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U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993, Water-Resources Investigations Report 94-4015

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Numerical Simulation of Downward Movement of Solutes During a Natural-Gradient Tracer Test in Sand and Gravel, Cape Cod, Massachusetts

by

Denis R. LeBlanc (U.S. Geological Survey, Marlborough, Mass.) and Michael A. Celia (Dept. of Civil Engineering and Operations Research, Princeton Univ., Princeton, N.J.)

Abstract

A numerical, finite-element, solute-transport model was used to test the hypothesis that the downward movement of a tracer cloud observed during a natural-gradient tracer test in sand and gravel on Cape Cod, Mass., was caused, in part, by density-induced sinking. The tracer solution, which included the nonreactive tracer, bromide, was 0.1 percent denser than the ambient ground water. The center of mass of the bromide cloud moved vertically downward about 3.2 meters during 237 days of transport. The model simulated density-dependent flow and solute transport along a two-dimensional vertical section 25 meters high and 136 meters long aligned with the direction of ground-water flow. Transport of the bromide cloud was simulated for a period of 237 days divided into 191 time steps. The temporal pattern of recharge applied to the top boundary of the model was determined from daily precipitation and estimates of evapotranspiration. On the basis of an analysis of spreading of the bromide cloud during the tracer test, dispersivity was increased with time in the simulation asymptotically from 0.05 to 0.96 meters. The simulated downward movement after 237 days was 2.1 meters (about two thirds of the observed movement). The simulation showed that density-induced downward movement was most important during the first 37 days of transport when the density difference between the ambient ground water and the tracer cloud was greatest. Earlier work indicated that the difference between the observed and simulated downward movement may be due, in part, to simulation in only two dimensions of the three-dimensional flow that occurred around the tracer cloud as it moved downward through the ambient ground water. Additional simulations during this and earlier studies showed how the amount of downward movement is affected by the size and shape of the initial bromide cloud; aquifer properties, such as dispersivity and anisotropy of hydraulic conductivity; and the type of boundary used to represent the water table in the model.

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