding planes of the bedrock. Groundwater in the Tnbsl regional aquifer is hydraulically isolated from overlying shallow (Qt-Tnscl) aquifer by the Tnsci confining laver. Data from wells completed in the Tnbsi unit in the central GSA indicate that saturated conditions exist beneath most of the GSA. Mineral data from groundwater collected from ten wells completed exclusively in the Tnbsi hydrologic unit were used to characterize this unit. Groundwater in the Tnbsl water-bearing zone is dominated by the sodium cation and contains no dominant anions. In general, groundwater from the Tnbsi hydrologic unit is higher in sodium that water from the Qt-Tnscl hydrologic unit. Two groundwater monitor wells, W-873-01 and W-873-03, are located downgradient of Building 871. Well W-873-03 is completed in the shallow Qt- Tnscl aquifer and well W-873-01 is completed in the deeper Tnbsl regional aquifer. No aromatic hydrocarbons (BTXE compounds) have ever been detected in groundwater samples collected from these wells. No analysis for total fuel hydrocarbons has been performed on groundwater samples from these wells. - 9 -