A. Lynn Wood

Field‐scale evaluation of in situ cosolvent flushing for enhanced aquifer remediation

A comprehensive, field-scale evaluation of in situ cosolvent flushing for enhanced remediation of nonaqueous phase liquid (NAPL)-contaminated aquifers was performed in a hydraulically isolated test cell (about 4.3 m × 3.6 m) constructed at a field site at Hill Air Force Base, Utah. This sand-gravel-cobble surficial aquifer, underlain by a deep clay confining unit at about 6 m below ground surface, was contaminated with a multicomponent NAPL as a result of jet fuel and chlorinated solvent disposal during the 1940s and 1950s. The water table within the test cell was raised to create a 1.5 m saturated flow zone that contained the NAPL smear zone. The cosolvent flushing test consisted of pumping about 40,000 L (approximately nine pore volumes) of a ternary cosolvent mixture (70% ethanol, 12% n-pentanol, and 18% water) through the test cell over a period of 10 days, followed by flushing with water for another 20 days. Several methods for assessing site remediation yielded consistent results, indicating that on the average >85% mass of the several target contaminants was removed as a result of the cosolvent flushing; NAPL constituent removal effectiveness was greater (90–99+%) in the upper 1-m zone, in comparison to about 70–80% in the bottom 0.5-m zone near the clay confining unit. Various interacting factors that control the hydrodynamic sweep efficiency, and the NAPL removal effectiveness during cosolvent flushing in this unconfined aquifer are discussed.

Field test of high molecular weight alcohol flushing for subsurface nonaqueous phase liquid remediation

A pilot scale field test of non-aqueous phase liquid (NAPL) removal using high molecular weight alcohols was conducted at Operable Unit 1, Hill Air Force Base, Utah. Petroleum hydrocarbons and spent solvents were disposed of in chemical disposal pits at this site, and these materials are now present in the subsurface in the form of a light non-aqueous phase liquid (LNAPL). This LNAPL is a complex mixture of aromatic and aliphatic hydrocarbons, chlorinated solvents, and other compounds. The field experiment was performed in a 5 m by 3 m confined test cell, formed by driving interlocking sheet pile walls through the contaminated zone into an underlying clay. The test involved the injection and extraction of about four pore volumes (1 pore volume=7000 L) of a mixture of 80% tert-butanol and 15% n-hexanol. The contaminants were removed by a combination of NAPL mobilization and enhanced dissolution, and the results of postflood soil coring indicate better than 90% removal of the more soluble contaminants (trichloroethane, toluene, ethylbenzene, xylenes, trimethylbenzene, naphthalene) and 70–80% removal of less soluble compounds (decane and undecane). The results of preflood and postflood NAPL partitioning tracer tests show nearly 80% removal of the total NAPL content from the test cell. The field data suggest that a somewhat higher level of removal could be achieved with a longer alcohol injection.

Controlled release, blind test of DNAPL remediation by ethanol flushing

A dense nonaqueous phase liquid (DNAPL) source zone was established within a sheet-pile isolated cell through a controlled release of perchloroethylene (PCE) to evaluate DNAPL remediation by in-situ cosolvent flushing. Ethanol was used as the cosolvent, and the main remedial mechanism was enhanced dissolution based on the phase behavior of the water-ethanol-PCE system. Based on the knowledge of the actual PCE volume introduced into the cell, it was estimated that 83 L of PCE were present at the start of the test. Over a 40-day period, 64% of the PCE was removed by flushing the cell with an alcohol solution of approximately 70% ethanol and 30% water. High removal efficiencies at the end of the test indicated that more PCE could have been removed had it been possible to continue the demonstration. The ethanol solution extracted from the cell was recycled during the test using activated carbon and air stripping treatment. Both of these treatment processes were successful in removing PCE for recycling purposes, with minimal impact on the ethanol content in the treated fluids. Results from pre- and post-flushing partitioning tracer tests overestimated the treatment performance. However, both of these tracer tests missed significant amounts of the PCE present, likely due to inaccessibility of the PCE. The tracer results suggest that some PCE was inaccessible to the ethanol solution which led to the inefficient PCE removal rates observed. The flux-averaged aqueous PCE concentrations measured in the post-flushing tracer test were reduced by a factor of 3 to 4 in the extraction wells that showed the highest PCE removal compared to those concentrations in the pre-flushing tracer test.

COSOLVENT EFFECTS ON SORPTION AND MOBILITY OF ORGANIC CONTAMINANTS IN SOILS

Abstract Batch equilibrium and column miscible displacement techniques were used to investigate the influence of an organic cosolvent (methanol) on the sorption and transport of three hydrophobic organic chemicals (HOCs) — naphthalene, phenanthrene, and diuron herbicide — in a sandy surface soil (Eustis fine sand). Equilibrium sorption constant (K) values calculated from batch and column data exhibited an inverse log-linear dependence on the volume fraction (fc) of methanol in the mixed solvent. The slope of the log-linear plot was approximately equal to the logarithm of the ratio of the HOC solubilities in neat cosolvent and water. K values obtained from breakthrough curves (BTCs) were comparable to those estimated from equilibrium sorption isotherms. Long-term exposure to methanol-water mixtures had little effect on sorption and transport properties of the soil, but column retardation factors were influenced by the short-term solvent exposure history prior to solute elution.

Changes in contaminant mass discharge from DNAPL source mass depletion: Evaluation at two field sites

Changes in contaminant fluxes resulting from aggressive remediation of dense nonaqueous phase liquid (DNAPL) source zone were investigated at two sites, one at Hill Air Force Base (AFB), Utah, and the other at Ft. Lewis Military Reservation, Washington. Passive Flux Meters (PFM) and a variation of the Integral Pumping Test (IPT) were used to measure fluxes in ten wells installed along a transect down-gradient of the trichloroethylene (TCE) source zone, and perpendicular to the mean groundwater flow direction. At both sites, groundwater and contaminant fluxes were measured before and after the source-zone treatment. The measured contaminant fluxes (J; ML(-2)T(-1)) were integrated across the well transect to estimate contaminant mass discharge (M(D); MT(-1)) from the source zone. Estimated M(D) before source treatment, based on both PFM and IPT methods, were approximately 76 g/day for TCE at the Hill AFB site; and approximately 640 g/day for TCE, and approximately 206 g/day for cis-dichloroethylene (DCE) at the Ft. Lewis site. TCE flux measurements made 1 year after source treatment at the Hill AFB site decreased to approximately 5 g/day. On the other hand, increased fluxes of DCE, a degradation byproduct of TCE, in tests subsequent to remediation at the Hill AFB site suggest enhanced microbial degradation after surfactant flooding. At the Ft. Lewis site, TCE mass discharge rates subsequent to remediation decreased to approximately 3 g/day for TCE and approximately 3 g/day for DCE approximately 1.8 years after remediation. At both field sites, PFM and IPT approaches provided comparable results for contaminant mass discharge rates, and show significant reductions (>90%) in TCE mass discharge as a result of DNAPL mass depletion from the source zone.

Evaluation of in situ cosolvent flushing dynamics using a network of spatially distributed multilevel samplers

A network of multilevel samplers was used to evaluate the spatial patterns in contaminant extraction during an in situ cosolvent flushing field test. The study was conducted in an isolation test cell installed in a fuel contaminated site at Hill Air Force Base, Utah. Partitioning tracer tests, conducted before and after the cosolvent flush, were used to estimate the spatial distribution of nonaqueous phase liquids (NAPL) and the effectiveness of cosolvent flushing for removing NAPL. Samples collected during the cosolvent flushing test were used to visualize the extraction process. The results of these two analyses showed similar spatial trends in mass removal and were in general agreement with observations based on soil core data. In general, the cosolvents were more effective in the upper portion of the flow domain and had slightly lower mass removal effectiveness in the lower portion of the flow domain. In this region, tracers indicated slower transport rates and higher NAPL saturations. The spatial analysis also indicated that cosolvent was trapped in the capillary fringe increasing the time required to displace the cosolvent from the aquifer. These results demonstrate the value of spatial information for performance assessment and improving in situ flushing design strategies.

Remediation of NAPL Source Zones: Lessons Learned from Field Studies at Hill and Dover AFB

Innovative remediation studies were conducted between 1994 and 2004 at sites contaminated by nonaqueous phase liquids (NAPLs) at Hill and Dover AFB, and included technologies that mobilize, solubilize, and volatilize NAPL: air sparging (AS), surfactant flushing, cosolvent flooding, and flushing with a complexing-sugar solution. The experiments proved that aggressive remedial efforts tailored to the contaminant can remove more than 90% of the NAPL-phase contaminant mass. Site-characterization methods were tested as part of these field efforts, including partitioning tracer tests, biotracer tests, and mass-flux measurements. A significant reduction in the groundwater contaminant mass flux was achieved despite incomplete removal of the source. The effectiveness of soil, groundwater, and tracer based characterization methods may be site and technology specific. Employing multiple methods can improve characterization. The studies elucidated the importance of small-scale heterogeneities on remediation effectiveness, and fomented research on enhanced-delivery methods. Most contaminant removal occurs in hydraulically accessible zones, and complete removal is limited by contaminant mass stored in inaccessible zones. These studies illustrated the importance of understanding the fluid dynamics and interfacial behavior of injected fluids on remediation design and implementation. The importance of understanding the dynamics of NAPL-mixture dissolution and removal was highlighted. The results from these studies helped researchers better understand what processes and scales are most important to include in mathematical models used for design and data analysis. Finally, the work at these sites emphasized the importance and feasibility of recycling and reusing chemical agents, and enabled the implementation and success of follow-on full-scale efforts.

Enhanced removal of DNAPL trapped in porous media using simultaneous injection of cosolvent with air: influencing factors and removal mechanisms

Factors influencing dense non-aqueous phase liquid (DNAPL) removal by concurrent injection of cosolvent and air were evaluated using micromodels and visualization techniques. Cosolvent (ethanol/water) was injected simultaneously with air into glass micromodels containing residual perchloroethylene (PCE). Impacts of the air flow rates and PCE solubility in the remedial fluid on PCE removal processes were examined. Although two major processes, immiscible displacement and dissolution, may contribute PCE removal from porous media during cosolvent-air (CA) flooding, PCE displacement occurred only in the initial flooding period and was independent of the air flow rate and ethanol content. However, faster airflow through the porous medium improved remedial fluid distribution and dynamics and resulted in enhanced dissolution of the DNAPL. Dissolution rates were directly related to PCE solubility in the remedial fluid. Enhanced contact between cosolvent and DNAPL during CA flooding was observed in a non-homogeneous micromodel with random flow paths.

Effects of pure and dyed PCE on physical and interfacial properties of remedial solutions

Hydrophobic dyes have been used to visually distinguish dense non-aqueous phase liquid (DNAPL) contaminants from background aqueous phases and soils. The objective of this study was to evaluate the effects of a dyed DNAPL, 0.5 g Oil-Red-O/l of PCE, on the physical properties of remedial solutions: water, co-solvents (50, 70, and 90% (v/v) ethanol), and surfactants (4% (w) sodium dihexyl sulfosuccinate). This study compared the densities, viscosities, and interfacial tensions (IFTs) of the remedial solutions in contact with both dyed and undyed PCE. The presence of the dye in PCE substantially alters the IFTs of water and ethanol solutions, while there is no apparent difference in IFTs of surfactant solutions. The remedial solutions saturated with PCE showed higher viscosities and densities than pure remedial solutions. Solutions with high ethanol content exhibited the largest increases in liquid density. Because physical properties affect the flow of the remedial solutions in porous media, experiments using dyed DNAPLs should assess the influence of dyes on fluid and interfacial properties prior to remediation process analysis.

The uncertainty of mass discharge measurements using pumping methods under simplified conditions.

Mass discharge measurements at contaminated sites have been used to assist with site management decisions, and can be divided into two broad categories: point-scale measurement techniques and pumping methods. Pumping methods can be sub-divided based on the pumping procedures used into sequential, concurrent, and tandem circulating well categories. Recent work has investigated the uncertainty of point measurement methods, and to a lesser extent, pumping methods. However, the focus of this study was a direct comparison of uncertainty between the various pumping method approaches that have been used, as well as a comparison of uncertainty between pumping and point measurement methods. Mass discharge measurement error was investigated using a Monte Carlo modeling analysis as a function of the contaminant plume position and width, and as a function of the pumping conditions used in the different pumping tests. Results indicated that for the conditions investigated, uncertainty in mass discharge estimates based on pumping methods was 1.3 to 16 times less than point measurement method uncertainty, and that a sequential pumping approach resulted in 5 to 12 times less uncertainty than the concurrent pumping or tandem circulating well approaches. Uncertainty was also investigated as a function of the plume width relative to well spacing. For a given well spacing, uncertainty decreased for all methods as the plume width increased, and comparable levels of uncertainty between point measurement and pumping methods were obtained when three wells were distributed across the plume. A hybrid pumping technique in which alternate wells were pumped concurrently in two separate campaigns yielded similar uncertainty to the sequential pumping approach. This suggests that the hybrid approach can be used to capitalize on the advantages of sequential pumping yet minimize the overall test duration.