ARCHIVED REPORTS_2003 FEASABILITY STUDY REPORT AND SITE ASSESSMENT WORK PLAN

Home Browse Search

S E C O R 2003 Feasibility Study Report and Site Assessment Work Plan July 18, 2003 Page 15 more accurately reflect the assumed source location of TPHd to the locations of the groundwater monitoring wells. 4.4.5 Model Limitations AT123D is an analytical groundwater transport capable of computing the spatial-temporal concentration distribution of contaminants (wastes) in the aquifer system and predicts the transient spread of a contaminant plume through groundwater. The model can be used as an assessment tool to help estimate the migration of dissolved concentration of CDCs. The accuracy of model results is dependent on the assumptions incorporated into the model, the assumed limited heterogeneity within the modeled area, and the current definition of groundwater impacts. In practice, lithologic heterogeneities and multiple sources have been identified at the site, which may result in lower or higher migration rates and persistence of COCs than what has been simulated by this model. In addition, the persistence of COC sources and the short- and long-term effects of onsite remedial efforts (i.e., source removal by excavation, DPE, ISO-GENT") could not be accounted for in the model. The presence of heterogeneities combined with the unknowns surrounding COC source(s) and effects of remedial efforts most likely accounts for differences in calibrated model results and known site conditions. However, based on conservative approach to modeling the long-term migration of COCs at the site,the simulations presented here are considered to be reasonable. 4.5 Simulation Results Results of the AT123D simulations are summarized in Table 2. The results for all the different cases predict that all the COCs simulated by AT123D do not migrate beyond well MW-59 at concentrations above 1 ug/L. For the TPHd scenario, the model predicts that TPHd concentrations would reach a distance analogous to the position of well MW-57 in approximately 20 years. However, the concentrations would be below a concentration of 0.02 ug/L. In addition, further analysis using extended model simulations also predicts that within 20 years, concentrations of TPHd would begin to degrade and TPHd would not migrate beyond MW-57 at concentrations above laboratory method reporting limits (MRLs). For the TPHg scenario, the model predicts that TPHg concentrations above MRLs would not reach a distance analogous to the position of well MW-57 in approximately 30 years. In addition, on site concentrations would begin to degrade immediately after the TPHg source was exhausted. For the MTBE scenario, the model predicts that the maximum MTBE concentrations that would reach distances analogous to the positions of wells MW-57 and MW-59 would be less than 1 ug/L. In addition, onsite and offsite MTBE concentrations would begin to degrade immediately after the MTBE source was exhausted and would not migrate past MW-59 at concentrations above 0.5 ug/L. I:\Chevron\1001621\RE PORTS\2003\2003FS\FS 2003 revD.doc