ARCHIVED REPORTS_XR0003132 - Laserfiche WebLink

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FATE AND TRANSPORT MODELING In its letter dated June 7, 1996, SJCPHS requested that this closure report include a calculated model capable of demonstrating that the residual petroleum impacted soils will not further degrade ground water in the future As discussed previously, it does not appear that the petroleum impacted soils at 305 South Hutchins are the source of petroleum hydrocarbons in the ground water beneath the site As requested by SJCPHS, and in accordance with the model and modeling parameters suggested by SJCPHS, a vertical transport model was used to evaluate whether petroleum impacted soils would degrade ground water in the future The vertical movement of a representative gasoline hydrocarbon compound, ethylbenzene, in the unsaturated (vadose) zone was simulated using Version 2 5 of SESOIL for Windows Ethylbenzene was used as the representative gasoline hydrocarbon compound instead of benzene because in the critical region to be modeled, benzene was not present in the vadose zone soils The Seasonal Soil Compartment model (SESOIL) is a one-dunensional, vertical transport model for the unsaturated soil zone It estimates pollutant concentrations in the soil profile following introduction Version 2 5 of SESOIL allows the user to specify initial adsorbed-soil concentrations in any sublayers It also includes the option of estimating average concentrations of the pollutant in the ground water that may result from movement down through the unsaturated zone to the water table SESOIL was used to simulate ethylbenzene transport in the vadose zone in the vicinity of GB-5 where vadose zone contamination is closest to the water table The water table was assumed to be about 55 feet below the surface The soil between the ground surface and the water table was assumed to be alLsand, even though several thin clay layers occur Three major layers were used in the model The uppermost layer represents the vadose zone from the ground surface to the top of the contaminated soil zone The second layer represents the contaminated soil zone An initial concentration of adsorbed ethylbenzene equivalent to that observed in the field was input to the soil of layer two The third or bottom layer represents the vadose zone beneath the contaminated soil zone These layers are 790, 550, and 330 cm thick, respectively, in the model The total model thickness is 1670 cm The bottom layer in the model was divided into 10 sublayers to provide better resolution of the advance of the pollutant front 9