Technology Status Report: In Situ Flushing - CLU-IN

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In Situ Flushing Project Summaries GWRTAC Case Study Database surface (bgs). At Hill AFB, OU1, the LNAPL has over 200 identified components. The major constituents of the Hill OU1 LNAPL on a mass basis are decane, 1,3,5-trimethyl benzene and undecane. Seasonal fluctuations in the water table smeared the LNAPL over the interval 5 m (16.4 feet) bgs to 8 m (26.2 feet) bgs. The sheet pile-enclosed cell measured 2.8 m x 4.6 m (9.2 x 15.1 feet) and penetrated 3.7 m (12.1 feet) into the clay confining unit. Leak testing of the sheet pile cell confirmed that leakage of the sheet pile cell was insignificant. The water table within the test cell was maintained at 4.9 m bgs. The test cell held fifteen multilevel samplers, three extraction wells and four injection wells. The injection wells and extraction wells were screened from 4.9 m to 7.9 m (16.1 to 25.9 feet) bgs. At each of the fifteen multilevel sampler locations, from five to eight individual samplers were bundled to yield a total of 96 multilevel sampling points. Peristaltic pumps maintained a flow rate of approximately one pore volume per day. The extent of contamination within the test cell was assessed by collecting soil samples during well installation and analyzing for a selected group of target analytes present in the NAPL. The test cell contamination was also characterized by conducting a 10-pore volume tracer test with a group of partitioning and non-partitioning tracers. The test cell had an average NAPL saturation of about 0.06 prior to SPME flushing. Water samples were also collected and analyzed for the target analytes, however, nearly all the contaminants were below detection limits. Finally, before the SPME flood, contamination in the test cell was characterized using interfacial tracers. The method estimates the contact area between NAPL and water in the test cell. The interfacial tracer method, developed at the University of Florida, was tested for the first time in the SPME test cell at Hill AFB. The result indicated that the NAPL-water contact area can vary more than one order of magnitude within the test cell. This variability in NAPL-water contact area could have significant impacts on in-situ flushing extraction efficiency. If there are mass transfer limitations that are magnified with low NAPL-water contact area this will reduce flushing efficiency, or if the NAPL is simply non-uniformly distributed, as evidenced by a low NAPL-water contact area, this can lead to inefficiencies. The SPME flood was conducted over a 18-day period by pumping nine pore volumes of the precursor solution with some flow interruption periods. In the field test, a non-reactive tracer test was conducted first to determine the hydrodynamic performance of the cell and then set the flow rate for the rest of the flushing test. Next, a 0.16 pore volume pulse of partitioning tracers, methanol (2700 mg/L) and 2, 2-dimethyl 3-pentanol (930 mg/L), flowed through the sheet pileenclosed cell before the SPME test. The distribution and morphology of NAPL in the test cell was also characterized with interfacial tracers that were first developed and tested during this project. After the pre-flood tracer tests, 3.5 pore volumes of water were flowed through the test cell. Nine pore volumes of the surfactant and alcohol mixture (3% Brij 97 (polyoxyethylene (10) oleyl ether) and 2.5% n-pentanol) were perfused through the test cell. The viscosity of the surfactant + alcohol mixture, measured in the field, was 1.66 centipoise. The surfactant/alcohol mixture was followed by surfactant alone, a water flood, and a final partitioning tracer test. To assess NAPL removal, the investigators collected soil cores before and after the addition of the surfactant and alcohol mixture; they also conducted pre- and post-flush NAPL partitioning tracer tests. Fluid samples were collected from the extraction wells (540 total) and multilevel samplers (6500 total). Eighty-five pre-flush soil core samples and 40 post flush soil core samples were collected from the test cell. Soil cores were sub-sampled and immediately extracted in the field. Soil core and fluid samples were analyzed for target LNAPL components (p-xylene, n-decane, nundecane, n-dodecane, n-tridecane, 1, 2 - dichlorobenzene, 1, 2, 4 - trimethylbenzene, Ground-Water Remediation Technologies Analysis Center Operated by Concurrent Technologies Corporation Appendix - Page 38 of 164 Copyright GWRTAC 1998 Revision 1 Tuesday, November 17, 1998
In Situ Flushing Project Summaries GWRTAC Case Study Database naphthalene, trichlorobenzene). The use of multiple methods of evaluation provides a more comprehensive assessment of the technology and therefore better comparison with other technologies tested in the SERDP project. During the SPME flushing study, samples of effluent (combined from the three extraction wells) were collected for a laboratory investigation of methods to minimize and reduce cost of waste management. The use of salt to separate the waste into oil and water rich phases for more economic disposal was investigated at a small pilot scale. Following the SPME flood, a post-flushing partitioning tracer test was conducted. The method and tracers used were the same as the pre-flushing test. The average NAPL saturation in the test cell following the SPME flood was 0.018, producing a NAPL reduction of about 72%. After the tracer test was completed, post-flushing core samples were collected. The core samples were compared to average values obtained during the pre-flush sampling to estimate mass removal for each target analyte. The mass removal based on core estimates ranged from 64 to 96%, but the three largest constituents present in the NA.PL, n-undecane, n-decane, and 1,3,5-trimethylbenzene, were removed more than 90%. The final method used to evaluate the SPME effectiveness was a mass balance on the target analytes. Using estimates of initial mass from the core data and partitioning tracers, the mass recoveries were calculated. Using the partitioning tracers to estimate initial mass, the mass removal estimates ranged from 62 to 82%. When the core data are used to estimate the initial mass only two constituents, n-decane and n-undecane, can be compared for removal effectiveness and the removals for these were 93 and 105%, respectively. Once again, an apparent difference in the effectiveness is seen between the results based on tracers and soil cores. A pitch fraction model of the NAPL is proposed as a likely explanation for the differences observed. The methods outlined above demonstrate that the SPME process was capable of affecting significant mass removal of the complex multi-component NAPL. Simulations of the SPME flushing study were conducted following the experiment. In this effort both the tracer tests and the SPME flushing results were used. The simulator UTCHEM was used. The model was capable of simulating the processes observed in the experimental data. It was evident that the model required the use of a mass transfer term to capture the observed behavior. While the model simulation may have limited use in helping to design experiments conducted in a controlled test cell, they can be very important for designing hydraulically controlled systems. Of greater utility are simulations to assess cost and efficiencies of the SPME process at larger scales than the test cell. Report(s)/Publication(s) (Additional Information Sources): AATDF Spring 1998 Newsletter: Summary "Lab and Field Evaluation of Single-Phase Microemulsions for Enhanced In-Situ Remediation of Contaminated Aquifers". Annable, M.D., Jawitz, J.W., Rao, P.S.C., and Rhue, R.D. 1996c. "Field Testing of a Surfactant- Alcohol Mixture for In-Situ NAPL Solubilization as a Microemulsion." Presented at the 2nd Annual Conference on Remediation, Houston, Texas, pp. 75-77. Ground-Water Remediation Technologies Analysis Center (GWRTAC), 1996: Surfactants/Cosolvents, Technology Evaluation Report TE-96-002, Surfactants/Cosolvents, Dr. Ground-Water Remediation Technologies Analysis Center Operated by Concurrent Technologies Corporation Appendix - Page 39 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.
Page 35 and 36: Site Remediation 25% Figure 4. In S
Page 37 and 38: NAPL Removal 34% Figure 6. In Situ
Page 39 and 40: Figure 8. In Situ Flushing - Distri
Page 41 and 42: Number of Projects 35 30 25 20 15 1
Page 43 and 44: Number of Case Studies 45 40 35 30
Page 45 and 46: 25 ft and 50 ft and 10 ft and 100 f
Page 47 and 48: Unconfined Aquifer 65% Figure 16. I
Page 49 and 50: In Situ Flushing Project Summaries
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