UTCHEM Model of DNAPL Contamination of an Aquifer
(Link to nontechnical description of this UTCHEM Model of DNAPL Contamination of an Aquifer.)
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UTCHEM
The purpose of this webpage is to describe a UTCHEM simulation of DNAPL contamination of an aquifer. UTCHEM is a Fortran program developed by staff at the University of Texas in Austin. It is described in the technical documentation (RERPC, 2000). The software has been under development since the 1970s. Originally it was for simulation of enhanced recovery of oil using surfactant and polymer processes. Later, it was expanded so that it can be used to simulate aquifer remediation, especially remediation of aquifers contaminated by non-aqueous phase liquids. It is capable of simulating the downward movement of dense non-aqueous phase liquids resulting from leaks and spills into aquifers, and it can simulate the effects of complex conditions. These conditions include inhomogeneous and anisotropic permeability, groundwater movement in response to pressure gradients, chemical reaction of the DNAPL with the ambient groundwater, effect of interfacial tension on movement of the non-wetting DNAPL phase, and dispersivity of the DNAPL phase. It is also capable of simulating the behavior of LNAPLs.
UTCHEM DNAPL CONTAMINATION MODEL
The UTCHEM simulation discussed on this webpage is based on a representation of a vertical slice of aquifer using a two-dimensional finite-difference grid. The input for this model was supplied by the University of Texas along with the source code for for testing the compiled executable. It is labeled "Jin Minquan's exp. data for PCE." The grid consists of rectangular parallelepipeds with 49 grid blocks in the x-direction (horizontal) and 24 grid blocks in the z-direction (vertical). The blocks are uniform with x, y, and z dimensions of 3.28 feet, 49.21 feet, and 1.64 feet, respectively. So the grid extends 160.72 feet in the horizontal direction and 39.36 feet in the vertical direction, and the section is 49.21 feet thick (the simulated aquifer volume has three dimensions). The grid is shown in Figure 4. The horizontal permeability values assigned to the grid blocks are contoured in Figure 1. The contours were calculated using a computer program for contouring gridded data called CONTOUR developed by Harbaugh (1990). The permeability values range from 1.9 to 48.4 Darcies. They are compatible with sand and silty sand (Freeze and Cherry, 1979), but the sand is inhomogeneous with respect to permeability. The vertical permeability is half the horizontal permeability in all blocks. The porosity is 0.34 in all blocks. The mechanical dispersivity of the DNAPL phase is related to the properties of the porous medium. The longitudinal dispersivity for the PCE phase is 0.1 feet, and the transverse dispersivity is 0.03 feet. The same dispersivities are applied to the water phase. Considerable uncertainty is generally associated with dispersivity because it is affected by the scale of the model, phase saturation, porous media texture, and grid block size. The boundary conditions of the model are set up so that all blocks remain completely saturated and there is a pressure gradient producing flow in the x-direction (left to right) of 0.328 psi across the model. Since the vadose zone is not inlcuded, there is no simulation of the effect of volitization above the water table.