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1 Rr <br /> Through experimentation, it was determined that precipitation, evapotranspiration and <br /> permeability have a significant effect on contaminant transportation. A precipitation of 53 <br /> cm/year was used in the models. The actual average precipitation in Stockton is <br /> approximately 36 cm/year. A conservative evapotranspiration rate of 0.35 cm/day was used. <br /> ' The evaporation rate obtained from the Stockton Water Treatment Plant is approximately <br /> 0.50 cm/day. Both these parameters were overestimated for the purpose of these tests. <br /> Therefore, the actual potential for the migration of contaminants is Iower than predicted. <br /> ' The model was then run using a rainfall twice actual, a permeability 100 times predicted and <br /> a contaminant load 10 times the amount indicated from site assessment data.The model still <br /> showed no contamination reaching groundwater (Appendix C). Only the input data sheets <br /> and summarized output are included in Appendices B and C. <br /> 3.0 CONCLUSIONS & RECOMMENDATIONS <br /> Calculations presented in the SESOIL modeling program indicate a very low probability of <br /> contaminants from the impacted soil at the site reaching groundwater. Even under worst <br /> case conditions, no contaminants were predicted to impact groundwater. <br /> ' From the calculations for remaining petroleum hydrocarbons presented in the CSR <br /> addendum letter, 90 percent of the remaining contaminants are located off-site, under <br /> ' Harding Way and the sidewalk. The surface above this area is covered with asphalt or <br /> concrete limiting the infiltration of surface water; this decrease in water infiltration was not <br /> incorporated into the models used. The effect of this surface cover should be similar to <br /> decreasing the total rainfall, and thus decreasing the potential for migration of contaminants. <br /> Future construction at the site will most likely result in the paving of the site, forming a <br /> surface seal, and preventing infiltration of almost all surface water. <br /> Approximately 83 per cent of contaminants at the site are in clayey soil. Subsurface <br /> Y <br /> remediation of clays is difficult due to the impermeable nature of finer-grained soils. The <br /> most common method of in-situ soil remediation, vapor extraction, is shown to have little <br /> effect of hydrocarbons trapped in clayey soils. Excavation of the impacted soil would require <br /> closing the east-bound lanes of Harding Way and disturbing and replacing numerous <br /> ' underground utilities. The costs involved would be enormous and this excavation would not <br /> be allowed by the City of Stockton. <br /> Due to the low probability of contaminants impacting groundwater and the enormous costs <br /> and ineffectiveness of remediation alternatives, we recommend the site be certified for <br /> closure. <br /> 4.0 LIMITATIONS <br /> Our professional services were performed using that degree of care and skill ordinarily <br /> exercised by environmental consultants practicing in this or similar localities. The findings <br /> 0-601 A.da S. ki IK. <br /> Yelbw 5.6a+.ri.e SESOIL 1.93 <br />