DNAPL Remediation - Pump and Treat (T)
UTCHEM MODEL OF DNAPL REMEDIATION WITH A PUMP AND TREAT SYSTEM
By Darrel Dunn Ph.D., PG (Consulting hydrogeologist. Professional Synopsis)
INITIAL DNAPL CONTAMINATION
This webpage describes the UTCHEM simulation of Pump and Treat (PT) remediation of the hypothetical 30-day PCE spill described on the webpage titled DNAPL Contamination. The liquid PCE distribution at the end of the spill is characterized by the 10 percent locus shown on Figure 1. This purple contour encloses the part of the aquifer where the oleic phase PCE fills more than 10 percent of the pore space. Figure 1 also shows the inhomogeneity of the aquifer with respect to permeability.
Figure 1. Initial oleic PCE saturation characterized by the 10 percent contour (purple), plus permeability contours.
PUMP AND TREAT SYSTEM
The hypothetical pump and treat (PT) system used to illustrate the effects of this type of remediation on oleic PCE saturation and aqueous PCE concentration is indicated in Figure 3. It has been called a divergent line-drive well pattern. In practice, a line of extraction wells is constructed on opposite sides of the contaminated area, and a line of injection wells is placed in the contaminated area between the extraction wells. One advantage of this configuration is that it can provide hydraulic containment of the mobilized contaminants. It also minimizes any potential for stagnant flow zones between the extraction wells. The system simulated for this webpage assumes that 2000 cubic feet per day (10.39 gpm) of potentially contaminated water is removed by each extraction well (Well 1 and Well 3), treated to pure water, and injected at a rate of 4000 cubic feet per day (20.78 gpm) in the middle of the contaminated area (Well 2). The two extraction wells are located 29.5 and 52.5 feet up-gradient and down-gradient from the injection well, which is located at the spill site (Figure 3). All three wells completely penetrate the aquifer and are open to the entire thickness of the aquifer. The simulated radius of the wells is 1 foot.
PUMP AND TREAT RESULTS, 30 DAYS
Figures 2 and 3 provide a comparison of the movement of the oleic phase PCE after cessation of the spill without PT and with PT. Figure 2 shows the locus of the contours enclosing the part of the aquifer containing oleic PCE saturations greater than 10 percent at successive times of 5, 15, and 30 days after the cessation of the spill without PT. Figure 3 shows the same contours with the PT system put into operation at the cessation of the spill. The effect of the injection well is to reduce the oleic PCE saturation near the well at 5 and 15 days, but downward migration results in accumulation at the bottom of the aquifer similar to the accumulation without injection. Without PT, 996.79 cubic feet of oleic PCE remains at 30 days after cessation of the spill. With PT, this amount is reduced to 979.95 cubic feet. The amount of oleic PCE in the aquifer at the cessation of the spill and the beginning of the PT was 998.15 cubic feet of the original 1000 cubic feet spilled.
Figure 2. Isosurfaces of 10 percent oleic PCE saturation 5, 15, and 30 days after the spill with no PT.
Figure 3. Isosurfaces of 10 percent oleic PCE saturation 5, 15, and 30 days after the spill ceased and PT started.
Figure 4 shows the position of the 3.0E-9 v/v PCE concentration isosurface in the aqueous phase after 30 days of PT compared to the same isosurface at that time without PT. The effect of the injection and pumping is the spread the dissolved PCE outward from the injection well. Indeed the isosurface moved about 10 feet past the downgradient pumping well and even farther past the upgradient pumping well. The pumping rates of the wells are not great enough to develop strong horizontal pressure gradients to capture all of the dissolved PCE. However, the injection rate in Well 2 is great enough to create horizontal pressure gradients away from the well extending to both lateral boundaries.
Figure 4. Comparison of the with and without PT isosurfaces of 3.0E-9 v/v concentration of PCE in the aqueous phase at 60 days after beginning of the spill.
PUMP AND TREAT RESULTS, 300 DAYS
Figure 5 shows the locus of the contours enclosing the part of the aquifer containing oleic PCE saturations greater than 10 percent at 30 and 300 days after the cessation of the spill with PT. The saturation is reduced at the injection well and the oleic PCE has spread along the base of the aquifer. It has passed the upgradient pumping well. It has spread farther in the downgradient direction, but the 10 percent saturation has not passed the downgradient pumping well. This combination of well location, injection rate, and extraction rates allows PCE to escape past the extraction wells. It does not form a closed system, in that hydraulic containment is not provided.
Figure 5. Comparison of 10 percent oleic PCE saturation 30 days and 300 days after cessation of spill and start of remediation.
PUMP AND TREAT, CLOSED SYSTEM
A closed system providing hydraulic containment would require one or more of the following changes: (1) reduced injection rate in Well 2, (2) increased extraction rate in Well 1 and/or Well 3, or (3) increased distance from the injection well to Well 1 and/or Well 3. Figure 6 shows successive aqueous PCE concentration for a closed system formed by reducing he injection rate in Well 2 to 2000 cubic feet per day, increasing the extraction rates in Well 1 and Well 3 to 4000 cubic feet per day, and moving Well 1 farther from the injection well. This scenario assumes that 6000 cubic feet per day of treated water is discharged to waste rather than being injected. The dissolved PCE reaches both extraction wells within 30 days and then the 3.0E-9 v/v concentration isosurfaces stabilize near the extraction wells. They do not change position between 150 and 300 days, indicating that hydraulic containment is provided.