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Supplemental Investigation Report BP Oil Site No 11193 C A M B R I A Stockton, California October 13, 2000 Fate and Transport Modeling Fate and transport modeling was performed to evaluate the potential for BTEX and MTBE migration to the Caltrans well The AT123D model modified for input of source concentrations rather than a mass loading was used The primary assumptions of the model include (1) the aquifer thickness is homogeneous, isotropic, and comprised of a single, horizontal layer of uniform thickness, and (2) the hydraulic gradient is uniform and constant within the model domain Input parameters were estimated from the previous hydraulic testing, groundwater monitoring and other available data These parameters are presented in Appendix H and summarized on the tables below Table A— Model Input Parameters Parameter Value Comment Hydraulic conductivity 0 39 meters per day Site maximum from hydraulic testing Hydraulic gradient 0 007 northeastward July 14, 2000 monitoring event with extended well network Effective porosity 030 Typical of silty sand Bulk density 17 grams per milliliter T real of silty sand Fraction of organic carbon 00005 Analytical results from boring C13-4 Constituent Concentration Benzene 0 084 mg/1 Maximum reported during Toluene 0 002 mg/l July 14, 2000 monitoring event Ethylbenzene 0 015 mg/1 Xylenes 0 018 mg/l MTBE 250 mg/l The release of BTEX and MTBE was simulated in the model by introducing a steady concentration over a fixed time interval and source area A source duration of 5 years was used based on the interval between the initial reports of MTBE in 1993 and tank replacement in 1998 The source area was assumed to encompass the entire UST complex with dimensions of 1 meter thick by 10 meters long by 10 meters wide The model simulated migration over 100 years, calculated dispersivity values and assumed no degradation of these constituents 12