Nonaqueous Phase Liquid Source Zone Research Articles

Simulating field-scale thermal conductive heating with the potential for the migration and condensation of vapors

Thermal conductive heating (TCH) is an in-situ thermal treatment (ISTT) technology for treating non-aqueous phase liquid (NAPL) source zones. Numerical models can be useful tools for improving remedial performance, but traditional multiphase flow models are rarely used to simulate mass recovery during ISTT applications at the field scale due to their computational expense. This study developed a 3D model based on macroscopic invasion percolation to simulate the vaporization of NAPL, and the subsequent vapor migration and potential condensation at the field scale. The model was used to simulate the mass recovery of trichloroethene (TCE) from a NAPL source zone under seven scenarios of different heater placements, including three scenarios with an undersized target treatment zone (TTZ). Simulation results showed that TCH was effective in removing NAPL within the TTZ, but the treatment zone did not extend far from the perimeter heaters. In addition, during heating, NAPL condensation outside the TTZ due to the escaping vapor was observed in all scenarios. Overall, the resulting mass recovery was lower in the three scenarios with an undersized TTZ (91–95%) than in the other four scenarios (≈ 99%). Moreover, the locations of unrecovered/condensed NAPL could be inferred by monitoring mass recovery tailing at individual extraction wells.

Significance of Mass–Concentration Relation on the Contaminant Source Depletion in the Nonaqueous Phase Liquid (NAPL) Contaminated Zone

This study develops a new numerical model adopting a generic relation between the nonaqueous phase liquid (NAPL) mass and aqueous-phase NAPL concentration to simulate the relationship between NAPL contaminant mass discharge and contaminant mass reduction in the source zone, which plays a critical role to assist with site management decisions on contaminated zone remediation. The model can accommodate any contaminant mass and concentration relations and applicable to the situations when groundwater flowrate in the NAPL source zone varies in any form temporally. Therefore, the combined effects of mass–concentration relation and groundwater flowrate variations can be examined. It is hypothesized that the NAPL mass–concentration relations reflect the spatial variability of porous media in the subsurface. The developed model is compared with results from field monitoring sites and found to exhibit high flexibility and capability in capturing the observed complex NAPL source zone dynamics. Using six contaminant mass and concentration functions of varying shapes, we show that contaminant mass and concentration relation has pronounced effects on contaminant mass discharge dynamics in addition to the groundwater flowrate temporal variations. In general, the coupled mass–concentration relation and groundwater flowrate variations demonstrate stronger capability in capturing the NAPL source zone dynamics under a wide range of field porous medium conditions reported in the literature than the models that only consider groundwater flux variations. In particular, the concave mass–concentration models can be used in less heterogeneous porous media, while the convex mass–concentration models are more appropriate in more heterogeneous site conditions.

An empirical model for the evaluation of the dissolution rate from a DNAPL-contaminated area.

This paper investigates dynamic variation in the morphologic distribution of dense non-aqueous phase liquids (DNAPLs), which take into account the coupled mass transfer. Experiments were carried out in a 2D tank representing a reconstructed aquifer model. DNAPL dissolution rates were investigated over a wide range of DNAPL saturations, several source configurations, and different hydraulic conditions. Morphometric indexes are presented that take into consideration further factors affecting the dissolution process. Local information regarding transport parameters related to the characteristics of the medium was obtained through a neural network and an optimization algorithm applied to experimental tracer tests. The history of DNAPL source architecture, in terms of saturation, indentation grade, and orientation, was determined by image analysis. Dissolved concentrations were registered and mass transfer rate coefficients were obtained for a wide range of source-zone configurations. A statistical analysis was performed to develop a constitutive equation that is descriptive of the mass transfer rate as a function of source-zone metric characteristics. A new empirical dissolution model using the proposed morphometric parameters is presented and compared with other models. The mass transfer correlation reported incorporates morphometric parameters and considers the complex and variable architecture of non-miscible contaminants. The proposed correlation can be used for an initial assessment of non-aqueous phase liquid (NAPL) dissolution rates over a wide range of saturation (residual and non-residual) conditions and different aqueous phase velocities within the NAPL source zone.

A numerical model to simulate the NAPL source zone remediation by injecting zero-valent iron nanoparticles

A continuum-scale numerical model is developed to simulate the flow, mass-transfer, and reactive processes taking place during the injection of zero-valent iron nanoparticles (nZVI) in a water-saturated porous medium, containing a chlorinated non-aqueous phase liquid (NAPL) at its residual saturation (source zone). Firstly, the transport of the nanoparticles (NP) is modelled by coupling the nanoparticles transport with their deposition in a porous medium. A systematic sensitivity analysis is done to examine the individual effect of each parameter on the spatial and temporal evolution of nanoparticle concentration along the column length. The parameters quantifying the kinetics of attachment/detachment between the nanoparticles and the porous medium (sand grains) are estimated via inverse modeling. Secondly, the kinetics of the reaction of nZVI with dissolved tetrachloroethylene (PCE) is based on the numerical predictions of the statistical shrinking-core model that couples the mass-transfer with the reactive processes taking place at a nanoparticle scale. The continuum NP transport model is then extended to include the reactive flows by combining the dynamics of PCE ganglia dissolution and nZVI reactions with mass balances for the residual PCE saturation, the dissolved PCE concentration, and the nZVI concentration in the aqueous phase. Sensitivity analyses are performed on the NP transport-NAPL remediation model with a view to clarify the effects of dimensionless parameters (dimensionless flow velocity, Damköhler numbers) on the fate of nZVI and trapped PCE ganglia. Finally, the residual PCE remediation efficiency is numerically predicted as a function of the injected NP mass. This efficiency is compared with the respective experimental results of a PCE source zone remediation test performed in a sand column, by using as adjustable parameters the kinetic coefficients of the nanoparticles deposition on sand grains. The remediation efficiency of the PCE source zone is maximized when the injected nZVI maintains its reactivity throughout its flow in the porous medium. To design an efficient NAPL source zone remediation strategy, the parameters must be set to values which will ensure that the nZVI is kept to a higher than zero concentration throughout the length of the porous medium.

Temperature-dependent dissolution of residual non-aqueous phase liquids: model development and verification

The use of heat storages in the subsurface, especially in urbanized areas, may conflict with existing subsurface contaminations of non-aqueous phase liquids (NAPL). In this work, available data and models regarding temperature influences on parameters for kinetic NAPL dissolution of trichloroethene (TCE) are summarized, discussed and implemented into a numerical simulator. As systematic data on temperature-dependent TCE solubility, diffusion coefficients and dissolution rates are sparse, a set of high-resolution quasi-2D laboratory NAPL dissolution experiments using TCE was conducted at 10, 20, 40 and 70 degrees C. Because the experimental data show incomplete dissolution of the residual TCE-NAPL, two different classes of TCE-NAPL blobs representing fast and slow dissolution kinetics were introduced in the model. A good agreement of model simulations and experimental measurements of TCE mass flow rates could thus be obtained for each temperature investigated. The numerical model thus can be applied to simulate kinetic dissolution of residual NAPL source zones in groundwater under variable temperature conditions.

Temporal Dynamics of NAPL Source Zone Strength: Relationship between Groundwater Flux and Contaminant Mass Discharge

AbstractUse of contaminant mass discharge (CMD) or mass-flux measurements to characterize site conditions and assess remediation performance of nonaqueous phase liquid (NAPL) is becoming popular. The main objective of this study is to determine how groundwater flux variations in the source zone can affect nonaqueous phase liquid (NAPL) dissolution dynamics. In particular, the authors develop interplays among groundwater flux variations, NAPL aqueous concentration, NAPL CMD, and other source strength dynamics. A power-law functional form is used to describe groundwater flux variations in illustrating how NAPL source strength functions can be affected. The developed analytical models can capture a wide range of NAPL source zone dynamics encountered in real-world applications. The results demonstrate the significance of groundwater flux variations in influencing the NAPL source dynamics. If groundwater flux decreases with time, the CMD declines initially at higher rate, but the rate decreases at later stages...

Predictions of bioenhancement of nonaqueous phase liquid ganglia dissolution using first- and zero-order biokinetic models

The bioenhanced dissolution of nonaqueous phase liquid (NAPL) contaminants that occurs as a result of an increased concentration gradient is influenced by several factors, including the biokinetics. This is important because available data suggest that at typical NAPL source zone concentrations, descriptions of dissolution bioenhancement may require kinetic expressions ranging from first- to zero-order. In this work, an analytical model for the bioenhancement factor, E, is developed for NAPL ganglia dissolution with zero-order kinetics, and compared to a model for E with first-order kinetics. The models are analyzed and an illustrative example is provided to demonstrate the importance of using the correct biokinetics when estimating the potential magnitude of the bioenhancement of NAPL ganglia dissolution.

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.

Using radon as environmental tracer for the assessment of subsurface Non-Aqueous Phase Liquid (NAPL) contamination – A review

The radioactive noble gas radon has an ambivalent nature: on the one hand is it of main concern with regard to radiation protection, on the other hand can it be applied as powerful tracer tool in various fields of applied geosciences. Due to its omnipresence in nature, its chemical and physical properties, and its uncomplicated detectability radon fulfils all requirements for being used as environmental tracer. This application is discussed in the paper with focus on the use of radon as tracer for subsurface contamination with Non-Aqueous Phase Liquids (NAPL). After a short introduction in the ambivalence and ubiquitous presence of radon in nature, the theoretical background of its suitability as NAPL tracer is summarized. Finally three potential applications are discussed. Background information and practical examples are given for (i) the investigation of residual NAPL contamination in soils, (ii) the investigation of residual NAPL contamination in aquifers and (iii) the monitoring of the remediation of dissolved NAPL contamination in groundwater. The presented information reveals that radon is an ideal tracer for the assessment of a wide range of subsurface NAPL contamination. Still, its application is not without restrictions. Problems may occur due to mineralogical heterogeneity of the soil or aquifer matrix. Furthermore, local changes in the permeability of the subsurface may be associated with preferential groundwater or soil gas flow paths bypassing isolated sub-domains of an investigated NAPL source zone. Moreover, NAPL aging may result in alterations in the composition of a complex NAPL mixture thus giving rise to significant changes of the radon partition coefficient between NAPL and water or soil gas. However, since radon shows a strong affinity to NAPLs in general, semi-quantitative results will always be possible.

An evaluation of Sherwood–Gilland models for NAPL dissolution and their relationship to soil properties

Predicting the longevity of non-aqueous phase liquid (NAPL) source zones has proven to be a difficult modeling problem that has yet to be resolved. Research efforts towards understanding NAPL depletion have focused on developing empirical models that relate lumped mass transfer rates to velocities and organic saturations. These empirical models are often unable to predict NAPL dissolution for systems different from those used to calibrate them, indicating that system-specific factors important for dissolution are not considered. This introduces the need for a calibration step before these models can be reliably used to predict NAPL dissolution for systems of arbitrary characteristics. In this paper, five published Sherwood-Gilland models are evaluated using experimental observations from the dissolution of two laboratory-scale complex three-dimensional NAPL source zones. It is shown that the relative behavior of the five models depends on the system and source zone characteristics. Through a theoretical analysis, comparing Sherwood-Gilland type models to a process-based, thermodynamic dissolution model, it is shown that the coefficients of the Sherwood-Gilland models can be related to measurable soil properties. The derived dissolution model with soil-dependent coefficients predicts concentrations identical to those predicted by the thermodynamic dissolution model for cases with negligible hysteresis. This correspondence breaks down when hysteresis has a significant impact on interfacial areas. In such cases, the derived dissolution model will slightly underestimate dissolved concentrations at later times, but is more likely to capture system-specific dissolution rates than Sherwood-Gilland models.

The role of geological heterogeneity and variability in water infiltration on non-aqueous phase liquid migration

This study investigates the influence of two factors—geological heterogeneity and variability in water infiltration—on non-aqueous phase liquid (NAPL) migration in the unsaturated zone. NAPL migration under three-phase flow conditions resulting from a ground surface spill is modeled for multiple heterogeneous realizations of a porous medium with various water infiltration scenarios. Increased water infiltration before the spill has two counteracting effects: NAPL relative permeability (k rn) increases with increasing water saturation (S w) for a given NAPL saturation, while higher S w in the soil near the NAPL source zone leads to less NAPL mass infiltration. It is found that the former effect is overwhelmed by the latter effect, the net effect being that with longer infiltration durations before the spill, both the infiltrated NAPL mass and the depth of the front migration decrease. Simulation results also show strong effect of the medium heterogeneity. Results suggest that total infiltrated mass, front depth and plume spread increase with an increasing standard deviation of log-permeability. Also variability in modeling results among realizations is largely impacted by the log-permeability standard deviation. Spatial correlation in permeability also strongly influences NAPL infiltration. An increase in the isotropic correlation length from 0.75 to 1.5 m leads to a decrease in total infiltrated mass, plume migration depth as well as vertical spread. Lateral spread in this case is not shown to be affected by the correlation length.

Aquitard contaminant storage and flux resulting from dense nonaqueous phase liquid source zone dissolution and remediation

A one‐dimensional diffusion model was used to investigate the effects of dense nonaqueous phase liquid (DNAPL) source zone dissolution and remediation on the storage and release of contaminants from aquitards. Source zone dissolution was represented by a power law source depletion model, which served as a time variable boundary condition to the diffusion equation used to describe mass transport in the aquitard. Two key variables were used to assess source zone dissolution behavior on aquitard mass storage and release: the power law exponential term Γ, which reflects the influence of the source zone architecture, and a new variable defined herein as the source to aquitard mass transfer coefficient β, which reflects the influences of both the source characteristics and the aquitard media properties. As Γ increased or as β increased because of more rapid source dissolution, the aquitard concentrations, depth of penetration, and long‐term back‐diffusion flux decreased. However, when β increased because of increased sorption, concentrations and back diffusion increased but penetration decreased. The duration of aquitard mass loading was found to be significantly less than the duration of mass release. Moreover, the mass per unit area stored in the aquitard was 3 or more orders of magnitude less than the initial DNAPL source zone mass per unit area, and the back diffusion flux from the aquitard was typically 4 or more orders of magnitude less than the initial source zone flux. Additionally, the effects of partial source zone remediation were investigated, and the results suggest that source remediation can have a favorable effect on long‐term back‐diffusion risk.

Quantification of experimental subsurface fluid saturations from high‐resolution source zone images

Laboratory‐scale aquifer cells are commonly used to investigate processes governing contaminant fate and transport in heterogeneous subsurface systems. In recent years, dramatic improvements in image processing methods have led to increasingly refined image resolution in these experiments. With these enhanced imagining methodologies, system parameters, such as nonaqueous phase liquid (NAPL) saturation, can now be quantified at the submillimeter scale. This fine level of resolution, however, is generally inconsistent with the typical scale of a representative elementary volume (REV), the averaging volume associated with property evaluation in continuum‐based flow and transport models. Such inconsistency of scales calls into question the practice of directly comparing laboratory observations with continuum‐based model simulations. This work explores the application of alternative approaches to image data processing for characterization of NAPL source zone architecture in aquifer cells and examines the implications of data processing on the interpretation of experimental‐model comparisons. The utility of two alternative upscaling methods, a continuity‐equation based (CB) and discrete‐block based (DBB) approach are considered. Examination of the stability of point averages of saturation demonstrates that a REV length scale can be defined as ∼30 × d50 for the thickness‐averaged, two‐dimensional aquifer cell experiments examined. Quantification of upscaled source zone metrics, including the average saturation, the second spatial moment in the vertical direction, and the ganglia‐to‐pool (GTP) mass ratio reveals that the GTP is strongly sensitive to observation scale. Use of GTP values computed from upscaled images is shown to improve mathematical model predictions of experimental effluent concentrations by nearly 50%.

WITHDRAWN: Influence of Porous Media Heterogeneity on NAPL Dissolution Fingering and Upscaled Mass Transfer

AbstractThe utility of existing models for describing upscaled mass transfer from non‐aqueous phase liquid (NAPL) were examined when preferential dissolution pathways form in NAPL‐contaminated zones. Such models are needed because the coarse discretizations often used in simulators cannot capture the details of local‐scale preferential dissolution pathways.Laboratory experiments were conducted in two well‐characterized, heterogeneous packings that differed only in the correlation lengths of the permeability field. Experimental results were used to validate a numerical simulator for capturing the growth of centimeter‐scale preferential NAPL dissolution patterns. Using data from these experiments and simulations, three methods for upscaling the mass transfer rate coefficient for NAPL dissolution [Imhoff et al., 2003; Saenton and Illangasekare, 2007; Christ et al., 2006] and an equilibrium stream tube model for predicting contaminant flux [Basu et al., 2008] were evaluated. These models account for the influence of dissolution fingering [Imhoff et al., 2003] or NAPL architecture [Saenton and Illangasekare, 2007; Christ et al., 2006; Basu et al., 2008] on contaminant flux from NAPL source zones, and were applied here to local‐scale preferential NAPL dissolution. When the correlation length of permeability perpendicular to the mean water flow direction was 6.0 cm, greater than the scale of the dissolution fingers, only 4.2% of the NAPL resided in pools. Dissolution fingers formed in this experiment and all models predicted effluent concentrations with similar accuracy, with root mean square errors (RMSE's) between 0.03 and 0.05. When the correlation scale was smaller (1.0 cm), 31.3% of the NAPL was in pools and preferential dissolution pathways were dominated by channeling, preferential dissolution caused by spatial variations in aqueous phase velocity and NAPL dissolution rates. For this experiment all models performed poorly, with RMSEs between 0.12 and 0.35. Dissolution fingering was important when the permeability correlation length was sufficiently large that dissolution finger formation was not disrupted and NAPL pools were not dominant.

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

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

Iron-mediated trichloroethene reduction within nonaqueous phase liquid

Aqueous slurries or suspensions containing reactive iron nanoparticles are increasingly suggested as a potential means for remediating chlorinated solvent nonaqueous phase liquid (NAPL) source zones. Aqueous-based treatment approaches, however, may be limited by contaminant dissolution from the NAPL and the subsequent contaminant transport to the reactive nanoparticles. Reactions occurring within (or at the interface) of the NAPL may alleviate these potential limitations, but this approach has received scant attention due to concerns associated with the reactivity of iron within nonaqueous phases. Results presented herein suggest that iron nanoparticles are reactive with TCE–NAPL and exhibit dechlorination rates proportional to the concentration of (soluble) water present within the NAPL. Reactivity was assessed over a 12-day period for five water contents ranging from 0.31 M to 4.3 M, with n-butanol used to enhance water solubility in the NAPL. Rates of dechlorination were generally slower than those reported for aqueous-phase dechlorination, but were not observed to slow over the course of the 12-day period. The lack of observed deactivation may indicate the potential that highly efficient (with respect to utilization of available electrons) dechlorination reactions can be engineered to occur within nonaqueous liquids. These results suggest a need for subsequent investigations which focus on understanding the mechanisms of the reactions occurring within NAPL, as well as those assessing the utility of controlling both the iron and water content within a NAPL source zone.

Analysis of tracer tomography using temporal moments of tracer breakthrough curves

Hydraulic/partitioning tracer tomography (HPTT) was recently developed by Yeh and Zhu [Yeh T-CJ, Zhu J. Hydraulic/partitioning tracer tomography for characterization of dense nonaqueous phase liquid source zones, Water Resour Res 2007;43:W06435. doi:10.1029/2006WR004877.] for estimating spatial distribution of dense nonaqueous phase liquids (DNAPLs) in the subsurface. Since discrete tracer concentration data are directly utilized for the estimation of DNAPLs, this approach solves the hyperbolic convection–dispersion equation. Solution to the convection–dispersion equation however demands fine temporal and spatial discretization, resulting in high computational cost for an HPTT analysis. In this work, we use temporal moments of tracer breakthrough curves instead of discrete concentration data to estimate DNAPL distribution. This approach solves time independent partial differential equations of the temporal moments, and therefore avoids solving the convection–dispersion equation using a time marching scheme, resulting in a dramatic reduction of computational cost. To reduce numerical oscillations associated with convection dominated transport problems such as in inter-well tracer tests, the approach uses a finite element solver adopting the streamline upwind Petrov–Galerkin method to calculate moments and sensitivities. We test the temporal moment approach through numerical simulations. Comparing the computational costs between utilizing moments and discrete concentrations, we find that temporal moments significantly reduce the computation time. We also find that tracer moment data collected through a tomographic survey alone are able to yield reasonable estimates of hydraulic conductivity, as indicated by a correlation of 0.588 between estimated and true hydraulic conductivity fields in the synthetic case study.

Slow gas expansion in saturated natural porous media by gas injection and partitioning with non-aqueous phase liquids

The partitioning of volatile non-aqueous phase liquid (NAPL) compounds to a discontinuous gas phase can result in the expansion of that gas phase, and the resulting gas flow can significantly affect the mass transfer from NAPL source zones. This recently reported gas flow generated by the spontaneous expansion of a discontinuous gas phase has not been extensively characterized in the literature. This study measured the expansion rate of a single gas cluster in a 1.1 mm sand above a pool of trans-1,2-dichloroethene (tDCE) in small-scale flow cell experiments. To characterize the gas flow, gas injection experiments in three sizes of sand were conducted at very slow injection rates typical of gas flow rates produced by gas expansion due to NAPL partitioning. Gas cluster spontaneous expansion rates above a tDCE pool were found to be 0.34 ± 0.02 and 0.29 ± 0.01 mL/day in duplicate experiments, which is sufficiently slow to result in discontinuous gas flow in porous media with a grain size diameter greater than 0.02 mm. Measured capillary pressures during gas injection showed patterns consistent with discontinuous gas flow, and identified multiple fragmentation events and expansion by coalescence with trapped clusters. The combination of pressure data and light transmission images were used to identify fragmentation and obtain direct measurements of the critical cluster length (i.e. the length at which withdrawal of the gas phase from a pore space occurs) in quasi-two-dimensional porous media for the first time. The measured critical cluster lengths were 1.4–3.6, 3.2–6.0 and 2.8–6.5 cm in 1.1, 0.7 and 0.5 mm sands, respectively. These values agreed well with estimates of the critical cluster length made using previously reported equations, and parameters derived from the medium’s capillary pressure-saturation relationship.

Wettability hysteresis and its implications for DNAPL source zone distribution

Subsurface heterogeneity at sites contaminated with nonaqueous phase liquids (NAPLs) reduces the effectiveness of traditional remediation measures. One cause may be the increased proportion of NAPL that is hydraulically isolated due to capillary trapping in heterogeneously-wetted materials. This study examines the wettability of ten materials, ranging from minerals, such as calcite and dolomite, to carbonaceous materials, such shale and coal, in air and water, NAPL and air, and NAPL and water systems. The wettability differed depending on which phase the solid material was initially immersed in: the less crystalline solids, if initially contacted by water were water-wet, but if initially contacted by NAPL were NAPL-wet. This difference, termed here wettability hysteresis, was observed for a suite of halogenated NAPLs and was independent of equilibration time. The degree of wettability hysteresis was greatest in the NAPL and water systems, with the magnitude of the difference increasing with the carbonaceous materials. Since the degree of capillary trapping in subsurface materials is related to wettability, the phenomenon of wettability hysteresis suggests that system history is a factor that may increase the heterogeneity of NAPL source zones.

Microbial activity and distribution during enhanced contaminant dissolution from a NAPL source zone

Laboratory experiments were conducted to assess microbial reductive dechlorination in one-dimensional sand columns containing a 10 cm long source zone of uniformly distributed residual tetrachloroethene (PCE) nonaqueous phase liquid (NAPL), a 10 cm long transition zone directly down-gradient of the source zone containing some nonuniformly distributed NAPL ganglia, and a 40 cm long plume region down-gradient of the transition zone. The activity and distribution of S ulfurospirillum multivorans, a PCE-to-1,2- cis-dichloroethene ( cis-DCE) dechlorinating bacterium, was evaluated in columns containing either a mixed-NAPL (0.25 mol/mol PCE in hexadecane) or pure PCE-NAPL. Significant dechlorination of PCE to cis-DCE was observed in the mixed-NAPL column, resulting in 53% PCE-NAPL mass recovery in the effluent with PCE-NAPL dissolution enhanced by up to 13.6-fold (maximum) and 4.6-fold (cumulative) relative to abiotic dissolution. Quantitative real-time PCR targeting pceA, the PCE reductive dehalogenase gene of S. multivorans, revealed that S. multivorans cells were present in the NAPL source zone, and increased in numbers (i.e., grew) throughout the source and transition zones. In contrast, minimal reductive dechlorination and microbial growth were observed in the column containing pure PCE-NAPL, where aqueous-phase PCE concentrations reached saturation. These results demonstrate that microbial growth within NAPL source zones is possible, provided that contaminant concentrations remain below levels toxic to the dechlorinating organisms, and that microbial growth can result in significant bioenhanced NAPL dissolution.