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_ _ s _ t+s i � PRELIMINARY Iri'DROGEdLOGIC AssnsMENT REPORT 5491 "F Street,Banta,California Aelta Project No.4G-88 81 Page 7 r- predominantly sand, clay,silty clay, and silty sand. The uppermost layer is a silty clay that is underlain f-' 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,premed in Figure 5,was constructed from the soil boring logs. The cross-section locations are shown in Figure 6. l'.`vpics of the soil boring logs are included in Appendix B. 3.3 Rg lona] H droeenl 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. UnconCtnP.d water can be found in alluvial fan,flood plains,and stream,channel deposits. Ground water is found at shallower depths in the central part of the Sacramento Valley Basin(5 feet),and deeper toward the basin margins,where it occurs at depths greater than 100 feet. Regional ground water flow at the site is in a northeasterly direction r toward the Tam Paine Slough. (DWR Bulletin 146,1967). Recharge to the water table occurs through deep percolation of water from streams, rain, snowfall,and irrigation. r 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). 3.4 Site HVdt 9e_Ql6t Y !.. 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 , monitoring well MW-1 was previously installed by E f S. to monitoring wells MW-2 and MW-3 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 Y g 1 .T casing elevations were altered and resurveyed. Monitorin well MW-3 was arbitrarily assigned an elevasion_� - - g 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 water level between i' the monitoring wells for an individual monitoring event are correlative. A water table contour map, t. based on the May 241989,water level monitoring event, is presented in Figure 7.