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4.3 GEOLOGY AND SOILS In 1991, Lowry and Associates drilled 17 test borings to determine soil and water conditions and to provide geotechnical design recommendations for development of the project. Nine of the borings,dug generally to a depth of 15 feet, were located in the area proposed for housing. The borings confirmed the presence of hardpan soils. Depth to cementation(hardpan)was found to vary from 18 to 36 inches. The hardpan layer consisted of light brown fine sandy silts (Unified Soil Classification ML) and cemented brown silty fine to medium sands (SM), extending to depths of 15 to 25 feet. No groundwater was observed when the borings were completed (August, 1991). Two deep borings made by Lowry were adjacent to the Mokelumne River at the proposed bridge crossing. The soils in the upper 35 feet consisted of mainly silty to fine sand (ML) to fine and medium sand (SM). Groundwater was observed at depths of 22 and 25 feet. Three borings drilled in the lake beds consisted mainly of brown clayey silts (ML) and organic silts (OH). Groundwater was observed in one boring (BH) at a depth of 14 feet. In connection with the present EIR, Questa Engineering conducted a reconnaissance soils investigation with emphasis on soil suitability for on-site sewage disposal (Figure 4.3-2). Soils were evaluated from a series of six hand-augered borings generally about four to five feet deep. Observed soil conditions agreed with those mapped by the SCS,with one notable exception. The soils observed west of Buckeye Drive appear to have very poor drainage and shallow groundwater conditions(i.e.,an estimated three feet or less to the water table), rather than well to excessively drained as indicated by the SCS mapping. ENVIRONMENTAL IMPACTS AND MITIGATION MEASURES Significance Criteria All identified impacts are considered significant adverse impacts. The corresponding mitigation measure, unless otherwise noted,would be sufficient to reduce impacts to a less-than-significant level. Although not required by CEQA, some less-than-significant impacts have been discussed because they are issues of local concern. While no mitigations are required by CEQA for less-than-significant impacts, in some cases mitigation measures are proposed that would reduce the level of impact. The CEQA guidelines define a significant impact on the geologic or soil environment as "... a substantial, or potentially substantial, adverse change in any of the physical conditions within the area affected by the project." The guidelines also stipulate that the EIR analyze significant environmental effects the project might cause by bringing development or people into the area affected by geologic or soil hazards. 4.3-3