Joseph A. Pedit

Multiphase flow and transport modeling in heterogeneous porous media: challenges and approaches

Abstract We review the current status of modeling multiphase systems, including balance equation formulation, constitutive relations for both pressure-saturation-conductivity and interphase mass transfer, and stochastic and computational issues. We discuss weaknesses and inconsistencies of current approaches based on theoretical, computational, and experimental evidence. Where possible, we suggest new or evolving approaches.

Heterogeneous Sorption Processes in Subsurface Systems. 2. Diffusion Modeling Approaches

The pore diffusion modeling approach is often used to model the sorption of organic chemicals by natural sorbents, implicitly assuming that the physical and sorptive properties of the sorbent are homogeneous. But application of such approaches to systems with significant variations in particle size and sorption equilibrium properties maybe inappropriate ; most natural systems have such variations. A general multiple-particle class pore diffusion model that accounts for variations in physical and sorptive properties was developed and used to interpret the results of a batch sorption rate study. Results indicated that multiple-particle class models provide a more accurate representation of long-term sorption rate data than traditional single-particle class approaches, the common approach of adding an instantaneous equilibrium fraction to single-particle class models leads to model parameters that are sensitive to the range over which data is collected, and tortuosity factors obtained with multiple-particle class modeling approaches were consistent with the range of values expected based upon mechanistic reasoning.

Mass transfer rate limitation effects on partitioning tracer tests

Abstract Partitioning tracer tests are often used to quantify the amount of non-aqueous phase liquid (NAPL) present in a porous medium. Results from such tests are usually interpreted by using models that assume local equilibrium exists between the NAPL and the aqueous phase or by using a moment method to solve for the fraction of the pore space occupied by the NAPL. We investigated the transport of partitioning alcohol tracers through heterogeneous sand porous medium systems containing a stationary trichloroethylene (TCE) phase for a range of aqueous phase velocities. NAPL saturations were quantified using a non-destructive X-ray absorption methodology. Experimental results were simulated using a numerical solution to an advective–dispersive model in which mass transfer between the NAPL and the aqueous phase was approximated using a dual-resistance approach consisting of a boundary layer mass transfer resistance and a diffusional resistance within the NAPL. The numerical model agreed well with the experimental data, and moment analysis yielded reasonable estimates of actual NAPL saturations.

Effectiveness of Source-Zone Remediation of DNAPL-Contaminated Subsurface Systems

Concerted efforts to remediate subsurface systems contaminated with dense nonaqueous-phase liquids (DNAPLs) have met with limited success when measured by comparing solute concentrations to drinking water quality standards. One-dimensional and three-dimensional laboratory experiments and a field-scale experiment are used to investigate the effectiveness of source-zone remediation and to assess factors that contribute to the observed results. The three-dimensional laboratory experiment and the field-scale experiment used a surfactant flush followed by vapor extraction to reduce the DNAPL saturation, while vertical DNAPL mobilization was controlled using a brine barrier. DNAPL mobilization and recovery in the field-scale experiment was relatively ineffective due in part to the low saturation levels of the DNAPL. The results show essentially that complete removal of a DNAPL is required to reach typical cleanup standards and that details of the morphology and topology of a DNAPL distribution, in addition to the saturation, play an important role in determining the rate of mass transfer. The results are interpreted in terms of guidance for remediation approaches, realistic expectations for source-zone remediation, and elements needed for improved models of such systems.

Household air pollution (HAP), microenvironment and child health: Strategies for mitigating HAP exposure in urban Rwanda

Exposure to household air pollution (HAP) from cooking and heating with solid fuels is major risk factor for morbidity and mortality in sub-Saharan Africa. Children under five are particularly at risk for acute lower respiratory infection. We use baseline data from randomized controlled trial evaluating a household energy intervention in Gisenyi, Rwanda to investigate the role of the microenvironment as a determinant of childrens HAP-related health symptoms. Our sample includes 529 households, with 694 children under five. We examine the association between likelihood of HAP-related health symptom prevalence and characteristics of the microenvironment including: dwelling and cooking area structure; distance to nearest road; and tree cover. We find that children residing in groups of enclosed dwellings, in households that cook indoors, and in households proximate to tree cover, are significantly more likely to experience symptoms of respiratory infection, illness with cough and difficulty breathing. On the other hand, children in households with cemented floors and ventilation holes in the cooking area, are significantly less likely to experience the same symptoms. Our findings suggest that in addition to promoting increased access to clean cooking technologies, there are important infrastructure and micro-environment related interventions that mitigate HAP exposure.