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2 MODELING OF SOIL AND GROUNDWATER CHEMISTRY <br />' EMCON has modeled the potential transport of the compounds found in the soil to <br /> determine whether these compounds could migrate to groundwater at the former Nestle <br /> site This section presents the procedures used for the modeling and the results of the <br />' modeling <br /> 2.1 Modeling Procedures <br /> For this assessment, a relatively simple transport model was selected because the <br />' stratigraphy of the unsaturated zone appears to be a relatively simple series of clayey, silty, <br /> and gravely sand alluvial deposits (EMCON, 1994) In general, the modeling used the <br /> concentration of each chemical detected in the soil to estimate its concentration in soil- <br /> pore liquid dust before it enters the groundwater underlying the site The model accounts <br /> for the mayor processes governing transport of dissolved constituents while requiring only <br /> a minimum of assumptions The assumptions that were made, however, were <br /> conservative, so predictions based on this model tend to overestimate rather than <br /> underestimate chemical concentrations <br />' The vertical and horizontal spread (VHS) model was chosen to estimate the infiltration of <br /> the chemicals detected in the soil downward toward the groundwater The details of this <br /> model and the derivation of the values used for the modeling are discussed in Appendix D, <br />' and the values for the modeling parameters are summarized in Table 3 Briefly, however, <br /> the distance to the water table was measured to be about 72 feet According to the <br /> SJCPHS, the water table in the vicinity of the site has historically risen to within 45 feet of <br /> the ground surface Using the value for the higher water table, the minimum distance from <br /> the soil impact to the water table is about 34 feet (10 4 meters) The modeling used the <br /> 100-year average precipitation for Stockton (about 14 9 inches per year) and assumed that <br /> the site would remain paved but that a portion (2 percent) of the precipitation would <br /> infiltrate through the pavement The soils encountered in borings B-1 and MW-1 were <br /> mostly clayey, silty, and gravely sands Values for porosity and bulk density typical of <br /> sands were used to estimate the more porous members of the soil column The total <br /> organic carbon content of the soil was conservatively estimated to be 1 percent <br /> r <br /> I IPMRXK00285 DOC 951jlc 1 Rev 0,8/28/95 <br /> 22074-001 001 2-1 <br />