H22C-01 INVITED
Prediction of Mass Flux Emission From DNAPL Source Zones Using Up-scaled Models of Mass Transfer
Organic waste chemicals that are in the form of dense non-aqueous phase liquids (DNAPLs), when released into the subsurface produce complex entrapment architecture containing pools and zones of residual saturation (ganglia and residues). A variety of factors contribute to the development of the DNAPL architecture and the subsequent mass transfer that occurs within the source zone, which eventually contributes to the plume development and longevity. Experimental and modeling studies were conducted to evaluate how source factors such as heterogeneity, soil texture, texture contrasts, entrapment architecture, entrapment morphology, source aging, source modification due to remedial action, composition of the DNAPL, and groundwater flow, contributes to mass transfer, with the goal of predicting the mass flux emission form source zones. The processes that incorporate these factors into simulation models are generally defined and characterized at scales other than the field scale where the predictions are to be made for remedial decision-making and site management. Questions as to which of these parameters are important and which have to be up-scaled when the mass transfer occurs under natural conditions and under remedial action had to be answered. The study involved the development of Gilland-Sherwood type empirical models for mass transfer at column scale. Experiments were conducted in intermediate-scale test systems under controlled conditions prior to, during and post remediation. The changes in the source zone architecture were accurately monitored using x-ray and gamma attenuation methods. DNAPL spills were conducted in test aquifers with different degrees of heterogeneity to create different entrapment architecture. Models of groundwater flow and transport coupled with various reaction packages simulating the physical, chemical and biological transformations that occur in source zone during remediation, were developed and validated. The hypothesis that Gilland-Sherwood empirical models developed for natural dissolution in conjunction with models of reaction and degradation kinetics associated with remediation could be used to predict mass transfer during and after remediation was tested. This knowledge in conjunction with modeling tools and experimental data were used to develop and validate an up-scaling method to simulate the field-scale effective mass transfer. The proposed technique uses dissolution parameters measured at the laboratory scale, parameters that describe the DNAPL entrapment architecture and the geostatistical parameters of the aquifer heterogeneity.
H22C-02 INVITED
Linking Contaminant Mass Discharge to DNAPL Source Zone Architecture and Mass Removal
To evaluate the relationship between partial dense nonaqueous phase (DNAPL) mass removal and plume behavior, laboratory-scale experiments were conducted in a two-dimensional aquifer cell containing a tetrachloroethene (PCE) source zone and a down-gradient plume region. PCE-DNAPL saturation distributions were quantified using a light transmission system and expressed in terms of a ganglia-to-pool (GTP) volume ratio. To achieve incremental mass removal, the aquifer cells were flushed with a 4% Tween 80 surfactant solution that increased the solubility of PCE by more than two orders-of-magnitude with minimal mobilization of entrapped PCE-DNAPL. For a ganglia-dominated source zone (GTP = 1.6) greater than 70% mass removal was required before measurable reductions in mass discharge were realized, while for pool-dominated source zones (GTP < 0.3) reductions in mass discharge versus mass removal approached a 1:1 correlation. Current experiments are designed to evaluate the potential for coupling aggressive mass removal with microbial reductive dechlorination.
H22C-03 INVITED
Prediction of down-gradient impacts of DNAPL source depletion using tracer techniques: Laboratory and modeling validation
Interwell application of coupled nonreactive and reactive tracers through aquifer contaminant source zones enables quantitative characterization of aquifer heterogeneity and contaminant architecture. Parameters obtained from tracer tests are presented here in a Lagrangian framework that can be used to predict the dissolution of nonaqueous phase liquid (NAPL) contaminants. Nonreactive tracers are commonly used to provide information about travel time distributions in hydrologic systems. Reactive tracers have more recently been introduced as a tool to quantify the amount of NAPL contaminant present within the tracer swept volume. Our group has extended reactive tracer techniques to also characterize NAPL spatial distribution heterogeneity. By conceptualizing the flow field through an aquifer as a collection of streamtubes, the aquifer hydrodynamic heterogeneities may be characterized by a nonreactive tracer travel time distribution, and NAPL spatial distribution heterogeneity may be similarly described using reactive travel time distributions. The combined statistics of these distributions are used to derive a simple analytical solution for contaminant dissolution. This analytical solution, and the tracer techniques used for its parameterization, were validated both numerically and experimentally. Illustrative applications are presented from numerical simulations using the multiphase flow and transport simulator UTCHEM, and laboratory experiments of surfactant-enhanced NAPL remediation in two-dimensional flow chambers.
H22C-04
Evaluation of Modeling Approaches to Describe Dense Nonaqueous Phase Liquid Dissolution in 3D Heterogeneous Permeability Fields
A suite of approaches was used to simulate dense nonaqueous phase liquid (DNAPL) dissolution from four different three-dimensional (3D) flow cells (25.5 by 9 by 8.5 cm3) packed with one of two spatially correlated random permeability fields, and containing one of two entrapped DNAPL volumes. The approaches are (i) a one- dimensional (1D) model using specific DNAPL-water interfacial areas estimated from magnetic resonance (MR) images of trapped DNAPL, (ii) the same 1D model that incorporates an empirical mass transfer (Sh) correlation to describe DNAPL dissolution, (iii & iv) both pseudo-steady state and transient versions of the 3D Multiple Analytical Source Superposition Technique (MASST), (v) a distributed streamtube model, and (vi) a 3D sub-grid-block pool dissolution (SGBPD) model. All of these approaches except (ii) require knowledge of the initial DNAPL saturation distribution, and approaches (i) and (iii) require knowledge of the DNAPL saturation distribution at each time step. Only approaches (i) and (vi) consider changes in mass transfer rates due to changes in relative permeability that occur when trapped DNAPL is present. In approach (i) this is accounted for by assuming the total DNAPL-water interfacial area for mass transfer is a simple function of DNAPL saturation, and in approach (vi) this is accounted for by solving the complete 3D flow field using a relative permeability function. In this talk, simulation results will be compared and used to determine trade-offs between model accuracy, data needs, and computational demands
H22C-05
Impact of Source Mass Depletion on the Contaminant Flux Distribution at the Source Control Plane
The impact of source depletion on the contaminant flux distribution (J) at the source control plane (CP) was investigated using model simulations and data from a field study. DNAPL dissolution and transport in three- dimensional, heterogeneous, spatially correlated, random permeability fields with emplaced sources were simulated with two numerical codes (ISCO3D and T2VOC). The DNAPL and J distributions were characterized using population statistics (mean, standard deviation, coefficient of variation) and spatial statistics (centroid, second moments, variograms). The mean and standard deviation of the Sn and J distributions decreased with source mass depletion by dissolution. The decrease in mean and standard deviation was proportional for the J distribution resulting in a constant coefficient of variation (CV), while for the Sn distribution, the mean decreased faster than the standard deviation. The spatial distributions exhibited similar behavior as the population distribution, i.e., the CP flux distribution was more stable (defined by temporally constant second moments and range of variograms) than the Sn distribution. These observations appeared to be independent of the heterogeneity of the permeability (k) field (variance of the lnk distribution = 1 and 2.45), correlation structure (positive vs. negative correlation between the k and Sn domains) and the DNAPL dissolution model (equilibrium vs. rate limited), for the cases studied. The temporal invariance of the contaminant flux distribution was also observed in the results from a flux monitoring field study (Hill Air Force Base, Utah) at a DNAPL source CP before and after source remediation. These observations suggest that the temporal evolution of the contaminant flux distribution can be estimated if the initial distribution is known. However, the findings are preliminary and broader implications to sampling strategies for remediation performance assessment and expected plume behavior need to be evaluated in additional modeling and experimental studies.
H22C-06 INVITED
Effectiveness of Various Source Zone and Dissolved Plume Remediation Measures for Sites with Residual DNAPL and DNAPL Pool Sources
A model is described for transient field-scale DNAPL dissolution kinetics with (i) source zone biodecay, (ii) dissolved phase transport downgradient of the source, and (iii) dissolved plume mass losses due to biodecay and volatilization. The source model considers effects of DNAPL "architecture" on the functional dependence of dissolution kinetics versus source mass depletion, based on results of a high-resolution numerical simulations and field studies by a number of researchers. A formulation is presented that is convenient for model calibration and for uncertainty analyses on forward predictions. The model is employed to evaluate differences in source depletion and dissolved plume attenuation over time for sources dominated by residual DNAPL ("ganglia") or by DNAPL pools or lenses. In addition to the case of natural attenuation, the effectiveness of various source zone and dissolved plume remediation strategies are investigated, including: partial source zone mass removal, partial source zone flow reduction, enhanced source zone biodecay, enhanced source zone mass transfer kinetics, enhanced dissolved plume biodecay, enhanced volatilization, and various combinations of these. For each source type and remediation scenario, source mass flux and concentrations at downgradient receptor locations are evaluated versus time. Times to achieve remediation criteria are compared for each case. Remediation times are generally greater for pool-dominated sources than for residual DNAPL sources under otherwise comparable conditions; however, successful remediation can be achieved even for more intransigent pool-dominated problems by using more aggressive source strategies and/or by combining multiple strategies, especially if compliance points are further downgradient from the source or risk-based cleanup levels can be employed.