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1 <br /> Appendix Vadose <br /> Zone Model <br /> P <br /> The Vadose Zone mode] in RISC simulates contaminant transport through unsaturated <br /> soil The transport equations are solved using the analytical solutions of the one- <br /> dimensional advective dispersive solute transport equation(van Genuchten and Alves, <br /> 1982) The model considers the following fate and transport processes (j) a well- <br /> mixed finite-mass source zone, (n) pseudo steady-state volatilization and diffusive <br /> vapor transport from the source to ground surface, (iij)leaching from the source zone, <br /> ' (iv) advective dissolved-phase transport, (v) dissolved-phase dispersion, (vi) <br /> • adsorption,and vii)first-order decay in the leachate <br /> ' This model is similar to the vadose zone model presented by Unlu et al (1992) with <br /> the exception of several significant differences that are discussed The most <br /> significant change is that the Vadose Zone transport model in RISC allows for the <br /> presence of a second soil layer located between the source zone and the ground <br /> surface, this layer may have soil properties different from the rest of the vadose zone <br /> ' which can dramatically affect vapor emissions to the surface <br /> ' The purpose of the Vadose Zone model is to predict (1) loading to groundwater and <br /> (2)volatilization losses The groundwater loading term may be used as a source input <br /> to the saturated zone model(Appendix B) The volatilization losses may be used as a <br /> ' source for the box air model to calculate concentrations in outdoor air (Appendix F) <br /> Figure A-1 shows the processes simulated by the Vadose Zone model <br /> i <br /> 1 <br /> 1 <br /> A-1 <br /> 1 <br />