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' RISC User's Manual Version 4 0 <br /> Unlu et at (1992) assumes there is always residual phase hydrocarbon present and <br /> ' uses equation A-19 to estimate losses from the source This approach is realistic for <br /> the types of waste zones that Until et al equations were developed for-- waste sludge <br /> pits with high levels of hydrocarbons However, RISC also allows the user to <br /> estimate the behavior of sources for which immiscible phase is not initially present <br /> (c g,equation A-15a is satisfied) <br /> A 4 7 Source --When Residual Phase Hydrocarbon is Not Present <br /> When an immiscible phase is not present it is assumed that the concentrations of the <br /> chemical in each phase are in equilibnum The equilibrium partitioning equation is <br /> G = C,!'h (A-23) <br /> ' �' (phKaF. +6„, +OuKH� <br /> • which when inserted into Equation A-14 yields <br /> 9 + DCffKl1 =Qw + (A-24) <br /> (phK,.F,. +8M +9QKI)L. (p,K�Fx +B„ +6aKH)LdL,. <br /> ' where all the variables are as defined previously <br /> Y d F <br /> hydraulic <br /> Following Unlu et al (1992), the RISC Vadose Zone model uses a unit h y <br /> gradient approximation, estimating the unsaturated hydraulic conductivity with the <br /> Brooks and Corey (1964) model The unit gradient approach assumes that the <br /> pressure(suction) head in the soil profile is constant, as is the moisture content This <br /> ' is a mayor simplification of the real processes that control flow in the unsaturated <br /> zone However, the goal in modeling the concentration is to estimate average <br /> conditions over long time periods(for purposes of risk assessment 7 to 30 years) For <br /> 1 <br /> A-'14 <br /> 1 <br />