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PRELRYDNARY HYDROGEOLOGIC ASSESSMENT REPORT 5491 "F Street, Banta, California Delta Project No. 44-88-481 Page 7 predominantly sand, clay, silty clay, and silty sand. The uppermost layer is a silty clay that is underlain by fine- to medium-grained sand. A silty clay/clayey silt occurs below the sand and is underlain by sand and silty sand. The sand and silty sand were the deepest units penetrated at the site. A geologic cross- section, presented in Figure 5, was constructed from the soil boring logs. The cross-section locations are shown in Figure 6. Copies of the soil boring logs are included in Appendix B. 3.3 Regional Hydro eog logy Ground water in the Sacramento Valley occurs in both confined and unconfined aquifers. Confined ground water tends to be located in older and deeper formations. Unconfined water can be found in alluvial fan, flood plains, and stream channel deposits. Ground water is found at shallower depths in the et),and deeper toward the basin margins,where it occurs central part of the Sacramento tiyalley Basin (5 fe at depths greater than 100 feet. Regional ground water flow at the site is in a northeasterly direction toward the Tom Paine Slough. (DWR Bulletin 146, 1967). Recharge to the water table occurs through deep percolation of water from streams, rain, snowfall, and irrigation. .s Ground water quality throughout the valley is sufficient for most uses. Ground water in the eastern half of the valley is of better quality, with respect to mineral content, than water from the western side of the valley. On the west side,where rocks of the Coast Range contain many soluble minerals and saline water, high concentrations of sodium, chloride, and sulfate in the ground water may render it unsuitable for irrigation and drinking (DWR Bulletin 118-6, 1978). 3A Zite Hydrogeology On April 28, 1989, two monitoring well borings were advanced and completed as ground water monitoring wells MW-2 and MW-3. The construction details of these wells are presented in Appendix A_ In addition to monitoring wells MW-2 and MW-3, monitoring well MW-1 was previously installed by ETS. The depth to ground water at the site was measured on May 12, May 24, and June 27, 1989. Measurements were made from the top of the casing. After the May 12, 1989, monitoring event, the casing elevations were altered and resurveyed. Monitoring well MW-3 was arbitrarily assigned an elevation of 100 feet above sea level. As a result the water level data from the May 12, 1989, event is noncorrellative with data from the May 24, 1989, event (Table 5). Differences in the , ater level between the monitoring wells for an individual monitoring event are correlative. A water table contour map, based on the May 24, 1989, water level monitoring event, is presented in Figure 7.