Numerical model for analyzing slug tests in vertical cutoff walls

Analysis of a slug test estimating the hydraulic conductivity of a vertical cutoff wall is complicated by the high compressibility of backfill materials and by the proximity of a well to the edge of the cutoff wall. An implicit finite-difference program, named Slug_3, was developed to analyze results of slug tests in the vertical cutoff wall. The program uses block-centered mesh formulation, considers variable hydraulic conductivity and specific storage, and has automatic time-step control and mesh generation. The geometry and flux-boundary condition in the well-intake section is fully considered, and the interface between a cutoff wall and natural soil formation is modeled as a constant head-boundary condition. Also, a filter cake can be simulated in Slug_3. Slug_3 is verified by comparing results with an analytical solution for a partially penetrating well in aquifers and another numerical code, MODFLOW-96, for a vertical cutoff wall. The program provides a new analytic tool for analyzing slug-test results from vertical cutoff walls and is unique in the ability to simulate variable hydraulic properties, which can be particularly important for highly compressible materials such as soil-bentonite backfill in a cutoff wall.