Hydrogeology. The Santa Xxxxx Valley Basin is a large, northwest trending structural depression between the San Andreas and Hayward faults. The valley is bordered on the west by the Santa Xxxx Mountains and on the east by the Diablo Range. Regionally, the Santa Xxxxx Valley contains up to 1,500 feet of interbedded alluvial, fluvial, and estuarine deposits (Tetra Tech,1998a). The shallow aquifer (upper 250 feet) is subdivided into the A, B, and C aquifers. The A aquifer consists of sands and gravels found between depths of approximately 5 and 60 feet below ground surface (“bgs”). It is divided into the Al- and A2- aquifer zones by a discontinuous low-permeability horizon (A1/A2 aquitard) located between approximately 25 and 30 feet (Tetra Tech, ). In general, the groundwater flow direction in the A aquifer is toward San Francisco Bay (north) with a horizontal gradient of 0.004 to 0.005 feet per foot (ft/ft). Vertical gradients between the Al- and A2- aquifer zones are weak and locally variable. Depth to groundwater ranges from 5 to 12 feet bgs (Tetra Tech, 1998a). The A/B aquitard is a 5-7 foot thick clay zone encountered between the depths of approximately 65 to 70 feet bgs and may be locally continuous under the western portion of ARC (PRC, 1996). The B aquifer (70-120 feet bgs) includes permeable deposits characterized by interbedded fine- to medium-grained sands, and clayey sands. Significant upward vertical gradients exist between the B aquifer and the overlying A2-aquifer in the ARC. A laterally extensive clay aquitard (B/C aquitard) effectively isolates the C aquifer (160 to 250 feet below ground surface) (Tetra Tech, 1998a). The MEW Companies interpret the hydrogeology of the Site differently from the description above. Specifically, they refer to the A2-aquifer zone as the B1-aquifer zone and thus interpret the B aquifer as extending from approximately 30 to 120 feet bgs.
Hydrogeology. Within the northern Santa Xxxxx Valley groundwater basin, watershed boundaries are defined by drainage divides in the Santa Xxxx Mountains and Diablo Range. The contact between the bedrock and Quaternary alluvium defines the extent of the groundwater basin (Tetra Tech, 1998c). Regionally, the Quaternary water-bearing deposits are divided into a deep, confined aquifer, and a shallow, unconfined aquifer based on the extent of a regional confining layer (Tetra Tech, 1998c). Four regional subdivisions of the upper 250 feet of Quaternary sediments are as follows: • Holocene (Recent Interglacial Period) alluvium (A Aquifer zone) • Late Pleistocene (Wisconsinan Glacial Period) alluvium (B aquifer zone) • Late Pleistocene (Sangamon) Interglacial deposits (B/C acquitard) • Pleistocene (Illinoian Glacial Period) alluvium (C aquifer zone). The shallow aquifer (upper 250 feet) is subdivided into the A, B, and C aquifers. A laterally extensive clay aquitard (B/C aquitard) effectively isolates the C aquifer (160 to 250 feet below ground surface [bgs]) from the upper aquifers. The A/B aquitard may be locally discontinuous. The remaining discussion focuses on the A aquifer zones beneath the NRP because the aquifer is most accessible and likely to be impacted by contamination and because of the relative lack of contamination in the deeper aquifers. The A aquifer extends from a depth of 5 to 65 feet bgs at the western side of Xxxxxxx Field. The A aquifer is divided into the Al- and A2- aquifer zones by a discontinuous, low-penneability horizon (A1/A2 aquitard) located between 25 and 30 feet bgs (Tetra Tech, 1998a). The aquifer consists of sands and gravels with gravel comprising 20 to 90 percent of the coarse material. In general, groundwater flow is toward San Francisco Bay (north) with a horizontal gradient of 0.004 to 0.005 feet per feet (ft/ft) (PRC, 1996). Depth to groundwater ranges from 5 to 12 feet bgs (Tetra Tech, 1998a)
Hydrogeology. Describe the regional and site specific hydrogeologic setting including any information concerning local aquifers, ground water levels, gradients, flow direction, hydraulic conductivity, and velocity. Include potentiometric surface contour maps. Describe the beneficial uses of the ground water (e.g., drinking water supply, agricultural water supply, etc.). Plot ground water elevations on the geologic cross-sections and indicate ground water flow directions and likely contaminant pathways. Describe temporal variations (seasonal and historical).
Hydrogeology. The main San Xxxxxxx Basin comprises approximately 167 square miles of water -bearing valley land. The maximum depth of alluvial fill is unknown, although it is expected to be between 2,000 and 4,000 feet. The estimated total storage capacity of the main San Xxxxxxx Basin is 10.44 million acre -feet; however, because of the great depth of the basin and the subsequent inaccessibility of much of the groundwater, the available supply of the basin is much less. The majority of natural inflow to the main Xx x Xxxxxxx Basin is in the form of surface water, originating as precipitation and entering through stream channels or as overland flow. Subsurface flow crosses into the SGV from the Xxxxxxx Ground Water Basin, across the Xxxxxxx fault on the northwest , and from the Chino Groundwater Basin on the east.
Hydrogeology. The groundwater system in the pit area is highly complex and includes flows derived from rainfall infiltration into the upper regions of the caldera, sea water intrusions and erratic high pressure areas caused by volcanic activity. An extensive system of depressurizing, dewatering and cooling water/steam wxxxx exist in and around the pit.
Hydrogeology. Based upon a review of local topography and physiographic information, groundwater is at or just above the water level of the Ohio River, and will fluctuate with the water level in the river. The groundwater flow direction is towards the Ohio River, and also with a westward component, with the flow direction of the Ohio River. The Ohio River is one quarter mile south of the INDOT excess properties. Pigeon Creek runs north to south and passes just west of the project area boundary.
