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amec— <br /> beneath <br /> this zone are generally one to two orders of magnitude lower (< 210 pg/L) <br /> between the Upper A-zone and the top of the Corcoran Clay (approximately 200 <br /> feet bgs). CDCs from deeper groundwater samples (below 60 feet bgs) are likely <br /> the result of vertical movement through conduit wells or from offsite releases within <br /> the Study Area, because the observed chemical stratification among groundwater <br /> samples from beneath the Site suggests that vertical movement of chemicals <br /> through Site soils between aquifer zones has not occurred at locations where these <br /> samples have been collected. <br /> • Groundwater extraction at well E-2 (519 Stockton Avenue) should be terminated. <br /> Nestle should re-consider long term groundwater extraction at E-6, also at <br /> 519 Stockton Avenue. <br /> 4.0 STUDY AREA <br /> This section of the SCM report focuses on geologic, hydrogeologic, and anthropogenic <br /> controls on groundwater flow beneath the Study Area. The SCM reconstructs historical <br /> groundwater flow patterns to further understand the distribution of COCs beyond the Site <br /> boundaries. <br /> 4.1 SHALLOW STRATIGRAPHY OF THE RIPON AREA <br /> Multiple revisions and interpretations of the local stratigraphy have occurred with the <br /> installation of new monitoring wells since the 1990s, however, this updated SCM is the first <br /> attempt to correlate local stratigraphic units with the regional geologic units described in Burow <br /> et al. (2004). <br /> As part of this SCM update, the available boring logs for wells installed on behalf of Nestle <br /> were reviewed, along with the geophysical logs and driller's logs from municipal supply wells, <br /> and compared with the descriptions of the geologic formations described above. A map of well <br /> locations is provided in Appendix D. A three-dimensional Plexiglas® model was assembled to <br /> assess the stratigraphic information in three dimensions; this model is kept at the field office <br /> located at 519 South Stockton Avenue. Stratigraphic analysis and correlation with the regional <br /> geologic formations were conducted with the intention of: <br /> • Further assessing geologic controls on the lateral and vertical movement of <br /> groundwater. Geologic features such as paleosols, ash layers and other clays may <br /> impair vertical groundwater movement, and stacked channel sequences may allow <br /> for greater horizontal and vertical movement of groundwater; <br /> • Assessing the three-dimensional orientation of geologic contacts underlying the <br /> area to assist with spatial correlation among boring logs; and <br /> • Developing a further understanding of the soil type and mineralogy as it may relate <br /> to natural groundwater quality issues, such as arsenic mobility and intrinsic <br /> remediation potential for chlorinated VOCs. <br /> AMEC Geomatrix, Inc. <br /> hDoc_Safe\9000s\9837.005\4000 REGULATORY\SCM_01.30.09\1_text\SCM Report Final.doc 25 <br />