Nonaqueous Phase Liquid Remediation Research Articles

Enhanced mass transfer of residual NAPL by convection in stagnant zone

Cosolvent flushing is a promising method to improve NAPLs (non-aqueous phase liquids) removal. Nevertheless, the removal performance is undermined by the tailing phenomenon, i.e., the contaminant concentration tends to remain unchanged in later period as NAPL is residually trapped in the stagnant zone. High proportion of stagnant zone leads to more severe tailing phenomenon and how to accelerate dissolution of NAPL in the stagnant zone is important for NAPL remediation. However, few work has studied the remediation process at pore-scale due to limited observation techniques. As a result, the fundamental process of convection enhancement remains unclear. In this work, dynamics of NAPL removal in the stagnant zone is quantitatively investigated in transparent micromodels via the use of Raman spectroscopy. Impacts of injection condition and properties of the porous media are systematically investigated. Results indicate that convection dominants the mass transfer in aqueous phase and enhances the NAPL dissolution. Increasing the flushing velocity can enhance convection, but when the flushing velocity exceeds 3 cm/min, convection in the region away from the dominant channel is little affected by the flushing velocity. Differently, increasing the pore throat size will enhance convection in the entire stagnant zone and accelerate the whole dissolution process.

Modeling of Complex, Multi‐Component NAPL Remediation for Decision Support

AbstractIn situ remediation of nonaqueous phase liquid (NAPL)‐impacted sites is difficult and costly. Even with enhancements (e.g., chemical and thermal) and partial NAPL recovery, mass transfer constraints associated with partitioning from residual NAPL typically control the depletion rate of sources and attainment of cleanup goals. Practical methodologies are needed to support strategic evaluation, planning, and implementation of cost‐effective remedial approaches that are considered protective. A modeling system averaging over the NAPL‐impacted saturated soil volume was developed and demonstrated at the former Williams Air Force Base. The system combines upscaled, physically based mass transfer coefficients for multi‐component NAPL dissolution with theoretical enhancements specific to multiple remediation processes. Dissolution increases in the presence of aqueous phase reactions, according to first‐ or second‐order kinetics, by increasing concentration gradients. Slow biological processes are considered first order in modeling natural attenuation and enhanced biological degradation. Fast reactions associated with chemical oxidation are considered second order. These enhancement models are equally applicable to numerical simulations of NAPL remediation. Pump and treat enhances dissolution in proportion to increases in the characteristic velocity associated with dissolution. The demonstration yielded realistic predictions, with greater certainty, for outcomes from of multiple technologies intended to reduce remedial timeframes and life cycle costs. The enhanced dissolution modeling provides a site‐specific, quantitative assessment of changes in NAPL source discharge concentration and mass discharge over time for various remedial options that is equivalent to assessments from complex numerical transport models, given typical input data limitations.

An experimental study of in-situ remediation of NAPL-polluted soils by ozonation

The effectiveness of ozonation as a method for the in situ remediation of non-aqueous phase liquid (NAPL)-polluted soils is examined. Soils were polluted artificially with a model NAPL consisting of equal mass fractions of n-decane, n-dodecane and n-hexadecane at two concentrations 1g/kg-soil, and 10g/kg-soil. The polluted soils were placed inside a stainless steel column and treated with the injection of ozone-rich gas, produced from oxygen by a corona discharge ozone generator, controlled by a gas mass flow controller, and monitored by an ozone analyzer. Soil samples collected from the soil column were used after pre-treatment to measure the concentrations of the total organic carbon (TOC), and total petroleum hydrocarbons (TPH) as function of time. The concentrations of carbon dioxide (CO2), carbon monoxide (CO), nitrogen oxides (NOx), and volatile organic compounds (VOCs) were detected on-line by a multi-sensor and portable gas analyzer. .

Dissolution and remobilization of NAPL in surfactant-enhanced aquifer remediation from microscopic scale simulations

In this paper, the dissolution and mobilization of non-aqueous phase liquid (NAPL) blobs in the Surfactant-Enhanced Aquifer Remediation (SEAR) process were upscaled using dynamic pore network modeling (PNM) of three-dimensional and unstructured networks. We considered corner flow and micro-flow mechanisms including snap-off and piston-like movement for two-phase flow. Moreover, NAPL entrapment and remobilization were evaluated using force analysis to develop the capillary desaturation curve (CDC) and predict the onset of remobilization. The corner diffusion mechanism was also applied in the modeling of interphase mass transfer to represent NAPL dissolution as the dominant mass transfer process. In addition, the effect of pore-scale heterogeneity on mass transfer rate coefficient and recovered residual NAPL was considered in the simulations. Sodium dodecyl sulfate (SDS) and Triton X-100 were used as the surfactant for the SEAR process. The results indicate that although surfactants enhance NAPL recovery during two-phase flow, surfactant-enhanced remediation of residual NAPL through dissolution is highly dependent on surfactant type. When SDS ─as a surfactant with high critical micelle concentration (CMC) and low micelle partition coefficient (Km)─ was injected into a NAPL contaminated site, the mass transfer rate coefficient decreased (due to considerable changes in interface chemical potentials) which leads to a significant reduction in NAPL recovery after the end of two-phase flow. In contrast, Triton X-100 (with low CMC and high Km) improved NAPL recovery, by enhancing solubility at surfactant concentrations greater than CMC which overcompensates the interphase mass transfer reduction.

Petroleum NAPL Depletion Estimates and Selection of Marker Constituents from Compositional Analysis

AbstractSpills and releases of hydrocarbons may result in zones of nonaqueous phase liquids (NAPL) within soils and groundwater. The NAPL will change, or “weather” over time due to a range of physical, chemical, and biological processes. Hydrocarbon constituents in environmental samples collected from NAPL‐impacted groundwater wells, sediments, or soils vary in composition over time due to weathering. The changing composition can be used to estimate mass depletion rates and trends for the bulk NAPL and for individual constituent chemicals relative to marker constituents which are less susceptible to weathering. Methods for the selection of marker constituents and for quantitatively and conservatively estimating NAPL depletion rates and trends over time are shown. Estimates are included for two sites with different NAPL mixtures present (crude oil and gasoline/diesel‐range product), for which depletion of half the initial total NAPL is estimated at 13.6 ± 2.9 years and 7.3 ± 1.8 years respectively, and with no active NAPL remediation at either site. Similar methods for oil or NAPL depletion estimates have often relied on a prior‐identified suite of presumed‐conserved marker constituents. The method presented here includes steps which identify the best set of analyzed candidate marker constituents in a NAPL mixture. This can confirm prior‐selected markers but is particularly useful for NAPL mixtures in which no prior‐identified marker constituents are present.

Selective solvent filters for non-aqueous phase liquid separation from water

Injectable filters permeable to water but impermeable to non-polar solvents were developed to contain non-aqueous phase liquids (NAPL) in contaminated aquifers, hence protecting downstream receptors during NAPL remediation. Filters were produced by injecting aqueous solutions of 0.01% chitosan, hydroxyethylcellulose and quaternized hydroxyethylcellulose into sand columns, followed by rinsing with water. Polymer sorption onto silica was verified using a quartz-crystal microbalance with dissipation monitoring. Fluorescence and gas chromatography mass spectroscopy showed low ppm range concentrations of non-polar solvents (e.g., hexane and toluene) in water eluted from the filters (in the absence of emulsifiers). The contact angles between polymer-coated surfaces and hexane or toluene were > 90°, indicating surface oleophobicity. Organic, polar solvents (e.g. tetrahydrofuran and tetrachloroethylene, TCE) were not separated from water. The contact angles between polymer-coated surfaces and TCE was also > 90°. However, the contact area with polymer coated surfaces was greater for TCE than non-polar solvents, suggesting higher affinity between TCE and the surfaces. Emulsifiers can be used to facilitate NAPL extraction from aquifers. Emulsion separation efficiency depended on the emulsifier used. Emulsions were not separated with classical surfactants (e.g. Tween 20 and oleic acid) or alkaline zein solutions. Partial emulsion separation was achieved with humic acids and zein particles.

Remediation of trapped DNAPL enhanced by SDS surfactant and silica nanoparticles in heterogeneous porous media: experimental data and empirical models.

The remediation of nonaqueous phase liquids (NAPLs) enhanced by surfactant and nanoparticles (NP) has been investigated in numerous studies. However, the role of NP-assisted surfactants in the dissolution process is still not well discussed. Besides, there is a lack of empirical dissolution models considering the effects of initial residual saturation Strap, NAPL distribution, and surfactant concentration in NAPL-aqueous phase systems. In this work, micromodel experiments are conducted to quantify mass transfer coefficients for different injected aqueous phases including deionized water, SDS surfactant solutions, and NP-assisted solutions with different levels of concentrations and flow rates. Observations reveal that silica nanoparticles (SNP) can significantly enhance interphase mass transfer, while SDS surfactant reduces the mass transfer coefficient. In addition, Strap and intrinsic interfacial area ai, as an indicator of dense nonaqueous phase liquids (DNAPL) distribution, influence the interphase mass transfer. The ai is also independent of DNAPL saturation SNAPL except for SNAPL < 7% when ganglia breakup occurs. Based on these observations, new empirical dissolution models are proposed in the presence and the absence of SDS surfactant and SNP in which ai, Strap, and surfactant concentrations are introduced as new parameters. The evaluated mass transfer rate coefficients using the proposed models show a significant improvement compared to available empirical models. The finding of this study might be attractive for application in field-scale simulations of surfactant-enhanced aquifer remediation (SEAR) and NP-assisted methods.

Surfactant Foam Selection for Enhanced Light Non-Aqueous Phase Liquids (LNAPL) Recovery in Contaminated Aquifers

Foam is promising for the remediation of non-aqueous phase liquids (NAPL) source zones; however, the production of foam and its behavior in porous media are poorly understood. A methodology for the selection of surfactants suitable for foam production applied to NAPL remediation was developed. Two criteria were initially used for surfactant selection: foamability as evaluated by the Ross–Miles test and interfacial tension reduction measured with the pendant drop method. Three promising surfactants were identified and used in sand column tests: Genapol LRO because it produced the highest foam height in the Ross–Miles test, Ammonyx Lo which exhibited the lowest interfacial tension with p-xylene and had the second highest foam height, and Tomadol 900 because it showed intermediate results in both tests. Viscosity was found to be proportional to foamability. Genapol LRO produced a foam so viscous that it destabilized by the end of the experiment. Ammonyx Lo produced a less viscous foam but with a stable front. Tomadol 900 produced an unstable foam with poor viscosity. Results from column tests gave indications of optimal conditions needed to produce a stable and viscous foam front.

Interaction between biofilm growth and NAPL remediation: A pore-scale study

In this paper, we introduce a pore-scale model to study the interaction between biofilm growth and non-aqueous-phase-liquid (NAPL) dissolution. Liquid flow and dissolved NAPL transport are coupled with a biofilm growth model to correctly describe the complex dynamics of the processes including fluid flow, NAPL dissolution/biodegradation and biofilm growth. Fluid flow is simulated using an immersed boundary-lattice Boltzmann (IB-LB) model; while solute transport is solved by a cut-cell finite volume method (FVM). A uniform dissolution approach is also adopted to capture the temporal evolution of trapped blobs. Spatio-temporal distributions of the biomass are investigated using a cellular automaton algorithm combined with the immersed boundary method (IBM). Simulations focused on NAPL dissolution in both abiotic and biotic conditions are conducted to assess the capability of the model. In abiotic conditions, we analyze the effects of the hydrodynamic regimes and the spatial distribution of NAPL blobs on the dissolution rate under different assumptions (i.e., blob size and Péclet number). In biotic conditions, a series of impact factors are also investigated (i.e., spatial distribution, reaction kinetics and NAPL-induced toxicity). Finally, the current model is used to evaluate the pore scale relevance of a local equilibrium assumption between fluid phase and biofilm phase in the vicinity of the NAPL source.

Time‐Dependent Pore Filling

AbstractCapillarity traps fluids in porous media during immiscible fluid displacement. Most field situations involve relatively long time scales, such as hydrocarbon migration into reservoirs, resource recovery, nonaqueous phase liquid remediation, geological CO2 storage, and sediment‐atmosphere interactions. Yet laboratory studies and numerical simulations of capillary phenomena rarely consider the impact of time on these processes. We use time‐lapse microphotography to record the evolution of saturation in air‐ or hydrocarbon‐filled capillary tubes submerged in water to investigate long‐term pore filling phenomena beyond imbibition. Microphotographic sequences capture a lively pore filling history where various concurrent physical phenomena coexist. Dissolution and diffusion play a central role. Observations indicate preferential transport of the wetting liquid along corners, vapor condensation, capillary flow induced by asymmetrical interfaces, and interface pinning that defines the diffusion length. Other processes include internal snap‐offs, fluid redistribution, and changes in wettability as fluids dissolve into each other. Overall, the rate of pore filling is diffusion‐controlled for a given interfacial configuration; diffusive transport takes place at a constant rate for pinned interfaces and is proportional to the square root of time for free interfaces where the diffusion length increases with time.

Recent advances in surfactant-enhanced In-Situ Chemical Oxidation for the remediation of non-aqueous phase liquid contaminated soils and aquifers

Abstract Protecting soils and groundwater from contaminants is a global challenge and remediating contaminated sites may require decades and many resources. Non-aqueous phase liquids (NAPLs) resulting from oil and gas exploration or drilling, coal gas plants, leaking storage tanks and wood treatment facilities are major contaminants of subsurface soil and groundwater. In-Situ Chemical Oxidation (ISCO) has certain limitations with respect to cost, time, and rebound of contaminants when it is applied to remediation of NAPLs. The development of plant-based biodegradable and environmentally friendly surfactants has opened the door to developing an innovative Surfactant Enhanced In-Situ Chemical Oxidation (S-ISCO) system for remediating hydrocarbon contaminated soils and aquifers. This review highlights the state of the art of this technology and compares it to ISCO. An overview on the three most important components of S-ISCO — surfactant/co-solvent mixtures, the catalyst and/or the oxidant and the free radicals is presented. The major factors affecting the application of S-ISCO for the remediation of contaminated sites are highlighted. This review delivers a detailed explanation on the recent case studies and field applications of S-ISCO and factors affecting its implementation.

Optimizing Ethanol Enhanced NAPL Remediation Using Evolutionary Algorithms

In the past decades, subsurface non-aqueous phase liquid (NAPL) contamination has been recognized as one of the most widespread and challenging environmental problems. Thus, researchers have focused their efforts on developing and testing the efficiency of remediation methodologies, able to address the unique nature of these contaminants. Recently, in-situ flooding techniques for the accelerated removal of NAPLs trapped in the subsurface have been proposed, where additives are injected together with water upgradient of the NAPL-contaminated area in order to alter the physio-chemical properties of the contaminants, such as interfacial tension, and enhance their solubilities. In this work, the efficiency of ethanol enhanced NAPL remediation is addressed. To this end, a non-linear, multi-objective optimization strategy is developed by combining a multiphase flow simulation model with evolutionary algorithms. Two conflicting optimization objectives are considered: minimizing operation cost and maximizing remediation efficiency, while preventing uncontrolled NAPL mobilization. More specifically, the first objective involves the operation cost of the procedure, which is directly proportional to the pumping rate, duration and ethanol volume used. The second represents the environmental considerations of the problem that, in this work, are described by the maximization of free product removal and the prevention of DNAPL vertical spreading.

Implications of soil mixing for NAPL source zone remediation: Column studies and modeling of field-scale systems

Soil remediation is often inhibited by subsurface heterogeneity, which constrains contaminant/reagent contact. Use of soil mixing techniques for reagent delivery provides a means to overcome contaminant/reagent contact limitations. Furthermore, soil mixing reduces the permeability of treated soils, thus extending the time for reactions to proceed. This paper describes research conducted to evaluate implications of soil mixing on remediation of non-aqueous phase liquid (NAPL) source zones. The research consisted of column studies and subsequent modeling of field-scale systems. For column studies, clean influent water was flushed through columns containing homogenized soils, granular zero valent iron (ZVI), and trichloroethene (TCE) NAPL. Within the columns, NAPL depletion occurred due to dissolution, followed by either column-effluent discharge or ZVI-mediated degradation. Complete removal of TCE NAPL from the columns occurred in 6–8 pore volumes of flow. However, most of the TCE (>96%) was discharged in the column effluent; less than 4% of TCE was degraded. The low fraction of TCE degraded is attributed to the short hydraulic residence time (<4days) in the columns. Subsequently, modeling was conducted to scale up column results. By scaling up to field-relevant system sizes (>10m) and reducing permeability by one-or-more orders of magnitude, the residence time could be greatly extended, potentially for periods of years to decades. Model output indicates that the fraction of TCE degraded can be increased to >99.9%, given typical post-mixing soil permeability values. These results suggest that remediation performance can be greatly enhanced by combining contaminant degradation with an extended residence time.

Dielectric barrier discharge plasma used as a means for the remediation of soils contaminated by non-aqueous phase liquids

A plane-to-grid dielectric barrier discharge (DBD) reactor operating with air at atmospheric pressure was used to investigate the removal of non-aqueous phase liquids (NAPLs) from soil layers. A mixture of n-C10, n-C12 and n-C16 was used as a model NAPL that polluted the soil at a very high initial concentration (100,000mg/kg-soil). The effect of treatment time, energy consumption, and soil thickness on the NAPL removal efficiency was investigated, the plasma active species were identified, and the macroscopic gas temperature was determined. The NAPL remediation efficiency found to be as high as 99.9% after 60–120s of plasma treatment, depending on soil thickness. The energy density required to remediate completely the NAPL was about 600J/g-soil and was practically independent of the soil thickness, indicating that the DBD-based plasma has the potential to become a highly cost-effective technology for the remediation of NAPL-contaminated soils. N2+, N2∗, NOx and O3 were identified as plasma-induced reactive species, a maximum gas temperature close to 300°C was recorded, and the total carbon detected in exhaust gases, in the form of CO and CO2, was ca 40% of that contained in the NAPL removed from the soil. The main mechanisms of NAPL removal by plasma found to be the evaporation of organic compounds coupled with their oxidation in liquid and gas phase. Using ATR-FTIR in combination with high-throughput organic profiling analysis by GC–MS, ketones and alcohols were identified as the main intermediate products of alkanes oxidation in soil matrix.

Three-Phase Fractional Flow Analysis for Foam-Assisted Non-aqueous Phase Liquid (NAPL) Remediation

Among numerous foam applications in a wide range of disciplines, foam flow in porous media has been spotlighted for improved/enhanced oil recovery processes and shallow subsurface in situ NAPL (non-aqueous phase liquid) remediation, where foams can reduce the mobility of gas phase by increasing effective gas viscosity and improve sweep efficiency by mitigating subsurface heterogeneity. This study investigates how foams interact with and displace oleic contaminants in remediation treatments by using MoC (Method of Characteristics)-based three-phase fractional flow theory. Six different scenarios are considered such as different levels of foam strength (i.e., gas mobility reduction factors), different initial conditions (i.e., initially oil/water or oil/water/gas present), foam stability affected by water saturation $$({S}_{\mathrm{w}})$$ and oil saturation $$({S}_{\mathrm{o}})$$ , and uniform versus non-uniform initial saturations. The process is analyzed by using ternary diagrams, fractional flow curves, effluent histories, saturation profiles, time–distance diagrams, and pressure and recovery histories. The results show that the three-phase fractional flow analysis presented in this study is robust enough to analyze foam–oil displacements in various conditions, as validated by an in-house numerical simulator built in this study. The use of numerical simulation seems crucial when the foam process becomes very complicated and faces multiple possible solutions.

Non-aqueous phase liquid-contaminated soil remediation by ex situ dielectric barrier discharge plasma

Non-thermal dielectric barrier discharge plasma is examined as a method for the ex situ remediation of non-aqueous phase liquid (NAPL)-contaminated soils. A mixture of equal mass concentrations (w/w) of n-decane, n-dodecane and n-hexadecane was used as model NAPL. Two soil types differing with respect to the degree of micro-heterogeneity were artificially polluted by NAPL: a homogeneous silicate sand and a moderately heterogeneous loamy sand. The effect of soil heterogeneity, NAPL concentration and energy density on soil remediation efficiency was investigated by treating NAPL-polluted samples for various treatment times and three NAPL concentrations. The concentration and composition of the residual NAPL in soil were determined with NAPL extraction in dichloromethane and GC-FID analysis, while new oxidized products were identified with attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). The experimental results indicated that the overall NAPL removal efficiency increases rapidly in early times reaching a plateau at late times, where NAPL is removed almost completely. The overall NAPL removal efficiency decreases with its concentration increasing and soil heterogeneity strengthening. The removal efficiency of each NAPL compound is inversely proportional to the number of carbon atoms and consistent with alkane volatility. A potential NAPL degradation mechanism is suggested by accounting for intermediates and final products as quantified by GC-FID and identified by ATR-FTIR.

Microbially enhanced dissolution and reductive dechlorination of PCE by a mixed culture: Model validation and sensitivity analysis

Reductive dechlorination catalyzed by organohalide-respiring bacteria is often considered for remediation of non-aqueous phase liquid (NAPL) source zones due to cost savings, ease of implementation, regulatory acceptance, and sustainability. Despite knowledge of the key dechlorinators, an understanding of the processes and factors that control NAPL dissolution rates and detoxification (i.e., ethene formation) is lacking. A recent column study demonstrated a 5-fold cumulative enhancement in tetrachloroethene (PCE) dissolution and ethene formation (Amos et al., 2009). Spatial and temporal monitoring of key geochemical and microbial (i.e., Geobacter lovleyi and Dehalococcoides mccartyi strains) parameters in the column generated a data set used herein as the basis for refinement and testing of a multiphase, compositional transport model. The refined model is capable of simulating the reactive transport of multiple chemical constituents produced and consumed by organohalide-respiring bacteria and accounts for substrate limitations and competitive inhibition. Parameter estimation techniques were used to optimize the values of sensitive microbial kinetic parameters, including maximum utilization rates, biomass yield coefficients, and endogenous decay rates. Comparison and calibration of model simulations with the experimental data demonstrate that the model is able to accurately reproduce measured effluent concentrations, while delineating trends in dechlorinator growth and reductive dechlorination kinetics along the column. Sensitivity analyses performed on the optimized model parameters indicate that the rates of PCE and cis-1,2-dichloroethene (cis-DCE) transformation and Dehalococcoides growth govern bioenhanced dissolution, as long as electron donor (i.e., hydrogen flux) is not limiting. Dissolution enhancements were shown to be independent of cis-DCE accumulation; however, accumulation of cis-DCE, as well as column length and flow rate (i.e., column residence time), strongly influenced the extent of reductive dechlorination. When cis-DCE inhibition was neglected, the model over-predicted ethene production ten-fold, while reductions in residence time (i.e., a two-fold decrease in column length or two-fold increase in flow rate) resulted in a more than 70% decline in ethene production. These results suggest that spatial and temporal variations in microbial community composition and activity must be understood to model, predict, and manage bioenhanced NAPL dissolution.

Remediation of Nonaqueous Phase Liquid Polluted Sites Using Surfactant‐Enhanced Air Sparging and Soil Vapor Extraction

A two-dimensional laboratory sand tank was installed to study the remediation efficiency of surfactant-enhanced air sparging (-SEAS) coupled with soil vapor extraction (SVE) in nonaqueous phase liquid (NAPL) polluted sites. During initial stages of remediation, it was more reasonable to use conventional air sparging coupled with SVE. When most free NAPLs were removed and contaminant removal rate was maintained at a relatively low level, surfactant was added to the groundwater. During enhanced remediation, lower interfacial tension caused residual NAPLs in the porous media to slightly migrate, making the downstream contaminant concentration somewhat higher. The polluted area, however, was not more enlarged than before. The decrease in surface tension resulted in increased air saturation in the groundwater and the extent of the air influence zone. After 310 hours, 78.7% of the initial chlorobenzene mass had volatilized, 3.3% had migrated out of the sand profile, 17.5% was in the vadose zone, and 0.5% remained in the groundwater, thus revealing that SEAS/SVE can effectively improve the remediation of NAPL polluted sites.

Wettability contrasts between fresh and weathered diesel fuels

The remediation of non-aqueous phase liquid (NAPL) contaminated sites is impeded due to subsurface complexities, including wettability. Wettability quantifies which of two immiscible fluids preferentially coats a solid. At most contaminated sites water-wetting conditions are typically assumed despite mounting evidence that this is not always the case. In this study, wettability was examined for two NAPL samples of contrasting origin: a fresh and a field sample. Wettability was assessed through (i) cyclical, 'cumulative elapsed contact time' intrinsic contact angle measurements, (ii) interface jar tests, and (iii) cyclical, pseudo-static capillary pressure-saturation curves. The work as a whole demonstrated that while the fresh diesel sample was consistently water-wet, the field diesel sample exhibited repeatable cycles of wettability reversal between water drainage and imbibition. And while wettability hysteresis increased with contact time for the field diesel, the occurrence of wettability reversal at each change of saturation direction was independent of contact time. Such behavior is not easily assessed by standard wettability indices. Moreover, it contrasts with the permanent wettability alteration observed for complex organics (e.g., coal tar) observed in most studies. It is hypothesized that the cyclical wettability reversal is related to cyclical changes in intermediate pore wettability due to sorption of surface active compounds (causing NAPL-wetting imbibition) and rupturing of the soil grain water film (causing water-wet drainage). The wettability differences between the two NAPLs may be due to additives (i.e., a surfactant) in the original formulation and/or byproducts from subsurface weathering. These results support better characterization of site-specific wettability, improved model development and more realistic site conceptual models for improved remediation efforts.

A Bayesian approach to integrate temporal data into probabilistic risk analysis of monitored NAPL remediation

Upon their release into the subsurface, non-aqueous phase liquids (NAPLs) dissolve slowly in groundwater and/or volatilize in the vadose zone threatening the environment and public health over extended periods of time. The failure of a treatment technology at any given site is often due to the unnoticed presence of dissolved NAPL trapped in low permeability areas and/or the remaining presence of substantial amounts of pure phase NAPL after remediation efforts. The design of remediation strategies and the determination of remediation endpoints are traditionally carried out without quantifying risks associated with the failure of such efforts. We conduct a probabilistic risk analysis (PRA) to estimate the likelihood of failure of an on-site NAPL treatment technology. The PRA integrates all aspects of the problem (causes, pathways, and receptors) without resorting to extensive modeling. It accounts for a combination of multiple mechanisms of failure of a monitoring system, such as bypassing, insufficient sampling frequency and malfunctioning of the observation wells. We use a Bayesian framework to update the likelihood of failure of the treatment technology with observed measurements of concentrations at nearby monitoring wells.