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This report presents a new method of simulating transient leakage from confining units using the U.S. Geological Survey Modular Finite-Difference Ground-Water Flow Model. Transient leakage into or out of a compressible fine-grained confining unit results from ground-water storage changes within the unit. The importance of fine-grained units in analyses of transient ground-water flow is not always recognized. The new method of simulating transient leakage in the Modular Finite-Difference Ground-Water Flow Model is referred to as the Transient-Leakage Package, version 1, or the TLK1 Package. The TLK1 Package solves integrodifferential equations that describe the flow components across the upper and lower boundaries of confining units. The exact equations are approximated to allow efficient solution for the flow components. The flow components are incorporated into the finite-difference equations for model cells that are adjacent to confining units. Vertical hydraulic conductivity, thickness, and specific storage are specified in input arrays for each confining unit. Confining-unit properties can differ from cell to cell and a confining unit need not be present at all locations; however, a confining unit must be bounded above and below by model layers in which head is calculated or specified. The TLK1 Package was used for an example problem to simulate drawdown around a pumped well in a system with two aquifers separated by a confining unit. The pumping was limited to one of the two aquifers. The exact drawdown values determined from an analytical solution were compared with the simulated drawdown. The solution using the TLK1 Package closely matched the exact solution for drawdown values in excess of 1 centimeter in the pumped and unpumped aquifers. The problem also was simulated without the TLK1 Package by using a separate model layer to represent the confining unit. That simulation was further refined by using two model layers to represent the confining unit. The simulation made using the TLK1 Package was faster and more accurate than either of the simulations using model layers to represent the confining unit.

Documentation of a computer program to simulate transient leakage from confining units using the modular finite-difference, ground-water flow model