Residual Dense Nonaqueous Phase Liquid Research Articles

Pulsed Air Sparging in Aquifers Contaminated with Dense Nonaqueous Phase Liquids

AbstractAir sparging was evaluated for remediation of tetrachloroethylene (PCE) present as dense nonaqueous phase liquid (DNAPL) in aquifers. A two‐dimensional laboratory tank with a transparent front wall allowed for visual observation of DNAPL mobilization. A DNAPL zone 50 cm high was created, with a PCE pool accumulating on an aquitard. Detailed process control and analysis yielded accurate mass balances and insight into the mass‐transfer limitations during air sparging.Initial PCE recovery rates were high, corresponding to fast removal of residual DNAPL within the zone influenced directly by air channels. The vadose zone DNAPL was removed within a few days, and the recovery in the extracted soil vapors decreased to low values. Increasing the sparge rate and pulsing the air injection led to improved mass recovery, as the pulsing induced water circulation and increased the DNAPL dissolution rate. Dissolved PCE concentrations both within and outside the zone of air channels were affected by the pulsing. Inside the sparge zone, aqueous concentrations decreased rapidly, matching the declining effluent PCE flux. Outside the sparge zone, PCE concentrations increased because highly contaminated water was pushed away from the air injection point.This overall circulation of water may lead to limited spreading of the contaminant, but accelerated the time‐weighted average mass removal by 40% to 600%, depending on the aggressiveness of the pulsing. For field applications, pulsing with a daily or diurnal cycling time may increase the average mass removal rate, thus reducing the treatment time and saving in the order of 40% to 80% of the energy cost used to run the blowers. However, air sparging will always fail to remove DNAPL pools located below the sparge point because the air will rise upward from the top of a screen, unless very localized geological layers force the air to migrate horizontally. Unrecognized presence of DNAPL at chlorinated solvent sites residual and pools could potentially hamper success of air sparging cleanups, since the presence of small DNAPL pools, ganglia or droplets can greatly extend the treatment time.

DNAPL Removal from a Heterogeneous Alluvial Aquifer by Surfactant‐ Enhanced Aquifer Remediation

AbstractA demonstration of surfactant‐enhanced aquifer remediation (SEAR) was conducted at Hill Air Force Base (AFB), Utah. Two surfactant floods were undertaken in a test section of a heterogeneous alluvial aquifer with a hydraulic conductivity range of 2.8 to 8.6 ft/day (10‐5 to 10‐4 m/sec) and a pore volume of approximately 15,000 gallons (57 m3). The wellfield installed for the demonstration consisted of lines of three injection and three extraction wells, a central monitoring well, and a single hydraulic control well. No physical barriers to flow, such as sheet‐pile walls, were employed; surfactant flooding was controlled entirely by hydraulic manipulation of the flow field. The inter‐well distance between injectors and extractors was 20 feet (6 m); the distance between individual injectors and extractors in line was 10 feet (3 m). The water table was 25 feet (7.6 m) below ground surface with a saturated zone approximately 19‐feet thick (5.8 m). Residual dense nonaqueous phase liquids (DNAPL) remained in a zone of alluvium 42 to 46 feet (13 to 14 m) below ground surface following extraction of free‐phase DNAPL. The injectors and extractors were screened in this DNAPL zone. Three partitioning interwell tracer tests (PITTs) and two surfactant floods were conducted over four months during 1996. The surfactant floods removed 341 out of 346 gallons of residual DNAPL (1290 of 1310 L), according to the PITTs. This represents a total recovery of about 98.5% of the DNAPL volume present in the zone of residual DNAPL as determined by comparing the initial and final PITTs. There was no reduction in hydraulic conductivity due to colloid mobilization during the surfactant floods; in fact, the hydraulic gradient across the test zone decreased as the floods progressed. Concentrations of dissolved total chlorinated hydrocarbons in the test section decreased from 1000 mg/L before the floods to less than 10 mg/L following the floods. This demonstration is evidence of the technical practicability of DNAPL removal from alluvium.

A controlled field experiment on groundwater contamination by a multicomponent DNAPL: creation of the emplaced-source and overview of dissolved plume development

A unique field experiment has been undertaken at the CFB Borden research site to investigate the development of dissolved chlorinated solvent plumes from a residual dense non-aqueous phase liquid (DNAPL) source. The “emplaced-source” tracer test methodology involved a controlled emplacement of a block-shaped source of sand containing chlorinated solvents below the water table. The gradual dissolution of this residual DNAPL solvent source under natural aquifer conditions caused dissolved solvent plumes of trichloromethane (TCM), trichloroethene (TCE) and perchloroethene (PCE) to continuously develop down gradient. Source dissolution and 3-D plume development were successfully monitored via 173 multilevel samplers over a 475-day tracer test period prior to site remediation research being initiated. Detailed groundwater level and hydraulic conductivity data were collected. Development of plumes with concentrations spanning 1–700,000 μg/l is described and key processes controlling their migration identified. Plumes were observed to be narrow due to the weakness of transverse dispersion processes and long due to advection and significant longitudinal dispersion, very limited sorptive retardation and negligible, if any, attenuation due to biodegradation or abiotic reaction. TCM was shown to be essentially conservative, TCE very nearly conservative and PCE, consistent with its greater hydrophobicity, more retarded yet having a greater mobility than observed in previous Borden field tests. The absence of biodegradation was ascribed to the prevailing aerobic conditions and lack of any additional biodegradable carbon substrates. The transient groundwater flow regime caused significant transverse lateral plume movement, plume asymmetry and was likely responsible for most of the, albeit limited, transverse horizontal plume spreading. In agreement with the widespread incidence of extensive TCE and PCE plumes throughout the industrialized world, the experiment indicates such solvent plumes are likely to be highly mobile and persistent, at least in aquifers that are aerobic and have low sorption potential (low f oc content).

Chromatographic Alteration of a Nonionic Surfactant Mixture During Transport in Dense Nonaqueous Phase Liquid Contaminated Sediment

Chromatographic alteration of a nonionic surfactant mixture during transport through DNAPL-contaminated aquifer sediment may occur due to differential loss of oligomers to sediment and to dense nonaqueous phase liquid (DNAPL). These losses may significantly alter the solubilizing properties of the mixture, which is a concern when the surfactant mixture is applied for the purpose of DNAPL solubilization. This study examined a nonionic surfactant mixture, which was characterized in terms of oligomer distribution within the mixture and the ability of the mixture to solubilize residual DNAPL. Three sediment-packed columns were connected in series to represent up-gradient, residual, and down-gradient zones, respectively, of a DNAPL-contaminated site. In the up-gradient column, greater retardation of high ethoxylate (EO) content oligomers was observed relative to low EO content oligomers, due to preferential sorption of high EO content oligomers by the sediment. In the residual-zone column, much greater retardation of low EO content oligomers relative to high EO content oligomers occurred, due to preferential sorption of low EO content oligomers to residual DNAPL. In the down-gradient column, retardation of only the high EO content oligomers was observed, due to lack of sorption of low EO content oligomers to sediment. Surfactant losses to sediment and DNAPL delayed the solubilization of DNAPL due to reduction of surfactant concentration and overall increased polarity of the surfactant mixture. Increased solution flow rate decreased surfactant sorption but resulted in an overall decrease in the mass of DNAPL solubilized due to kinetic limitations in DNAPL solubilization.

The First Vadose Zone Partitioning Interwell Tracer Test for Nonaqueous Phase Liquid and Water Residual

In 1995, a partitioning interwell tracer test was conducted in the vadose zone beneath two buried organic liquid disposal trenches at Sandia National Laboratories in New Mexico. The purpose was to estimate the amount and distribution of trichloroethylene (TCE) trapped by capillary forces as residual dense nonaqueous phase liquid (DNAPL). Screened injection and extraction wells, placed 16.8 m apart, and two monitor wells with multilevel sampling capability allowed vertical testing from 3.0 to 24.4 m below ground surface. Seven tracers were injected, but the most useful tracers in the final analysis were sulfur hexafluoride (nonpartitioning), perfluoro-1,3,5-trimethylcyclohexane (TCE-partitioning), and difluoromethane (water-partitioning). Both a TCE-partitioning tracer and a water-partitioning tracer were needed to determine average TCE DNAPL saturation. Average saturations of DNAPL and water were measured to be 0.11 ± 0.02% and 23 ± 2.0%, respectively, in the shallow zone between 3.0 and 10.7 m. Monitor w...

Dissolution and mass transfer of multiple organics under field conditions: The Borden emplaced source

The process that transfers mass from a subsurface source zone of residual dense nonaqueous phase liquid (DNAPL) to the flowing groundwater is a controlling factor in determining the time required to dissolve the source by noninvasive means. While mass transfer can be kinetic or equilibrium under laboratory conditions, aqueous concentrations in the field are generally found to be below equilibrium levels. To gain insight into the mass transfer process under field conditions, we simulated the dissolution of the emplaced DNAPL source at the Canadian Forces Base Borden, Ontario, which contains a mixture of three DNAPLs. The simulations clearly show that mass transfer at this site is equilibrium‐controlled during the 1000‐day observation period and that apparent tailing of one of the organic components is due to its declining solubility, rather than mass transfer kinetics. Flow lines passing through the source are focused in a narrow streamtube downstream of the source, and equilibrium concentrations are therefore observed only at the center of the effluent plume. Since the concentration peaks can be easily missed in the sampling, streamline focusing can explain the low concentrations observed in the field.

Characterization of a TCE DNAPL Zone in Alluvium by Partitioning Tracers

AbstractPartitioning interwell tracer tests (PITT) were used ID determine the spatial distribution and volume of residual trichloroethene (TCE) present in alluvium beneath the Portsmouth Gaseous Diffusion Plain in southern Ohio. Its first use at this site was in support of the design of a surfactant flood to remove the residual DNAPL (dense nonaqueous phase liquids) from the alluvial aquifer. The second application assessed the performance of the surfactant flood. The average DNAPL saturation in the first PITT was 0.1 to 0.2% in a swept pore volume of 4500 gallons (17.000 L). A second PITT was undertaken following the surfactant flood and yielded an average residual saturation of 0.06% in a swept pore volume of 3400 gallons (13.000 L), the reduction in pore volume being due to the confinement of the tracers to the lower sand and gravel unit of the alluvium. The design, operation, and analysis of the two PM Is provided strong evidence of a buried channel that controls the spatial distribution of the residual TCI: DNAPL in the basal sand and gravel aquifer and must be considered in the eventual full remediation of this aquifer.

Surfactant enhanced removal of PCE in a partially saturated, stratified porous medium

Dense nonaqueous phase liquids (DNAPLs), such as perchloroethylene (PCE), pose a significant threat to the environment, specifically to our drinking water present in aquifers. In many instances, a DNAPL will become trapped and form pools on top of confining layers while migrating through the vadose zone. Residual DNAPL and DNAPL pools form sources of long term contamination, which are difficult to remove by classical pump-and-treat remediation. A physically simulated PCE spill into a large, two-dimensional flow container, packed with a fine sand layer surrounded by coarse sand, was therefore studied in this work. A water table was maintained near the bottom of the flow container, such that the coarse sand just below the fine layer was unsaturated, but the bottom of the fine sand layer was at or near saturation. A PCE spill was applied at the center of the porous medium's surface and allowed to redistribute until static equilibrium was reached. The porous medium was then flushed with a surfactant solution (Triton X-100, 4.0% by volume), using the same application configuration as for the spill, while simultaneously extracting solution at one or more locations at the bottom of the porous medium. Effluent samples were analyzed for PCE concentrations. Volumetric water and PCE content values were determined several times at a number of locations by means of dual-energy gamma radiation measurements. The coarse sand in the upper portion of the porous medium was cleaned rapidly by the surfactant flushing. Removal of PCE from the fine sand layer, however, proved to be more difficult because most of the surfactant solution bypassed the PCE. Lateral spreading of the surfactant plume occurred near the saturated/nearly saturated zones in the fine and coarse sand. This lateral spreading, unfortunately, allowed for solubilized PCE, the concentration of which was enhanced by micelle formation, to move into regions previously uncontaminated. The pumping configuration was subsequently changed in an attempt to force the surfactant solution through the entire fine layer, i.e., to hydraulically control the surfactant flushing, while simultaneously increasing the amount of solubilized PCE.

Aquifer washing by micellar solutions:: 3 Field test at the Thouin Sand Pit (L'Assomption, Québec, Canada)

A field test was performed to recover DNAPL (Dense Non Aqueous Phase Liquid) in a shallow aquifer at the Thouin Sand Pit near Montreal, to evaluate a new technique of aquifer restoration involving surfactant solutions. Laboratory tests have shown that washing solutions containing alcohols, surfactants, and solvents are very efficient in recovering DNAPL as a miscible phase. The Thouin field test was designed to: (1) study in situ recovery of DNAPL; (2) evaluate an injection–pumping strategy; (3) test the use of polymer solutions to control the mobility of a washing solution slug and to improve the vertical sweep efficiency throughout the sand unit. The test was performed in a shallow medium sand aquifer containing both contaminated saturated and unsaturated zones. The washing experiment was done on 17 m 3 of the saturated zone with an average DNAPL initial concentration of 55 000 mg kg −1 dry soil. Solutions were injected through a central well and pumped into four wells arranged in a five-point square pattern. In the zone swept by the washing solution, 86% of residual DNAPL was recovered using only 0.9 pore volume of a surfactant solution. These results confirm laboratory sand column experiments although the washing solution used in the field experiment had not yet been optimized to meet site-specific criteria. The use of a polymer solution before and after injection of the washing solution proved beneficial in insuring that the washing solution effectively swept all the sand layers in spite of soil heterogeneities. As the rinsing cycles were not completed because of weather problems (freezing conditions), small but significant amounts of the washing solution ingredients were still present in the aquifer at the end of the test and their fate is being monitored. For this field test, DNAPL recovery could have been better if an optimal washing solution had been injected, a larger volume of the selected solution had been circulated, or a different injection–pumping strategy had been chosen. ©1997 Elsevier Science B.V.

The relevance of a DNAPL-tracer test conducted at Base Borden, Canada, to solvent groundwater contamination in the UK

Abstract The relevance of key conclusions obtained from a tracer test performed at the Base Borden research site in Canada to solvent contamination in the UK is indicated. The tracer test represented a world unique opportunity to study groundwater contamination arising from a known DNAPL source zone in a natural aquifer environment. The test involved the controlled emplacement of a source zone of residual solvent DNAPL (dense non-aqueous phase liquid) below the water table from which a dissolved plume of chlorinated solvents developed. Significant data were collected on DNAPL dissolution, plume transport and remediation. Conclusions from the test are shown to be relevant to the development, investigation and remediation of solvent contamination in UK aquifers.

Sensitivity Models and Design Protocol for Partitioning Tracer Tests in Alluvial Aquifers

AbstractZones of dense, nonaqueous phase liquids (DNAPLs) are difficult to characterize as to their volume, composition, and spatial distribution using conventional ground‐water extraction and soil‐sampling methods. Such incompletely characterized sites have negative consequences for those responsible for their remedial design, e.g., the uncertainties in the optimal placement of ground‐water extraction wells and in the duration of remediation. However, the recent use of the partitioning interwell tracer test (PITT) to characterize DNAPL zones at sites in New Mexico [unsaturated alluvium] and in Ohio, Texas, and Utah [saturated alluvium] demonstrates that the volume and spatial distribution of residual DNAPL can be determined with accuracy. The PITT involves injection of a suite of tracers which reversibly partition to different degrees between the DNAPL and the ground water or soil air resulting in the chromatographic separation of the tracer signals observed at the extraction well(s). The design of a PITT requires careful consideration of the hydrostratigraphic, hydraulic, and certain geochemical properties of the alluvium being tested. A three‐dimensional, numerical model of a heterogeneous alluvial aquifer containing DNAPL has been developed for use with the UTCHEM simulator to demonstrate partitioning tracer testing and to address questions that are frequently raised in its application. The simulations include (1) the estimation of DNAPL volume for the simple case where only residual DNAPL is present in heterogeneous alluvium, (2) sensitivity studies to demonstrate the effect of increasingly low residual DNAPL saturation on the tracer signal, and (3) the effect of free‐phase DNAPL on the estimation of the volume of DNAPL present. Furthermore, the potential interference of sedimentary organic carbon as a DNAPL surrogate on the tracer signal is considered and shown to be readily resolved by the careful choice of tracers. Finally, a protocol for the use of PITTs in alluvial aquifers is presented.

Aquifer remediation by pumping: A model for stochastic‐advective transport with nonaqueous phase liquid dissolution

The coupled effects of advection and rate‐limited mass transfer to the aqueous phase from residual dense nonaqueous phase liquid (DNAPL) in the saturated zone are investigated within an analytical stochastic‐advective framework. The transport domain is assumed to consist of a source zone where mass transfer takes place and a down‐gradient plume zone where transport is by advection only. The mass flux in the aquifer is evaluated from an analytical solution to coupled advection‐reaction and nonlinear mass transfer equations. The results emphasize the importance of the parameters associated with the source zone: its length in the mean flow direction, the residual DNAPL saturation, and the mass transfer rate coefficient. The sensitivity to the hydraulic conductivity variance is low for significantly rate‐limited mass transfer. It is found that advection in the plume zone has small effect on mass fluxes and cleanup times for the considered lengths of the source and plume regions.

Physicochemical Transport Processes Affecting the Removal of Residual DNAPL by Nonionic Surfactant Solutions

Aquifers contaminated with dense nonaqueous phase liquids (DNAPLs) are extremely difficult to remediate with standard pump-and-treat methods. Enhanced remediation methods, such as flushing with cosolvent or surfactant solutions, promise to reduce remediation times but result in complex physicochemical systems for which we still lack both fundamental understanding and reliable process-based models. The overall objective of this work was to observe and quantify various physicochemical transport processes acting during the removal of a typical residual DNAPL (tetrachloroethylene, PCE) by solutions containing a nonionic surfactant (Triton X-100). To achieve this goal, we measured the phase behavior of the water/PCE/Triton X-100 system in batch systems and performed a set of glass bead column experiments to investigate residual PCE removal mechanisms, nonreactive tracer transport, and Triton X-100 transport. We observed and quantified removal of residual PCE by a number of processes (dissolution, micellar emul...

Direct Detection of Residual Nonaqueous Phase Liquid in the Saturated Zone Using SF6 as a Partitioning Tracer

Sulfur hexafluoride (SF 6 ) was investigated as a potential detector of residual dense nonaqueous phase liquids (DNAPL) below the water table. In NAPL-water batch partitioning tests, SF 6 had a partitioning coefficient (K NW =concentration in NAPL÷ concentration in water) of 3211.33 to TCE, 2411.27 to DCM, and 45±1.89 to o-DCB. Tracer tests with SF 6 and bromide in packed sand columns containing residual DNAPL show that SF 6 breakthrough is retarded relative to bromide. The SF 6 retardation factor was 2.2 relative to bromide in a column containing 3.69% residual TCE and 2.6 in a column with o-DCB at 3.52% residual saturation. A relatively simple relationship based on equilibrium partitioning and retardation theory was developed that allows very accurate estimates of DNAPL residual content encountered by SF 6 during transport. Using observed retardation factors from the column tests and the experimentally determined K ow values, this simple calculation was found to predict DNAPL residual saturations to within 2.5% of that actually emplaced

Estimation of Residual Dense NAPL Mass by Inverse Modeling

AbstractGround water at many sites is contaminated by chlorinated solvents which are of relatively low solubility and denser than water, as well as other contaminants of similar properties. The presence of a dense nonaqueous phase liquid (DNAPL) in an aquifer can provide continued flux to the dissolved phase long after the initial discharge. The flux may be significant even when the nonaqueous phase has dispersed to a state of residual saturation, in which little or no free product can be detected or recovered. At an industrial site we required an estimate of the magnitude of a DNAPL source without undertaking invasive exploration. In theory, the presence and magnitude of a residual DNAPL source can be inferred from observations of downgradient concentrations by (1) developing an inverse modeling solution to estimate flux, and (2) matching estimated flux to a conceptual model of NAPL dissolution, which yields an estimate of residual mass. Such a method can provide a qualitative indication of the presence of a residual DNAPL source. However, the sensitivity of the solution to assumptions regarding physical characteristics of the source medium are such that a useful quantitative estimate cannot be made without observations over long periods of time.

Using pneumatic fracturing extraction to achieve regulatory compliance and enhance voc removal from low‐permeability formations

AbstractThe New Jersey Department of Environmental Protection and Energy (NJDEPE) has been developing cleanup regulations that focus on remediation, rather than extended delineation, and on integrating regulatory requirements with technological developments. To this end, the NJDEPE, under the regulatory aegis of the Environmental Cleanup and Responsibility Act (ECRA), is monitoring an innovative treatment technology pilot test at a TCE‐contaminated ECRA site in Hillsborough, New Jersey. The purpose of the study is to determine the applicability of pneumatic fracturing extraction (PFE) as a source‐removal technique for extracting volatile organic compounds (VOCs) trapped informations with low permeability.The technology being pilot tested is pneumatic fracturing extraction, a process for enhancing permeability to promote in‐situ removal and treatment of VOCs. The patented process uses high‐pressure air injected into an isolated subsurface zone at controlled rates and pressures. At a critical point, the geologic material ruptures, and fractures are created that radiate outward from the fracture location. At the pilot test site, formation air flow was increased from 400 percent to 700 percent.PFE is a key component of the overall remediation strategy at the Hillsborough site. Consistent with proposed NJDEPE regulations, a ground‐water pump‐and‐treat system will be installed for plume migration control. Once the pump‐and‐treat system has been established and shown to be effective, a more aggressive source removal program will be implemented using PFE. This program will include construction of a vadose zone PFE system and evaluation of the use of pneumatic fracturing to remove saturated zone residual dense nonaqueous phase liquids (DNAPL). Preliminary calculations suggest that if source zone concentrations can be reduced to 10 ppm of TCE, then TCE groundwater concentrations may be reduced to background levels at the property boundary compliance points.