Technology Status Report: In Situ Flushing - CLU-IN

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In Situ Flushing Project Summaries GWRTAC Case Study Database saturation was detected at less than one meter above the confining clay unit, in fine-grained sands with a hydraulic conductivity from 1 to 3 x 10-2 cm/s (permeabilities of 10 to 30 darcy) . From one to one and a half meters above the clay unit, no DNAPL was detected in the coarse sands (K >10-1 cm/s). DNAPL has not been detected in the clay unit. Initial borings and a partitioning interwell tracer test (PITT) indicated that about 0.079 m3 (21 gallons) of contaminant was present initially. This amount corresponds to 668 mg/kg soil. Thus, the demonstration area was not that of a pool but of a migration path of the contaminant to known pools located nearby which were sites for testing other remediation technologies. Subsurface heterogeneity is an important factor in the ultimate utility of surfactant-enhanced remediation. In a heterogeneous aquifer, surfactant and cosolvent solutions flow through the more conductive zones. After flushing with a surfactant or cosolvent solution in a heterogeneous aquifer, the NAPL mass remaining in the less permeable zones can still serve as a continuing source. As NAPL is removed from a high conductivity zone, the relative permeability of the high conductivity zone to the aqueous phase will increase, making the possibility of flushing the lower conductivity zone even more remote. If surfactant can be directed to more zones of a heterogeneous aquifer through mobility control, the sweep efficiency of surfactant-enhanced remediation will be improved. Less surfactant might be required to achieve the same level of NAPL removal or perhaps greater overall NAPL mass removal could be achieved. In the surfactant/foam process, alternating pulses of surfactant and air are used to create an in situ "foam". The first pulse of surfactant flows through the more permeable zones and removes residual NAPL. When air is next pulsed, it also flows into the more permeable zone, forming a foam. The third pulse, which is of surfactant, is now directed to the finer-grained soils with residual NAPL, because the foam is blocking pores in the more permeable zone. In this manner, the sweep efficiency of the surfactant/foam process is improved over that of the typical addition of surfactant to the subsurface. The surfactant/foam process was first optimized and tested in the lab, scaling up from a column to 2D models of increasing size and complexity. Regulation of pressure, rather than velocity, was determined to produce the optimum generation and propagation of foam in the laboratory-scale models. The surfactant system selected for the field test was MA-80, a dihexyl sulfosuccinate, at 3.5% and 10,250 ppm NaCl. The field test area was 6.1 m long x 4 m (20.0 x 13.1 feet) wide. The site contained three injection wells, three extraction wells, and two multilevel monitoring wells. The monitor wells were located near the center of the channel, 1/3 and 2/3 of the distance from the central injection well to the central extraction well. The injectors and extractors had a 1.5 m (4.9 foot) screened interval. The multilevel wells had three screened intervals: 0.3 m (0.98 ft), 1.2 to 1.8 m (3.9 to 5.9 ft), and 3 to 4 m (9.8 to 13.1 ft) above the clay unit. There were also two hydraulic control wells, one 3 m (9.8 ft) upgradient and one 3 m (9.8 ft) downgradient of the central injector and extractor, respectively. During installation of the wells, a small depression in the aquitard surface was discovered in the center of the well pattern with significant DNAPL contamination (>12% saturation). The removal of DNAPL from the test area was assessed by two measures, soil core analysis and partitioning tracer tests before and after the surfactant/foam flood. Fluid samples from the injection and extraction wells allowed the determination of a mass balance on the injected fluids. The multilevel wells were used to assess the propagation of the foam through the test area. Numerical Ground-Water Remediation Technologies Analysis Center Operated by Concurrent Technologies Corporation Appendix - Page 112 of 164 Copyright GWRTAC 1998 Revision 1 Tuesday, November 17, 1998
In Situ Flushing Project Summaries GWRTAC Case Study Database simulation aided in both the design and interpretation of the field test. At the outset of the field test, one-half pore volume of partitioning tracers, isopropanol and heptanol, was injected to estimate DNAPL mass in the test area. The tracer test was followed by a water flood (4.9 pore volumes) and a NaCl flood (1%, 1.1 pore volumes). After the injection of onehalf pore volume of surfactant, air injection (for two-hour durations each) alternated at each injection well. Air was injected through the upper portion of the screened interval to create a foam in the high permeability zone, while surfactant continued to be injected through the base of the screen. Approximately three pore volumes (total) of the surfactant solution, 3.5% sodium dihexyl sulfosuccinate and 10,250 ppm NaCl, was added. The surfactant solution also contained a conservative tracer. The surfactant/foam field test concluded with a one-half pore volume flood of 0.8% NaCl followed by a water flood (8.4 pore volumes) to break the foam. The postsurfactant/foam flood tracer test (1 pore volume of isopropanol and octanol) was followed by a 6.3 pore volume water flood. Both the soil core and partitioning tracers yielded similar estimates of initial DNAPL contamination in the test area. The core analysis estimated that 74.5 L to 107 L (20 to 28 gal) of DNAPL were present; the tracer analysis estimated 79.1 + 26.8 L (21+7 gal). A mass balance on the fluids that were injected during the field test showed that 99% of the surfactant, 110% of the NaCl, 84% of the pre-flood tracers, and 82% of the post-flood tracers were recovered. An increase in injection pressure and the production of foam from multiple levels of the monitor wells were evidence of the in situ generation of foam. The conservative tracer injected with the surfactant solution showed that there was a 50% reduction of the swept volume of the aquifer, which is another indication that foam was in place. (Subsequent waterflooding broke the foam and restored the swept volume to its initial value, as shown by the final PITT). During transport through the subsurface, the surfactant/NaCl solution underwent ion exchange with calcium present in the clays, resulting in a slightly over-optimum formulation and the production of a DNAPL-rich, high density microemulsion. One hundred and thirty nine L (36.5 gal) of DNAPL was produced in addition to the 42 L (11 gallons) of dissolved contaminant that would have been produced in a waterflood. Since this DNAPL recovery exceeds the 79 L (21 gallon) estimate of initial volume present, even allowing for uncertainty of 27 L (7 gallons) in the latter, it is thought that additional contaminant entered the pattern from the region beyond the injection wells, a conclusion supported by data from the extraction and monitoring wells. Some DNAPL remained in the center of the pattern, as shown by the detection of DNAPL in a post-flood soil core and in the two wells in the deepest part of the channel. Soil cores showed that 6.1 L (1.6 gal) of DNAPL remained in test pattern; partitioning tracers indicated that 9.8 + 7.6 L (2.6 + 2.0 gal) DNAPL remained. The final DNAPL concentration in the pattern averaged 0.03% or 77 mg/kg. This first field test of the surfactant/foam process demonstrated that foam could be generated in situ and propagated; the surfactant/foam was also responsible for an approximately 89% reduction in DNAPL in the test area. Report(s)/Publication(s) (Additional Information Sources): AATDF Spring 1998 Newsletter: Summary "Surfactant/Foam Process for Aquifer Remediation" Hirasaki, G.J., Miller, C.A., Szafranski, R., Lawson, J.B., and Akiya, N. 1996. "Surfactant/Foam Process for Aquifer Remediation," SPE paper no. 37257. To be presented at the SPE International Symposium on Oil Field Chemistry, Houston, Texas, February 18-21, 1997. Ground-Water Remediation Technologies Analysis Center Operated by Concurrent Technologies Corporation Appendix - Page 113 of 164 Copyright GWRTAC 1998 Revision 1 Tuesday, November 17, 1998
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Technology Status Report Technology
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ABSTRACT This technology status rep
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LIST OF TABLES Table Title Page 1 I
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1.0 INTRODUCTION / PURPOSE OF STATU
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a laboratory-scale project. As illu
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3.1 Flushing Solutions 3.0 ANALYSIS
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(approximately eight) full-scale pr
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Table 1. In Situ Flushing - Summary
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Table 3 In Situ Flushing - Geology
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Table 4 In Situ Flushing - Recovere
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Full-Scale/Commercial 26% Figure 2.
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Site Remediation 25% Figure 4. In S
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NAPL Removal 34% Figure 6. In Situ
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Figure 8. In Situ Flushing - Distri
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Number of Projects 35 30 25 20 15 1
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Number of Case Studies 45 40 35 30
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25 ft and 50 ft and 10 ft and 100 f
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Unconfined Aquifer 65% Figure 16. I
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In Situ Flushing Project Summaries
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