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RISC User's Manual Version 4.0 <br /> conservative)to assume that the source is constant(non-depleting)over the simulation <br /> time <br /> As an example, if the product spilled is fresh gasoline,the mass fraction of benzene in <br /> the product can be estimated based on published product data or based on measured <br /> data from similar sites If the mass fraction of benzene is assumed to be 0 03 in the <br /> product (fairly conservative, i e high, for gasoline), the effective solubility can be <br /> roughly calculated from the following equation <br /> CW =XS (B-10) <br /> where <br /> C,, = dissolved concentration of chemical adjacent to residual <br /> product [mg/1] <br /> S = aqueous solubility of pure component [mg/1] <br /> x = mole fraction of component in the hydrocarbon mixture <br /> [mol/mol] <br /> • This equation is also discussed in Appendix A (equation A-16) Of course, it is not <br /> easy to estimate the mole fraction of the chemical For chemicals and product <br /> mixtures that have similar molecular weights,the mole fraction can be replaced by the <br /> mass fraction This example assumes that benzene is 3%by mass of the product The <br /> pure chemical solubility for benzene is 1750 mg/l, therefore the effective solubility <br /> can be estimated as=0 03*1750 mg/I (solubility of benzene)=52 5 mg/l This is the <br /> concentration that would be specified for the source concentration <br /> B.4.3:Using the Model With Measured Concentration Data <br /> If there is a monitoring well in the source or dust down gradient, this information can <br /> be used to estimate the model source depth and the source concentrations If the <br /> monitoring well has a screen length of 2 meters and has benzene concentrations <br /> around 5 mg/1 then the source depth would be assumed to be 2 meters and the source <br /> concentration would equal the measured value Note, in many situations the residual <br /> can be located in the top few inches of the aquifer (at the water table) but the <br /> concentrations measured in the monitoring well average the concentration over the <br /> well screen length If the source is assumed to be only a few inches thick, then the <br /> B-10 <br />