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~ ..,. C'J N c.:... CJ SIMULATING EFFECTS OF PROPOSED SLURRY WALL ON GROUND-WATER FLOW AND SOLUTE TRANSPORT Preceding sections of this report describe the physical setting and hydrogeologic properties of the alluvial deposits, ground-water flow rates, and ground-water chemistry in the Whitney area. This section deals with the development of conceptual and mathematical models, using the infonnation described in previous sections. to describe the effect the proposed slurry wall could have on ground-water flow and solute transport in, and surrounding, the detention basin.'" ,'o.v Three conceptual models were developed and tested using mathematical (numerical) models. 't'o analyze effects of implementing the proposed slurry wall (which would extend through the entire thickness of the aquifer beneath the dike structure). The fin;t model is a planimetric ground-water flow model (solute transport is not included), which is designed to analyze flow characteristics in the vicinity of the detention basin. In other words, the model is intended to determine (1) whether ground water will continue to recharge the basin or whether, after some period of time. flow will be diverted around the basin as water levels rise behind the dike and slurry wall; and (2) how the distribution of hydraulic conductivity influences ground-water flow in the detention basin. These questions are addressed in the discussion of the planimetric model. The second model is a cross-sectional ground-water and solute-transport model (along segment X- Y of section W-Z, fig. 4). It is designed to analyze the sensitivity of several aquifer and transport properties, to (1) determine which properties greatly affect the flow and dissolved-solids distribution in the detention basin, and (2) describe the conditions under which the proposed slurry wall would work most effectively. The third model is an expansion of the second model to include downgradient areas from the slurry wall to Las Vegas Wash. with modifications to the program to incorporate seasonal variations in evapotranspiration at.the water table. Field-estimated properties were included in a simulation of flow and solutes to replicate conditions prior to construction of the slurry wall and to project how a proposed slurry wall might affect flow and transport in the study area. ' Figure II shows both a vertically exaggerated (A) and a true-scale section (no vertical exaggeration; B) through the Whitney area that is used in the latter two models, with the proposed slurry wall included. The vertical scale is exaggerated to show the flow and dissolved-solids distributions produced by the simulations, Before the development of each conceptual model is discussed, the numerical technique used to express the conceptual model in mathematicaltenns is presented. In addition. some definitions that describe transport phenomena are given. Mathematical Modeling Technique Used The computer program used to simulate flow and transport of solutes in the conceptual models is known as SUTRA (Saturated-Unsaturated TRAnsport), a U.S. Geological Survey numerical model developed by Voss (1984). Analytical models, although exact, are greatly limited in their application primarily because of their inherent simplistic boundary conditions and restriction to linear problems. SUTRA employs standard finite-element approximations for the governing equations describing fluid mass, solute mass, and energy. This program was selected over others for the analysis of ground-water flow and dissolved-solids transport in the Whitney area for several reasons: I. The program allows for geometric flexibility in application of boundary conditions and variable grid spacing. -25-