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y _ Binayak Acharya Project 2074-001.01 May 1 S, 1995 Page 3 MODELING PROCEDURES EMCON has modeled the potential transport of the compounds found in the soil and how these compounds could migrate to groundwater. For this assessment, a relatively simple transport model was selected because the stratigraphy of the unsaturated zone appears to be a relatively simple series of alluvial deposits of clayey, silty, and gravelly sands (EMCON, 1994). The model accounts for the major processes governing transport of dissolved constituents while requiring only a minimum of assumptions. The assumptions that were made, however, were made conservatively, so model predictions tend to overestimate rather than underestimate chemical concentrations at the surface of the water table. The vertical and horizontal spread (VHS) model was chosen to estimate the vertical infiltration of the chemicals detected in the soil at this site downward toward the groundwater. The details of the modeling are discussed in Appendix A. In general, however, the modeling used the concentration of each chemical detected in the soil samples to estimate its concentration in soil-pore liquid just before it enters the groundwater underlying the site, approximately 59 feet (1 S meters) beneath the zone of impact. The modeling used the average precipitation for Stockton (about 14 inches per year) and assumed that the site would remain paved but that a portion (2 percent) of the precipitation would infiltrate through the pavement. The soils encountered in borings B-1 and MW-1 were mostly clayey, silty, and gravely sands. Therefore, values for porosity and bulk density typical of sands were used. A sensitivity study was conducted by varying the value for the cross-sectional dimension of the source. The soluble concentration of each chemical are presented in Table 3. Maximum contaminant levels (MCLS) are also presented, where available, for comparison. The results in Table 3 show that, even for the worst-case scenario of an unrealistically large release (164 feet or 50-meter cross section), the concentrations of chemicals detected in site soil would not impact groundwater at levels in excess of their respective MCLs. The results in Table 3 also show the time estimated for each compound to reach the water table. The modeling results show that the compounds will take more than 20,000 years to reach the groundwater; due primarily to the low infiltration rate. This long migration is consistent with the limited vertical extent of impact detected in the soil borings, and provides ample time for degradation to further lower the concentrations of each compound during migration. It should be noted that the concentration of each chemical in groundwater would be expected to be much lower than the estimated concentrations, for the following reasons: JASSCT%P72120740010.1 BR-9511jt:2