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5.0 PRELIMINARY SITE CONCEPTUAL MODEL A Preliminary Site Conceptual Model (CSM) is a simplified representation of the vadose and groundwater flow system, frequently in the form of a block diagram or cross section (Anderson &Woessner, 1992). The purpose of the CSM is to establish an initial understanding of the vadose zone and groundwater systems and organize the associated field data so that the system can be analyzed more effectively. The CSM can be divided into three categories for the Site: (1) delineation of the hydrostratigraphic units; (2) evaluation of sources of chemicals in soil, soil gas, and groundwater; and; (3) evaluation of chemical migration. 5.1 Hydrostratigraphy The regional and local hydrogeology of the LNC area are presented in Section 3.2 of this workplan and are only briefly summarized here. Available data indicate that the sediments beneath the LNC site consist primarily of a heterogeneous, interbedded mixture of unconsolidated gravels, sand, silts, and clays to a depth of at least 120 feet bgs (Figure 6). Soil boring data indicate that the upper 15 feet of sediments consist of a heterogeneous mixture of interbedded sand, silty sand, clayey sand, and sandy silt. The shallow soils appear highly variable depending on location and historical land use. This variability may affect the distribution and detection of VOC vapors in shallow soil gas and on the parameters used to model potential VOC soil gas migration. Several intervals of sand have been identified that appear to be relatively continuous beneath the LNC site, although the thickness and permeability of the sand intervals vary(Figures 6 and 12). These sand intervals tend to coarsen and thicken towards the north (and the Mokelumne River), becoming thinner and finer-grained towards the south. Several fine-grained(predominantly silts and clay) intervals have been identified beneath the LNC site. These clay intervals appear to become more continuous and laterally extensive to the south of the LNC site. M ain\D:AEnvironmenta1\LN CV\10152007.wpd 30