Jason I. Gerhard

Variability of Point Source Infiltration Rates for Two-Phase Flow in Heterogeneous Porous Media

This study examines the influence of source release location, size, and strength on the infiltration rate and degree of lateral spreading of a dense nonwetting liquid infiltrating into an initially wetting liquid saturated, heterogeneous porous medium. It is demonstrated through numerical simulation in 25 realizations of a spatially correlated random hydraulic conductivity field that infiltration rates for point source releases are lognormally distributed with a variance equal to that of the underlying hydraulic conductivity distribution. The variability in infiltration rates is shown to decrease for sources larger than the correlation scale of hydraulic conductivity. In all cases, individual infiltration rates showed no tendency to converge to the ensemble average. A moment analysis demonstrates that the degree of lateral spreading of the nonwetting body for individual realizations varied greatly and also did not display a tendency to converge to an ensemble average. Numerical simulations carried out in an equivalent homogeneous porous medium incorporating large-scale anisotropy of intrinsic permeability provided infiltration rates below the ensemble average. For point source releases the degree of lateral spreading exhibited in the equivalent homogeneous porous medium was below the entire ensemble of heterogeneous results. A series of 10 simulations conducted in a single realization demonstrates that the degree of lateral spreading (second moment) along main drainage is a function of the average nonwetting phase saturation with greater degrees of lateral spreading at low capillary pressures. The practical implication of this study is that in addition to fluid and media properties the specific order of encounter of varying permeability lenses must be known in the immediate vicinity of a nonwetting phase release if infiltration rates are to be accurately predicted.

Measurement and prediction of the relationship between capillary pressure, saturation, and interfacial area in a NAPL-water-glass bead system

(1) In this work, the constitutive relationship between capillary pressure (Pc), saturation (Sw), and fluid-fluid interfacial area per volume (IFA) is characterized using computed microtomography for drainage and imbibition experiments consisting of a nonaqueous phase liquid and water. The experimentally measured relationship was compared to a thermodynamic model that relates the area under the PcSw curve to the total IFA, an, and the capillary-associated IFA, anw. Surfaces were fit to the experimental and modeled PcSwan and PcSwanw data in order to characterize the relationship in three dimensions (3D). For the experimental system, it was shown that the PcSwan relationship does not exhibit hysteresis. The model is found to provide a reasonable approximation of the magnitude of the 3D surfaces for an and anw, with a mean absolute percent error of 26% and 15%, respectively. The relatively high mean absolute percent errors are primarily the result of discrepancies observed at the wetting- and nonwetting-phase residual saturation values. Differences in the shapes of the surfaces are noted, particularly in the curvature (arising from the addition of scanning curves and presence of anSw hysteresis in the predicted results) and endpoints (particularly the inherent nature of thermodynamic models to predict significant anw associated with residual nonwetting-phase saturation). Overall, the thermodynamic model is shown to be a practical, inexpensive tool for predicting the PcSwan and PcSwanw surfaces from PcSw data. Citation: Porter, M. L., D. Wildenschild, G. Grant, and J. I. Gerhard (2010), Measurement and prediction of the relationship between capillary pressure, saturation, and interfacial area in a NAPL-water-glass bead system, Water Resour. Res., 46, W08512,

Self-Sustaining Smoldering Combustion for NAPL Remediation: Laboratory Evaluation of Process Sensitivity to Key Parameters

Smoldering combustion has been introduced recently as a potential remediation strategy for soil contaminated by nonaqueous phase liquids (NAPLs). Published proof-of-concept experiments demonstrated that the process can be self-sustaining (i.e., requires energy input only to start the process) and achieve essentially complete remediation of the contaminated soil. Those initial experiments indicated that the process may be applicable across a broad range of NAPLs and soils. This work presents the results of a series of bench-scale experiments that examine in detail the sensitivity of the process to a range of key parameters, including contaminant concentration, water saturation, soil type, and air flow rates for two contaminants, coal tar and crude oil. Smoldering combustion was observed to be self-sustaining in the range 28,400 to 142,000 mg/kg for coal tar and in the range 31,200 to 104,000 mg/kg for crude oil, for the base case air flux. The process remained self-sustaining and achieved effective remediation across a range of initial water concentrations (0 to 177,000 mg/kg water) despite extended ignition times and decreased temperatures and velocities of the reaction front. The process also exhibited self-sustaining and effective remediation behavior across a range of fine to coarse sand grain sizes up to a threshold maximum value between 6 mm and 10 mm. Propagation velocity is observed to be highly dependent on air flux, and smoldering was observed to be self-sustaining down to an air Darcy flux of at least 0.5 cm/s for both contaminants. The extent of remediation in these cases was determined to be at least 99.5% and 99.9% for crude oil and coal tar, respectively. Moreover, no physical evidence of contamination was detected in the treatment zone for any case where a self-sustaining reaction was achieved. Lateral heat losses to the external environment were observed to significantly affect the smoldering process at the bench scale, suggesting that the field-scale lower bounds on concentration and air flux and upper bound on grain size were not achieved; larger scale experiments and field trials where lateral heat losses are much less significant are necessary to define these process limits for the purposes of field application. This work provides valuable design data for pilot field trials of both in situ and ex situ smoldering remediation applications.

Volumetric scale-up of smouldering remediation of contaminated materials

Smouldering remediation is a process that has been introduced recently to address non-aqueous phase liquid (NAPL) contamination in soils and other porous media. Previous work demonstrated this process to be highly effective across a wide range of contaminants and soil conditions at the bench scale. In this work, a suite of 12 experiments explored the effectiveness of the process as operating scale was increased 1000-fold from the bench (0.003m(3)) to intermediate (0.3m(3)) and pilot field-scale (3m(3)) with coal tar and petrochemical NAPLs. As scale increased, remediation efficiency of 97-99.95% was maintained. Smouldering propagation velocities of 0.6-14×10(-5)m/s at Darcy air fluxes of 1.54-9.15cm/s were consistent with observations in previous bench studies, as was the dependence on air flux. The pilot field-scale experiments demonstrated the robustness of the process despite heterogeneities, localised operation, controllability through airflow supply, and the importance of a minimum air flux for self-sustainability. Experiments at the intermediate scale established a minimum-observed, not minimum-possible, initial concentration of 12,000mg/kg in mixed oil waste, providing support for the expectation that lower thresholds for self-sustaining smouldering decreased with increasing scale. Once the threshold was exceeded, basic process characteristics of average peak temperature, destructive efficiency, and treatment velocity were relatively independent of scale.

Evaluating four-dimensional time-lapse electrical resistivity tomography for monitoring DNAPL source zone remediation.

Practical, non-invasive tools do not currently exist for mapping the remediation of dense non-aqueous phase liquids (DNAPLs). Electrical resistivity tomography (ERT) exhibits significant potential but has not yet become a practitioners tool due to challenges in interpreting the survey results at real sites. This study explores the effectiveness of recently developed four-dimensional (4D, i.e., 3D space plus time) time-lapse surface ERT to monitor DNAPL source zone remediation. A laboratory experiment demonstrated the approach for mapping a changing NAPL distribution over time. A recently developed DNAPL-ERT numerical model was then employed to independently simulate the experiment, providing confidence that the DNAPL-ERT model is a reliable tool for simulating real systems. The numerical model was then used to evaluate the potential for this approach at the field scale. Four DNAPL source zones, exhibiting a range of complexity, were initially simulated, followed by modeled time-lapse ERT monitoring of complete DNAPL remediation by enhanced dissolution. 4D ERT inversion provided estimates of the regions of the source zone experiencing mass reduction with time. Results show that 4D time-lapse ERT has significant potential to map both the outline and the center of mass of the evolving treated portion of the source zone to within a few meters in each direction. In addition, the technique can provide a reasonable, albeit conservative, estimate of the DNAPL volume remediated with time: 25% underestimation in the upper 2m and up to 50% underestimation at late time between 2 and 4m depth. The technique is less reliable for identifying cleanup of DNAPL stringers outside the main DNAPL body. Overall, this study demonstrates that 4D time-lapse ERT has potential for mapping where and how quickly DNAPL mass changes in real time during site remediation.

Hydraulic Displacement of Dense Nonaqueous Phase Liquids for Source Zone Stabilization

Hydraulic displacement is a mass removal technology suitable for stabilization of a dense, nonaqueous phase liquid (DNAPL) source zone, where stabilization is defined as reducing DNAPL saturations and reducing the risk of future pool mobilization. High resolution three-dimensional multiphase flow simulations incorporating a spatially correlated, heterogeneous porous medium illustrate that hydraulic displacement results in an increase in the amount of residual DNAPL present, which in turn results in increased solute concentrations in groundwater, an increase in the rate of DNAPL dissolution, and an increase in the solute mass flux. A higher percentage of DNAPL recovery is associated with higher initial DNAPL release volumes, lower density DNAPLs, more heterogeneous porous media, and increased drawdown of groundwater at extraction wells. The fact that higher rates of recovery are associated with more heterogeneous porous media stems from the fact that larger contrasts in permeability provide for a higher proportion of capillary barriers upon which DNAPL pooling and lateral migration can occur. Across all scenarios evaluated in this study, the ganglia-to-pool (GTP) ratio generally increased from approximately 0.1 to between approximately 0.3 and 0.7 depending on the type of DNAPL, the degree of heterogeneity, and the imposed hydraulic gradient. The volume of DNAPL recovered as a result of implementing hydraulic displacement ranged from between 9.4% and 45.2% of the initial release volume, with the largest percentage recovery associated with 1,1,1 trichloroethane, the least dense of the three DNAPLs considered.

The impact of immobile zones on the transport and retention of nanoparticles in porous media: IMMOBILE ZONES AND NANOPARTICLE TRANSPORT AND RETENTION

Nanoparticle transport and retention within porous media is treated by conceptualizing the porous media as a series of independent collectors (e.g., Colloid Filtration Theory). This conceptualization assumes that flow phenomena near grain-grain contacts, such as immobile zones (areas of low flow), exert a negligible influence on nanoparticle transport and assumes that retention and release of particles depends only on surface chemistry. This study investigated the impact of immobile zones on nanoparticle transport and retention by employing Synchrotron X-ray Computed Microtomography (SXCMT) to examine pore-scale silver nanoparticle distributions during transport through three sand columns: uniform iron oxide, uniform quartz and well graded quartz. Extended tailing was observed during the elution phase of all experiments suggesting that hydraulic retention in immobile zones, not detachment from grains, was the source of tailing. A numerical simulation of fluid flow through an SXCMT dataset predicted the presence of immobile zones near grain-grain contacts. SXCMT-determined silver nanoparticle concentrations observed that significantly lower nanoparticle concentrations existed near grain-grain contacts throughout the duration of all experiments. In addition, the SXCMT-determined pore-scale concentration gradients were found to be independent of surface chemistry and grain size distribution, suggesting that immobile zones limit the diffusive transport of nanoparticles towards the collectors. These results suggest that the well-known overprediction of nanoparticle retention by traditional CFT may be due to ignoring the influences of grain-grain contacts and immobile zones. As such, accurate prediction of nanoparticle transport requires consideration of immobile zones and their influence on both hydraulic and surface retention. This article is protected by copyright. All rights reserved.

Application of self-sustaining smouldering combustion for the destruction of wastewater biosolids

Managing biosolids, the major by-product from wastewater treatment plants (WWTPs), persists as a widespread challenge that often constitutes the majority of WWTP operating costs. Self-sustained smouldering combustion is a new approach for organic waste treatment, in which the waste - the combustion fuel - is destroyed in an energy efficient manner after mixing it with sand. Smouldering has never been applied to biosolids. Column experiments, using biosolids obtained from a WWTP, were employed to identify if, and under what conditions, smouldering could be used for treating biosolids. The parameter space in which smouldering was self-sustaining was mapped as a function of key system metrics: (1) sand/biosolids mass fraction, (2) biosolids moisture content, and (3) forced air flux. It was found that a self-sustaining reaction is achievable using biosolids with water content as high as 80% (with a biosolids lower heating value greater than 1.6 kJ/g). Moreover, results suggest that operator-controlled air flux can assist in keeping the reaction self-sustaining in response to fluctuations in biosolids properties. This proof-of-concept demonstrates the potential for smouldering as a new energy efficient biosolids disposal method for very wet (i.e., minimally processed) biosolids that may offer WWTPs significant operating cost savings. This study emphasizes smoulderings usefulness as a novel waste management technique.

Modeling U(VI) Biomineralization in Single and Dual Porosity Porous Media

Uranium extraction, processing and storage have resulted in a legacy of uranium-contaminated groundwater aquifers worldwide. An emerging remediation technology for such sites is the in situ immobilization of uranium via biostimulation of dissimilatory metal reducing bacteria (DMRB). While this approach has been successfully demonstrated in experimental studies, advances in understanding and optimization of the technique are needed. The motivation of this work was to understand better how dual-porosity (DP) porous media may affect immobilization efficiency via interactions with the dominant geochemical and microbial processes. A biogeochemical reactive transport model was developed for uranium immobilization by DMRB in both single and DP porous media. The impact that microbial residence location has on the success of biostimulated U(VI) immobilization in DP porous media was explored under various porosity and mass transfer conditions. Simulations suggest that DP media are likely to show delayed U(VI) immobilization relative to single porosity systems. U(VI) immobilization is predicted to be less when microbial activity is restricted to diffusion- dominant regions but not when restricted to advective- dominant regions. The results further highlight the importance of characterizing the bioresidency status of field sites if biogeochemical models are to predict accurately remediation schemes in physically heterogeneous media.

Smoldering Remediation of Coal-Tar-Contaminated Soil: Pilot Field Tests of STAR

Self-sustaining treatment for active remediation (STAR) is an emerging, smoldering-based technology for nonaqueous-phase liquid (NAPL) remediation. This work presents the first in situ field evaluation of STAR. Pilot field tests were performed at 3.0 m (shallow test) and 7.9 m (deep test) below ground surface within distinct lithological units contaminated with coal tar at a former industrial facility. Self-sustained smoldering (i.e., after the in-well ignition heater was terminated) was demonstrated below the water table for the first time. The outward propagation of a NAPL smoldering front was mapped, and the NAPL destruction rate was quantified in real time. A total of 3700 kg of coal tar over 12 days in the shallow test and 860 kg over 11 days in the deep test was destroyed; less than 2% of total mass removed was volatilized. Self-sustaining propagation was relatively uniform radially outward in the deep test, achieving a radius of influence of 3.7 m; strong permeability contrasts and installed barriers influenced the front propagation geometry in the shallow test. Reductions in soil hydrocarbon concentrations of 99.3% and 97.3% were achieved in the shallow and deep tests, respectively. Overall, this provides the first field evaluation of STAR and demonstrates that it is effective in situ and under a variety of conditions and provides the information necessary for designing the full-scale site treatment.