ARCHIVED REPORTS_SITE CONCEPTUAL MODEL 2009

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amec— test results for extraction wells EU-3 and EU-4 (screened from 50 — 65 feet bgs in the Upper A-zone) yielded hydraulic conductivity values greater than 500 ft/d (ECM, 2008a). • Silts and sands are generally encountered from approximately 100 — 115 feet bgs (called the A-zone in previous Site reports, which is equivalent to the Lower A-zone under the new classification). The A-zone soils appear to be finer-grained than the overlying Middle and Upper A-zone sediments based on the CPT logs. A tracer dilution test performed at well M-1A was used to estimate a groundwater velocity of approximately 4 ft/day however this relatively high value may reflect the influence of nearby groundwater extraction at well EI-1 (Geomatrix, 2006). Slug tests were conducted in 1999 at Lower A-zone wells EU-1, EE-1, ED-1, ED-2, and M-1 A; hydraulic conductivity values ranging from 56 to 98 ft/d were estimated from these tests (IT, 2000). • A brown or grey silty clay (from Site boring logs) is encountered at a depth of approximately 110 — 115 feet bgs and extends to depths beyond 120 feet bgs and is interpreted as the base of the Upper Aquifer. Correlating lithologic units across borings separated by distances of 100 feet or more is challenging because fine-grained layers generally are not always laterally continuous beneath the Site. However, from the CPT borings, four relatively thin "layers" of cemented or over- consolidated fine-grained soils can be correlated across most borings at depths of approximately 38 —40 feet bgs, 80 — 85 feet bgs, 95-100 feet bgs, and 110 - >120 feet bgs, forming stratigraphic divisions for the Upper, Middle, and Lower A-zone units (Figures 4 and 5). These layers may represent buried caliche layers or ancient soil horizons (paleosols) where the sediments in between these layers represent fluvial sands and gravels and fine- grained overbank deposits associated with the ancient Stanislaus River. The boring log for well M-25C2 (total depth of 200 feet bgs) and associated geophysical log indicate that two coarse-grained water bearing units exist at depth intervals of approximately 148 — 158 feet bgs (called the C1-zone) and 178— 183 feet bgs (called the C2-zone). Blue green clay was encountered at a depth of approximately 196 feet bgs to the total depth of approximately 200 feet bgs. This clay zone is likely the top of the Corcoran Clay, which is an important regional aquitard in the Ripon area, as discussed in greater detail in later sections of this report. 3.3 WATER LEVELS The general directions of horizontal hydraulic gradients at the Site are impacted by several historical activities, including the pumping of several on and off-site wells, and recharge from multiple sources. Based on water level data collected in July 2008, the slope of the water table (B-zone) was generally to the north and northwest in the vicinity of the Site; with lower water levels at well M-1 B due to pumping from nearby extraction well EU-4 (Appendix B). AMEC Geomatrix, Inc. I:\Doc_Safe\9000s\9837.005\4000 REGULATORY\SCM_01.30.09\1_text\SCM Report Final.doc 17