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i 4. SUBSURFACE CONDITIONS Stratus developed geologic cross-sections based on observations in soil borings and CPT borings advanced during site investigation activities. The surface traces of the cross-sections are shown on Figure 2, and the cross-sections are presented in Figures 3 and 4. Boring logs, CPT logs, and well construction details are presented in Appendix A. 4.1 Geologic Conditions Lithologic materials encountered beneath the Apache site consist of a silty and clayey unit to approximately 95 to 105 feet bgs. Based on interpretation of the CPT boring logs, numerous discontinuous strata of sand were encountered throughout the silty/clayey unit. Several of the sand strata are up to 20 feet thick, and these thicker units may extend off-site to the northwest. Previous investigations logged strata of sand and silty sand (e.g. in the 15- to 30-foot bgs interval beneath the former Apache UST) that were not identified as such by subsequent CPT borings (in this example, boring CPT-9). A laterally continuous sandy unit underlies the silty/clayey unit aidepth approximately 95 to 105 feet bgs. This sandy unit ranges from 10 to 35 feet thick onsite to greater than 40 feet thick at offsite boring CPT-7, and is composed of sand and gravelly sand with some interbedded discontinuous strata of silt and clay. Fine-grained material (silt and clay) were encountered at the base of all borings that penetrated the sandy unit to a total explored depth of 150 feet bgs. 4.2 Hydrogeology Review of the subsurface geology and groundwater well elevation data indicate that although different transmissive (i.e. sandy) zones have been identified at various depths during this investigation, these zones are hydraulically connected and form one continuous water-bearing zone. The monitoring well network, as currently installed, was intended to target these sandy, more transmissive zones and potential migration pathways. During the most recent monitoring data (second quarter 2002), groundwater beneath the site was measured between 46.56 and 55.80 feet bgs. Groundwater levels in the monitoring wells have risen from approximately 68 feet bgs to 44 feet bgs since groundwater monitoring was initiated in 1989. PHS/BHD personnel have indicated that first encountered groundwater in the site vicinity was recorded at approximately 110 feet bgs in 1979. Historical groundwater elevation data is presented in Appendix B. Historical groundwater flow direction maps are also included in Appendix B. p.\Apache Plastics\Reports\Apache SCM.doc Page 6 Stratus