Reduction In Mass Flux Research Articles

Sensitivity of mass flux reduction and mass removal of perfluoroalkyl substances to groundwater flow and transport parameter variability and heterogeneity

The heterogeneity of soil hydraulic (e.g., hydraulic conductivity (KS), porosity (θS)) and chemical (e.g., solid-phase adsorption (Kd)) properties complicate contaminant transport by creating spatial variability in sources of contaminant leaching. There is a knowledge gap on the effect of the interplay between these properties on the retardation and transport of per- and polyfluoroalkyl substances (PFAS) with different properties including carbon–fluorine chain-length and functional groups even in water-saturated conditions. Breakthrough curves (BTCs) have been used to evaluate PFAS transport behavior through heterogeneous media, including arrival time, maximum concentration, and tailing behavior. Contaminant mass flux reduction (MFR) and mass removal (MR) correlations are also compared using numerical modeling herein to characterize PFAS transport through different source zones within a two-domain, heterogeneous system with comparison to homogeneous scenarios under water-saturated conditions. With heterogeneous properties, model sensitivity to KS was the highest among the other parameters and was controlled by the KS ratio between the different soils. The PFAS models in the homogeneous and heterogeneous scenarios were both sensitive to θS depending on PFAS chain length. However, long-chain PFAS were less sensitive to θS variability compared to short-chain PFAS, due to their higher Kd. The homogeneous and heterogeneous scenarios were equally sensitive to Kd variability, which was dependent on PFAS chain length.

Hydrologic cycle weakening in hothouse climates.

The hydrologic cycle has wide impacts on the ocean salinity and circulation, carbon and nitrogen cycles, and the ecosystem. Under anthropogenic global warming, previous studies showed that the intensification of the hydrologic cycle is a robust feature. Whether this trend persists in hothouse climates, however, is unknown. Here, we show in climate models that mean precipitation first increases with rising surface temperature, but the precipitation trend reverses when the surface is hotter than ~320 to 330 kelvin. This nonmonotonic phenomenon is robust to the cause of warming, convection scheme, ocean dynamics, atmospheric mass, planetary rotation, gravity, and stellar spectrum. The weakening occurs because of the existence of an upper limitation of outgoing longwave emission and the continuously increasing shortwave absorption by H2O and is consistent with atmospheric dynamics featuring the strong increase of atmospheric stratification and marked reduction of convective mass flux. These results have wide implications for the climate evolutions of Earth, Venus, and potentially habitable exoplanets.

Remediation of Pool Dominated Trichloroethene Source Zones in Heterogeneous Porous Media by Cosmetic Surfactant Flooding

<p>A series of flow-cell experiments were performed to investigate the performance of cosmetic rhamnolipid surfactant flooding on the relationship between source zone mass removal and mass flux reduction for pool-dominated DNAPL TCE source zones in heterogeneous porous media. The results were also compared to those of water-flood control experiments to assess the surfactant enhanced flushing efficacy. The flooding experiments were performed for silica sand and natural calcareous soil representing two different degrees of physical heterogeneity. The result from the flow-cell experiments showed that higher than 97% of TCE mass was removed during rhamnolipid flooding for both porous media scenarios. Although, rhamnolipid flooding experiment results showed successful remediation performance, DNAPL TCE dissolution and rhamnolipid-enhanced dissolution in heterogeneous porous media system exhibited multi-step mass-flux reduction/mass-removal behavior due to the presence of less hydraulically-accessible pool-dominated TCE source zone. However, mass removal and mass-flux reduction relationships for rhamnolipid flushing cases exhibited more ideal removal behavior, indicating the more efficient remediation performance compared to water flooding alone. For all cases, the later stage of mass removal was controlled by the more poorly-accessible mass associated with pool-dominated source zones. The results of this study revealed the impact of non-uniformity of the flow-field and effect of enhanced-solubilization agent on mass removal and mass-flux reduction behavior for DNAPL source zones in saturated porous media</p>

Mass-flux Reduction and Mass Removal Relationships (MFR-MR) of Pool-dominated Multicomponent DNAPLs in Heterogeneous Geological System

<p>Contamination of groundwater by chlorinated solvents such as trichloroethylene (TCE, tetrachloroethylene (PCE), and 1,1-dichloroethylene (1,1DCE) is a widespread problem that can seriously threaten human health and the environment. These compounds are often present as dense nonaqueous phase liquid (DNAPL) mixtures in the subsurface that serve as a long-term groundwater contamination sources. The objective of this study is to test the effectiveness of different in-situ flushing agents (SDS, Tween 80, MCD, and water) for the recovery of DNAPL mixtures (TCE, PCE, 1,1DCE) entrapped in heterogeneous aquifer media. A series of batch tests were first performed to assess the multiphase behavior of water-DNAPL mixture-reagent system. Intermediate scale tank experiments were then performed to compare the performance of the reagent solutions. The flushing breakthrough curves were also used to investigate the impact of solubilization agents on mass-flux reduction/mass-removal behavior relationships. Performance of agents on DNAPL mixture source zone remediation was found to be in the following order: Tween 80>SDS>MCD>>Water. Individual DNAPL constituents exhibited transient effective solubilities that depend on the reagent contents and DNAPL compositions. Although high fractions of DNAPL source zones in heterogeneous porous media were removed through chemical flushing (SDS, Tween 80, MCD, and water), the heterogeneous system exhibited an extended multi-step concentration behavior. The results emphasized that in the early stage, some portion of the organic liquid is hydraulically accessible (matrix) whereas the later stage of mass removal was controlled by the more poorly accessible mass (pool) associated with higher-saturation zones. Our results also showed that the distribution and the emplacement of organic liquid, flow-field heterogeneity, sorption and desorption, and flushing solution all influenced the DNAPL mixture removal. The results from this study underline the need for understanding the multiphase behaviour of a system, particularly when DNAPL mixtures are present, prior to field-scale remediation implementation.</p>

Trade-offs across values in cesspool management highlight challenges to policy making

On-site Sewage Disposal Systems (OSDS) are globally common, and in Hawai'i they present a risk of contamination to drinking water sources and nearshore waters. State legislation has commanded that all cesspools are to be banned by 2050, thus requiring tens of thousands of systems to be converted in the coming decades. This project followed a participatory structured decision-making (SDM) approach to collaboratively design cost-effective and equitable solutions for thousands of cesspools in the high elevation areas of north Maui, Hawai'i. Participatory workshops with a diverse group of stakeholders set ten objectives and brainstormed 33 alternatives, for which the technical team then modeled groundwater nutrients, costs, and equity. All alternatives posed trade-offs, though composting toilets performed best across most objectives, albeit with high maintenance burden. Discounting innovative toilets, the multi-objective analysis suggests that the state should invest in cluster sewering of high-density communities, followed by incentivizing septic tank solutions in properties with the highest effluent flow first, then expanding across the area. The total project cost (installation and operation/maintenance) would be $183–258 million, depending upon the sewer-septic combination. An efficiency frontier reveals sub-par combinations, including aerobic treatment units and passive absorption systems, which cost much more and deliver lower mass flux reduction than more cost-effective alternatives. This study contributes a novel case of rural sanitation to the literature in which decision support tools are used to facilitate evidence-based, collaborative decision-making for sanitation planning. The state could use a similar participatory SDM process when approaching other communities to discuss their cesspool upgrade strategies. Broadening the use of decision analytic techniques can have wider ecological, economic, and social benefits for the state and contexts beyond Hawai'i, as SDM provides a transparent and rigorous, evidence-based decision-theoretic framework to explore multiple values and strategies to address difficult resource management problems.

PFOS Mass Flux Reduction/Mass Removal: Impacts of a Lower-Permeability Sand Lens within Otherwise Homogeneous Systems.

Perfluorooctane sulfonic acid (PFOS) is one of the most common per- and polyfluoroalkyl substances (PFAS) and is a significant risk driver for these emerging contaminants of concern. A series of two-dimensional flow cell experiments was conducted to investigate the impact of flow field heterogeneity on the transport, attenuation, and mass removal of PFOS. A simplified model heterogeneous system was employed consisting of a lower-permeability fine sand lens placed within a higher-permeability coarse sand matrix. Three nonreactive tracers with different aqueous diffusion coefficients, sodium chloride, pentafluorobenzoic acid, and β-cyclodextrin, were used to characterize the influence of diffusive mass transfer on transport and for comparison to PFOS results. The results confirm that the attenuation and subsequent mass removal of the nonreactive tracers and PFOS were influenced by mass transfer between the hydraulically less accessible zone and the coarser matrix (i.e., back diffusion). A mathematical model was used to simulate flow and transport, with the values for all input parameters determined independently. The model predictions provided good matches to the measured breakthrough curves, as well as to plots of reductions in mass flux as a function of mass removed. These results reveal the importance of molecular diffusion and pore water velocity variability even for systems with relatively minor hydraulic conductivity heterogeneity. The impacts of the diffusive mass transfer limitation were quantified using an empirical function relating reductions in contaminant mass flux (MFR) to mass removal (MR). Multi-step regression was used to quantify the nonlinear, multi-stage MFR/MR behavior observed for the heterogeneous experiments. The MFR/MR function adequately reproduced the measured data, which suggests that the MFR/MR approach can be used to evaluate PFOS removal from heterogeneous media.

Enhanced-solubilization and dissolution of multicomponent DNAPL from homogeneous porous media

The presence of multicomponent dense nonaqueous phase liquid (DNAPL) mixtures in porous media can significantly limit the effectiveness of groundwater remediation. A series of column transport and flushing experiments were conducted to quantify the impact of various enhanced flushing agents on dissolution and removal of a multicomponent DNAPL source within a macroscopically homogeneous porous medium. The columns were established with NAPL saturations consisting of an equal mole mixture of cis-1,2-dichloroethene, trichloroethene, and tetrachloroethene. The solubilization agents used included two complexing sugars - hydroxypropyl-β-cyclodextrin (HPCD and methyl-β-cyclodextrin (MCD); a surfactant - sodium dodecyl sulfate (SDS); and a cosolvent - ethanol (EtOH). The chemical flushing agents greatly reduced the time needed to remove each DNAPL component, compared to flushing with water alone. Initial DNAPL-component elution concentrations were successfully predicted using Raoult's Law for MCD, HPCD, and water flushing, indicating that ideal dissolution was initiated by the lower-power enhanced-solubilization agents. EtOH was most efficient at removing the contaminants in terms of normalized mass recovery but least efficient based on a mass-ratio and mole-ratio of contaminant to reagent analysis. SDS was most efficient for contaminant removal when analyzed based on mass-contaminant to mass-reagent recovered and MCD was most effective based on a moles-contaminant to moles-reagent recovered efficiency evaluation. However, in terms of mass flux reduction analysis (i.e. removal metric), SDS was least efficient for contaminant removal compared to all other enhanced-flushing agents tested, especially during the initial stages of DNAPL removal. Results from this study indicate that ideal dissolution was initiated during enhanced-solubilization and that several criteria should be used to evaluate the removal effectiveness of flushing agents for multi-component NAPL systems.

Conditions for accretion disc formation and observability of wind-accreting X-ray binaries

Abstract We explore the effect of anisotropic wind driving on the properties of accretion onto black holes (BHs) in close binaries. We specifically focus on line-driven winds, which are common in high-mass X-ray binaries (HMXBs). In close binary systems, the tidal force from the companion star can modify the wind structure in two different ways. One is the reduction of wind terminal velocity due to the weaker effective surface gravity. The other is the reduction in mass flux due to gravity darkening (GD). We incorporate these effects into the so-called CAK theory in a simple way and investigate the wind flow around the accretor on the orbital scale. We find that a focused accretion stream is naturally formed when the Roche lobe filling factor is ${\gtrsim}0.8$ –0.9, analogous to that of wind Roche lobe overflow, but only when the velocity reduction is taken into account. The formation of a stream is necessary to bring in sufficient angular momentum to form an accretion disc around the BH. GD effects reduce the amount of accreted angular momentum, but not enough to prevent the formation of a disc. Based on these results, we expect there to be a discrete step in the observability of HMXBs depending on whether the donor Roche lobe filling factor is below or above ${\sim}$ 0.8–0.9.

Sound Production due to Swirl–Nozzle Interaction: Model-Based Analysis of Experiments

Indirect noise due to the interaction of flow inhomogeneities with a choked nozzle is an important cause of combustion instability in solid rocket motors and is believed to be important in aircraft engines. A previously published experiment (Kings, N., and Bake, F., “Indirect Combustion Noise: Noise Generation by Accelerated Vorticity in a Nozzle Flow,” International Journal of Spray and Combustion Dynamics, Vol. 2, No. 3, 2010, pp. 253–266.) demonstrated that interaction of a nozzle with time-dependent axial swirl can also be a source of sound. This axial swirl was generated by intermittent tangential mass injection upstream from a choked nozzle in a so-called vortex wave generator. The present work discusses the impact of swirl–nozzle interaction in this experiment on the acoustic waves detected downstream of the nozzle. The main source of sound appears to be the reduction in mass flux through the choked nozzle, which depends quadratically on the swirl number. This effect is quantitatively predicted by a quasi-steady and quasi-cylindrical analytical model. The model, combined with empirical data for the decay of axial swirl in pipe flows, predicts the observed influence of the distance between the vortex wave generator and the nozzle. The findings presented here contradict the hypothesis found in the literature, which presumes that sound production in the aforementioned experiment is due to the acceleration of vorticity waves through the nozzle.

Turbulent entrainment into a cylinder wake from a turbulent background

Abstract

Three-dimensional numerical study of direct steam generation in vertical tubes receiving concentrated solar radiation

Concentrated solar thermal energy is a good alternative to mitigate the environmental impacts generated by the growing use of energy. There are several commercial solar power tower plants in the world operating with direct steam generation. These plants operate with cavity-type and external tubular receivers. The conventional tubular receivers consist of a set of vertical circular tubes through which the working fluid circulates. This study is focused to analyze in detail the direct steam generation in a vertical tube receiving concentrated solar radiation. The mathematical model is based on the equations of continuity, momentum and energy for each phase. The modified model of Rensselaer Polytechnic Institute was used for the conditions of critical heat flux coupled to a Eulerian two fluid model. In the modified model of Rensselaer Polytechnic Institute, the total heat flux is divided into four components from the wall to the liquid: (a) liquid-phase convective heat flux, (b) heat transfer due to quenching, (c) heat flux due to evaporation and (d) convective heat flux of the vapor phase. The mathematical model was solved with a computational fluid dynamics software. The results were validated with experimental data reported in the literature and a parametric study was carried out to determinate the effect of the mass flux and heat flux incident on the wall, on: the steam quality, the volumetric fraction, the enthalpies and temperatures of liquid and steam. The reduction of mass flux by 44% increased the output mass flow of steam 4.28 times, whereas the increase of concentrated solar flux by 40% increased the output mass flow of steam 4 times.

MHD Accretion Disk Winds: The Key to AGN Phenomenology?

Accretion disks are the structures which mediate the conversion of the kinetic energy of plasma accreting onto a compact object (assumed here to be a black hole) into the observed radiation, in the process of removing the plasma’s angular momentum so that it can accrete onto the black hole. There has been mounting evidence that these structures are accompanied by winds whose extent spans a large number of decades in radius. Most importantly, it was found that in order to satisfy the winds’ observational constraints, their mass flux must increase with the distance from the accreting object; therefore, the mass accretion rate on the disk must decrease with the distance from the gravitating object, with most mass available for accretion expelled before reaching the gravitating object’s vicinity. This reduction in mass flux with radius leads to accretion disk properties that can account naturally for the AGN relative luminosities of their Optical-UV and X-ray components in terms of a single parameter, the dimensionless mass accretion rate. Because this critical parameter is the dimensionless mass accretion rate, it is argued that these models are applicable to accreting black holes across the mass scale, from galactic to extragalactic.

The horizontal reactive media treatment well (HRX Well®) for passive in situ remediation: Design, implementation, and sustainability considerations

AbstractA new in situ remediation concept termed a Horizontal Reactive Media Treatment Well (HRX Well®) is presented that utilizes a horizontal well filled with reactive media to passively treat contaminated groundwater in situ. The approach involves the use of a large‐diameter directionally drilled horizontal well filled with solid reactive media installed parallel to the direction of groundwater flow. The engineered contrast in hydraulic conductivity between the high in‐well reactive media and the ambient aquifer hydraulic conductivity results in the passive capture, treatment, and discharge back to the aquifer of proportionally large volumes of groundwater. Capture and treatment widths of up to tens of feet can be achieved for many aquifer settings, and reductions in downgradient concentrations and contaminant mass flux are nearly immediate. Many different types of solid‐phase reactive treatment media are already available (zero valent iron, granular activated carbon, biodegradable particulate organic matter, slow‐release oxidants, ion exchange resins, zeolite, apatite, etc.). Therefore, this concept could be used to address a wide range of contaminants. Laboratory and pilot‐scale test results and numerical flow and transport model simulations are presented that validate the concept. The HRX Well can access contaminants not accessible by conventional vertical drilling and requires no aboveground treatment or footprint and requires limited ongoing maintenance. A focused feasibility evaluation and alternatives analysis highlights the potential cost and sustainability advantages of the HRX Well compared to groundwater extraction and treatment systems or funnel and gate permeable reactive barrier technologies for long‐term plume treatment. This paper also presents considerations for design and implementation for a planned upcoming field installation.

Surfactant-Enhanced Permanganate Oxidation on Mass-Flux Reduction and Mass Removal (MFR-MR) Relationship for Pool-Dominated TCE Source Zones in Heterogeneous Porous Media

Chlorinated solvents generally enter the subsurface as dense nonaqueous phase liquids (DNAPL) and accumulation mostly occurs in aquifers as pool-dominated zones that can cause long-term aqueous phase groundwater contamination. In situ remediation of DNAPL source zones in such systems is crucial for protecting and/or restoring groundwater quality in these aquifer systems having significant groundwater potential. The objective of the project was to investigate the surfactant-enhanced permanganate oxidation efficacy for pool-dominated DNAPL source zones in heterogeneous aquifer media. A complementary objective of this study was to investigate the impact of nonuniform distribution of DNAPL source zones, surfactant-enhanced dissolution and surfactant-enhanced permanganate oxidation conditions on mass-flux reduction/mass-removal behavior relationships. A series of 2-D flow-cell tank experiments using various grain sizes silica sand and natural soil were conducted as part of this study. DNAPL trichloroethene (TCE) was used as a chlorinated solvent, and SDS (sodium dodecyl sulfate) and KMnO4 (potassium permanganate) were used as anionic surfactant and oxidant (remediation agents), respectively. The results were compared with a water-flooding experiment to test the remediation effort. Although, high fractions of TCE source zones in the heterogeneous porous media were removed by sodium dodecyl sulfate (SDS)-enhanced flushing, TCE removal in this system exhibited an extended multi-step concentration elution behavior. This nonideal behavior was observed for both the water-flood and SDS-flushing experiments. The results emphasized that in the early stage, some portion of the organic liquid is hydraulically accessible (matrix) whereas the later stage of mass removal was controlled by the more poorly accessible mass (pool) associated with higher saturation zones. Our results also showed that the distribution and the emplacement of organic liquid and flow-field heterogeneity had a significant influence on remediation and removal for both flushing solution (SDS and water). It was postulated that when SDS/MnO4 was applied with sufficient dosage and provided enough contact time, pool-dominated source zones could be remediated more efficiently compared to surfactant flushing alone. As a result, the performance of technology depends on the site characteristics which are critical to characterize effective DNAPL remediation strategies for contaminated sites.

Effect of Cyclodextrin-Enhanced Dissolution on Mass Removal and Mass-Flux Reduction Relationships for Non-uniformly Organic Liquid Distribution in Heterogeneous Porous Media

A series of 2-D tank experiments were conducted to investigate the impact of non-uniform organic-liquid distribution, type of porous media, and methyl-beta-cyclodextrin (MCD) flooding on the relationship between source zone mass removal and mass-flux reduction for heterogeneous porous media. Trichloroethene was used as the model organic liquid, and MCD was used as enhanced flushing reagent. The results were also compared to those of water-flood control experiments. The results showed that the representative heterogeneous system exhibited specific non-ideal multi-step mass-flux reduction/mass removal behavior depended upon different porous media configurations. This non-ideal behavior was observed for both the water-flood and MCD flooding experiments. For all cases, early stage of trichloroethene mass removal was controlled by greater relative hydraulically accessibility of the dense non-aqueous-phase liquid (DNAPL) source distributed within the porous media matrix compared to later stage removal which was controlled by poorly hydraulically accessible mass associated with higher saturated or pooled DNAPL source zones. The results of this study significantly revealed the impact of non-uniform organic liquid distribution, flow-field heterogeneity, type of porous media, and the concentration of MCD on mass removal and mass-flux reduction behavior.

The Horizontal Reactive Media Treatment Well (HRX Well®) for Passive In‐Situ Remediation

AbstractA new in‐situ remediation concept termed a Horizontal Reactive Media Treatment Well (HRX Well®) is presented that utilizes horizontal wells filled with reactive media to passively treat contaminated groundwater in‐situ. The approach involves the use of large‐diameter directionally drilled horizontal wells filled with granular reactive media generally installed parallel to the direction of groundwater flow. The design leverages natural “flow‐focusing” behavior induced by the high in‐well hydraulic conductivity of the reactive media relative to the aquifer hydraulic conductivity to passively capture and treat proportionally large volumes of groundwater within the well. Clean groundwater then exits the horizontal well along its downgradient sections. Many different types of solid granular reactive media are already available (e.g., zero valent iron, activated carbon, ion exchange resins, zeolite, apatite, chitin); therefore, this concept could be used to address a wide range of contaminants. Three‐dimensional flow and transport simulations were completed to assess the general hydraulic performance, capture zones, residence times, effects of aquifer heterogeneity, and treatment effectiveness of the concept. The results demonstrate that capture and treatment widths of up to tens of feet can be achieved for many aquifer settings, and that reductions in downgradient concentrations and contaminant mass flux are nearly immediate. For a representative example, the predicted treatment zone width for the HRX Well is approximately 27 to 44 feet, and contaminant concentrations immediately downgradient of the HRX Well decreased an order of magnitude within 10 days. A series of laboratory‐scale physical tests (i.e., tank tests) were completed that further demonstrate the concept and confirm model prediction performance. For example, the breakthrough time, peak concentration and total mass recovery of methylene blue (reactive tracer) was about 2, 35, and 20 times (respectively) less than chloride (conservative tracer) at the outlet of the tank‐scale HRX Well.

SL2B aptamer and folic acid dual-targeted DNA nanostructures for synergic biological and chemotherapy to combat colorectal cancer

The influence of chemical structure on NAPL mixture nonideality evolution, rate-limited dissolution, and contaminant mass flux was examined. The variability of measured and UNIFAC modeled NAPL activity coefficients as a function of mole fraction was compared for two NAPL mixtures containing structurally-different contaminants of concern including toluene (TOL) or trichloroethene (TCE) within a hexadecane (HEXDEC) matrix. The results showed that dissolution from the NAPL mixtures transitioned from ideality for mole fractions > 0.05 to nonideality as mole fractions decreased. In particular, the TCE generally exhibited more ideal dissolution behavior except at lower mole fractions, and may indicate greater structural/polarity similarity between the two compounds. Raoult's Law and UNIFAC generally under-predicted the batch experiment results for TOL:HEXDEC mixtures especially for mole fractions ≤ 0.05. The dissolution rate coefficients were similar for both TOL and TCE over all mole fractions tested. Mass flux reduction (MFR) analysis showed that more efficient removal behavior occurred for TOL and TCE with larger mole fractions compared to the lower initial mole fraction mixtures (i.e. < 0.2). However, compared to TOL, TCE generally exhibited more efficient removal behavior over all mole fractions tested and may have been the result of structural and molecular property differences between the compounds. Activity coefficient variability as a function of mole fraction was quantified through regression analysis and incorporated into dissolution modeling analyses for the dynamic flushing experiments. TOL elution concentrations were modeled (predicted) reasonable well using ideal and equilibrium assumptions, but the TCE elution concentrations could not be predicted using the ideal model. Rather, the dissolution modeling demonstrated that TCE elution was better described by the nonideal model whereby NAPL-phase activity coefficient varied as a function of COC mole fraction. For dynamic column flushing experiments, dissolution rate kinetics can vary significantly with changes in NAPL volume and surface area. However, under conditions whereby NAPL volume and area are not significantly altered during dissolution, mixture nonideality effects may have a greater relative control on dissolution (elution) and MFR behavior compared to kinetic rate limitations.

The Case for Flux‐Based Remedial Performance Monitoring Programs

The Case for Flux‐Based Remedial Performance Monitoring Programs

In situ stabilization of NAPL contaminant source-zones as a remediation technique to reduce mass discharge and flux to groundwater

Widely used flushing and in-situ destruction based remediation techniques (i.e. pump-and treat, enhanced-solubilization, and chemical oxidation/reduction) for sites contaminated by nonaqueous phase liquid (NAPL) contaminant sources have been shown to be ineffective at complete mass removal and reducing aqueous-phase contaminant of concern (COC) concentrations to levels suitable for site closure. A remediation method was developed to reduce the aqueous solubility and mass-flux of COCs within NAPL through the in-situ creation of a NAPL mixture source-zone. In contrast to remediation techniques that rely on the rapid removal of contaminant mass, this technique relies on the stabilization of difficult-to-access NAPL sources to reduce COC mass flux to groundwater. A specific amount (volume) of relatively insoluble n-hexadecane (HEXDEC) or vegetable oil (VO) was injected into a trichloroethene (TCE) contaminant source-zone through a bench-scale flow cell port (i.e. well) to form a NAPL mixture of targeted mole fraction (TCE:HEXDEC or TCE:VO). NAPL-aqueous phase batch tests were conducted prior to the flow-cell experiments to evaluate the effects of various NAPL mixture ratios on equilibrium aqueous-phase concentrations of TCE to design optimal NAPL (HEXDEC or VO) injection volumes for the flow-cell experiments. The NAPL-stabilization flow-cell experiments initiated and sustained significant reductions in COC concentration and mass flux due to a combination of both reduced relative permeability (increased NAPL-saturation) and via modification of NAPL composition (decreased TCE mole fraction). Variations in remediation performance (i.e. impacts on TCE concentration and mass flux reduction) between the different HEXDEC injection volumes were relatively minor, and therefore inconsistent with Raoult's Law predictions. This phenomenon likely resulted from non-uniform mixing of the injected HEXDEC with TCE in the source-zone. VO injection caused TCE concentrations and mass-flux to decrease more rapidly than with HEXDEC injections. This phenomenon occurred because the injected VO was observed to mix more uniformly with TCE in the source-zone due to a lower mobilization potential. The relative lower density differences (buoyancy effects) between VO and the flushing solution (water) was the primary factor contributing to the lower mobilization potential for VO. Overall, this study indicated that the delivery of HEXDEC or VO into the toxic TCE source-zone was effective in significantly reducing contaminant aqueous-phase concentration and mass-flux. However, the effectiveness of this in-situ NAPL stabilization technique depends on source delivery, uniform mixing of amendment, and that the amendment remains immobilized within and around the NAPL contaminant source.

Reduction in contaminant mass flux with induced oxide mineral precipitation in quasi-multidimensional systems

Dense non-aqueous-phase liquids (DNAPLs) are difficult to be removed from the highly heterogeneity aquifer. In situ chemical oxidation (ISCO) is one of the main DNAPL remediation technologies, but when using permanganate as oxidant to remediate the DNAPL in groundwater, the permeability within the DNAPL source area can be reduced due to the MnO2 solid-phase formation. Hence, the study of the complex dynamic change in the DNAPL source zone is meaningful for improvement in the ISCO efficiency. A developed one-dimensional column experiment was used in this study. The columns used in the experiments were parallel-connected. Limited time interval and low-concentration permanganate flushing were applied in the experiments to mimic real field condition. It was found that the permeability within the DNAPL source zone was reduced only for short time due to the precipitation and flocculation of MnO2 formed. The flow flux and mass flux within the DNAPL source zone decreased during permanganate oxidation while rebounded again after the oxidation. The experiment results illustrated that low concentration of permanganate oxidation can activate the DNAPL source area. Based on this phenomenon, an effective ISCO could be developed to remediate the source zone.