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<br />"f.!:.) <br />~ <br />_N <br />N 2, <br />0 <br />0 3, <br /> 4. <br /> 5. <br /> 6. <br /> <br />it ,J <br /> <br />The program can pertonn simulations using planimetric or vertical sections. <br /> <br />The program incOIporates single-species, density-dependent solute transport, <br /> <br />The program employs numerical techniques accurate enough to greatly diminish numerical error <br />when large concentration gradients and time steps are used, <br /> <br />The program can be used to simulate both steady-state and transient ground-water flow and <br />transport. " <br /> <br />The program is logically organized, easy to understand. and well documented;'JYowing for ~y <br />modifications to accommodate specialized problems. <br /> <br />These advantages are directly related to the needs defined by the hydrogeologic and geochemical <br />framework of the Whitney area. For example, incotporation of the dike and proposed slurry wall requires <br />variable spacing of the model grid because of the size of the wall relative to the section of aquifer being <br />simulated. Incotporation of densiry-dependent transport resulting from concentration gradients, over a period <br />of time (that is. transient flow and transport), was necessary to test the effectiveness of the detention basin <br />and its ability to reduce dissolved-solids loads to Las Vegas Wash. The ability of the program to handle <br />time-dependent boundaries allowed for the examination of flow and dissolved-solids distributions <br />immediately after implementation of the slurry wall during a single simulation. The ability to easily modify <br />time-dependent boundaries to incotporate evapotranspiration was necessary to reproduce field-measured <br />dissolved-solids distributions. SUTRA incotporates these attributes in a numerical model that solves the <br />mass-balance equation describing flow and transport phenomena through the aquifer system. <br /> <br />The equation for fluid and solute-mass balance presented here is a simplified fonn of the equation <br />used by Voss (1984, p. 61). The reasons for the simplification are fourfold. First, only saturated flow <br />conditions were simulated in the analysis of the hydrogeologic conditions in the Whitney area. Second, <br />energy transport was noI used; a constant temperature was assumed, both temporally and spatially. Third. <br />the adsotption of solutes on the aquifer sediments was not considered. Fourth, neither production nor decay <br />of the solute mass was a component of this study. The omission of theSe processes has reduced the <br />governing equation to the following: <br /> <br />de <br />EPdt"+Ev.ve-v'[Ep(D/+D)].ve = Q/C'-C). <br /> <br />(3) <br /> <br />where p(X,)'.I) = fluid density. in grams per cubic meter; <br /> E(X.)'.I) = porosity (dimensionless); <br /> v(x.y,l) = average fluid velocity, in meters per second; <br /> V = gradient operator; <br /> V. = divergence operator; <br /> e (x.y,t) = fluid solute mass fraction, in grams of solute per gram of fluid; <br /> D = apparent molecular diffusivity, including tortuosity effects. <br /> m <br /> in meters squared per second; <br /> I = identity tensor. I; <br /> D = dispersion tensor, in meters squared per second; <br /> Q/x.y.l) ~ fluid mass source, in grams of fluid per cubic meter per second; and <br /> C"(X,)'.f) = solute concentration of fluid sources, in grams of solute per gram of fluid. <br /> <br />-27- <br /> <br />~f <br />f~- <br />