UTCHEM Model of DNAPL Contamination of an Aquifer
By Darrel Dunn, Ph.D., PG, Hydrogeologist
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 (142). 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 (143). The permeability values range from 1.9 to 48.4 Darcies. They are compatible with sand and silty sand (Freeze and Cherry, 66), 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.
Figure 1. Permeability contours and 10 percent PCE saturation isosurfaces for the inhomogeneous sand aquifer.
The model simulates a PCE spill into a fresh water aquifer (TDS, 630 mg/l). PCE (C2Cl4) is an acronym for perchloroethylene (aka tetrachloroethylene, tetrachloroethene, perc, and other names). It is a volatile chlorinated hydrocarbon solvent. Its major use is as a dry-cleaning fluid, but it has other uses as well. It is toxic, and its density (specific gravity 1.62) causes it to sink in a fresh water aquifer because it is over 60 percent heavier than water. Other properties of PCE relevant to this model, and included in the input file, are PCE/water interfacial tension, 47.5 dynes/cm; equilibrium concentration of PCE in water, 0.00015 volume fraction (v/v); PCE residual saturation, 0.0; PCE dynamic viscosity, 0.89 cP (water viscosity, 1 cP); PCE vertical pressure gradient, 0.7036 psi/ft; and molecular diffusivity of PCE in water, 0.0 (ie. diffusivity was ignored). Biological and chemical degradation of the dissolved PCE is not included in the simulation. Model input specifies the capillary pressure curve shown in Figure 2. Normalized saturation (Sn) is the water saturation excluding the residual (irreducible) water saturation (142). The slope of the capillary curve is related to the pore-size distribution. The relative permeability curve for the PCE phase is shown in Figure 3. It is held constant. PCE is the non-wetting phase. These are Corey-type curves (144). Both phases are treated as incompressible. The irreducible water saturation is 0.24. The spill is simulated by injection into the top block in the center of the grid at 33.333 ft3/day for the entire 30-day spill.
Figure 2. Capillary pressure curve for UTCHEM simulation of PCE spill.
Figure 3. Relative permeability curve for UTCHEM simulation of PCE spill.
DNAPL SPILL, DAYS 1-30
Figure 1 illustrates the results of the simulation of the 30 day spill event. It shows the successive positions of the 0.1 PCE saturation isosurface (locus) at 5, 15, and 30 days while the spill is occurring. The downward percolation of the PCE does not appear to have been affected by the permeability variation. There is no suggestion of accumulation above the extensive horizontal layer of permeability less than 5 Darcies that is present above the tip of the 30-day isosurface. The 30-day isosurface extends to a depth of 34 feet, so the overall rate of downward penetration of this isosurface was greater than a foot per day.
Figure 4 shows the concentration of the 30-day spill event. PCE (v/v) dissolved in the aqueous phase at the end of the 30-day period of the spill. The red block is the source location. The 1.4E-3 (1.4x10-3) contour encloses the part of the aquifer where the concentration is close to the equilibrium with the oleic phase. The 3.0E-9 contour is the isosurface where the concentration is about the maximum contaminant level (MCL) set by the U.S. Environmental Protection Agency for public water systems. The dissolved PCE has spread farther in the down-gradient direction (about 26 feet at a depth of 7 feet) than in the up-gradient direction. At 30 days, 998.16 cubic feet of the 1000 cubic feet injected is still in the oleic phase. Only 0.18 percent of the PCE has partitioned to the aqueous phase.
Figure 4. Concentration of PCE dissolved in the aqueous phase at 30 days.
POST-SPILL DNAPL CONTAMINATION, DAYS 31-60
Figure 5 shows the successive positions of the 0.1 PCE oleic phase saturation isosurface at 35, 45, and 60 days. The DNAPL continued to move downward after the cessation of the spill, reaching the base of the aquifer at about 45 days after the beginning of the spill. The 60-day isosurface shows the oleic PCE starting to spread along the bottom. The spread is greater in the direction of groundwater flow. The downward migration of the DNAPL is irregular, and pockets of oleic PCE are left behind. The oleic saturation near the upper surface of the aquifer is decreasing.
Figure 5. Isosurfaces of 10 percent oleic PCE saturation at 35, 45, and 60 days.
Figure 6 shows the position of the 3.0E-9 v/v PCE concentration in the aqueous phase at 60 days from the beginning of the spill and 30 days from the cessation. Dissolved PCE has spread downward (as the oleic phase moved downward) and in the down-gradient direction.
Figure 6 Isosurface of the 3.0E-9 PCE concentration in the aqueous phase at 60 days.
At 60 days, 996.79 cubic feet of the 1000 cubic feet of PCE spilled is still in the oleic phase. Only 0.32 percent of the PCE has partitioned to the aqueous phase.
POST-SPILL DNAPL CONTAMINATION AT 300 DAYS
Figure 7 shows the positions of the 0.1 PCE oleic phase saturation isosurface at 60 days and 300 days. At 300 days, most of the oleic PCE has descended to the base of the aquifer and continued to spread (more in the down-gradient direction than in the up-gradient direction.).
Figure 7. Isosurfaces of the 10 percent PCE saturation at 60 and 300 days.
Figure 8 shows the position of the 3.0E-9 v/v PCE concentration in the aqueous phase at 60 and 300 days from the beginning of the spill. At 300 days, the aqueous concentration of PCE down-gradient from the spill location is all greater than 3.0E-9 v/v within the model boundaries. The concentration isosurface has moved up-gradient near the base of the aquifer where the oleic phase has spread upgradient.
Figure 8. Isosurfaces of the 3.0E-9 v/v aqueous PCE concentrations at 60 and 300 days.
At 300 days, 990.24 cubic feet of the 1000 cubic feet spilled is still in the oleic phase. Only 0.98 percent (less than 1 percent) of the PCE has partitioned to the aqueous phase.
Posted May 10, 2017.