H31A-0362 0800h
Analysis of Natural Attenuation at the Park-Euclid PCE Contaminated Site
Tetrachloroethe (PCE) and diesel contamination has been documented in the local perched and regional aquifers at the Park-Euclid WQARF site in Tucson, AZ. Monitored natural attenuation (MNA) is currently been evaluated as a potential remediation technology for the PCE contamination at the site. Physicochemical characterization of the immiscible liquid contamination has been conducted in prior studies. The focus of this study is to evaluate the biochemical processes active at the site. Analysis of the Arizona Department of Environmental Quality (ADEQ) database shows that the original contamination in the perched aquifer, where PCE was the single chlorinated ethene involved, has evolved into a mixture containing PCE and several transformation compounds typically associated with PCE (Trichloroethene, TCE, Dichloroethene, DCE, and Vinylchloride, VC). In general terms, PCE concentrations have decreased while TCE concentrations have been constant in time. Accumulation of DCE isomers (mainly cis-1,2-DCE) is observed in most of the wells. Conversely, VC has been observed in only three wells and only since 2001. This suggests that PCE is undergoing transformation and that DCE transformation is the limiting step in the mineralization of PCE. The regional aquifer does not show evidence of active PCE transformation processes.
H31A-0363 0800h
Complete Dissolution of Chlorinated-Solvent Immiscible Liquid in Saturated Sandy Media
Dense immiscible liquids are commonly found in the subsurface beneath hazardous waste sites and can serve as a long-term source of contamination. Complete removal of the immiscible liquid is dependent on dissolution processes. A series of column experiments was conducted to examine the influence of porous-media texture and residual saturation on complete dissolution. Trichloroethene (TCE), a chlorinated solvent, was used as a representative dense immiscible liquid in these experiments. A residual TCE saturation was created by imbibing TCE into the column and then flushing with a saturated aqueous TCE solution to remove the mobile phase. Once a residual saturation was established, the dissolution process was started by flushing the column with an artificial groundwater solution (0.005M CaCl2). The effluent was monitored until the concentration reached 0.1 mg/L. The elution waves exhibited three distinct regions of behavior with respect to the rate of change of concentration. An initial steady-state stage was followed by a transient stage characterized by an exponential decrease in concentration. This was followed by an extensive low-concentration (0.4 to 0.1 ug/L) tail. The former two stages are associated with immiscible-liquid dissolution, whereas the latter is associated with rate-limited sorption/desorption. The results indicate that the elution waves exhibit an asymptotic profile for a portion of the transient stage. The extent of the asymptotic behavior appears to be a function of the uniformity coefficient (Uc), with the greatest extent for the most poorly sorted medium. The asymptotic transient-stage elution effect observed in these experiments can increase the amount of time required to reach concentrations below the maximum contaminant level (MCL).
H31A-0364 0800h
Characterizing Dissolution Dynamics of Spatially Distributed Immiscible Liquid
Dissolution of immiscible liquid into the aqueous phase can be influenced by immiscible-liquid-water mass transfer, local-scale immiscible-liquid morphology and distribution, bypass flow, and dilution effects. The purpose of this work is to examine the effects of nonuniform distributions of immiscible liquid and porous-media heterogeneity on immiscible-liquid dissolution. The first set of experiments were conducted with an intermediate-scale flow cell, in which were positioned residual-saturated zones of dyed trichloroethene (TCE). The zones consisted of sand finer than the surrounding material. Additional experiments were conducted using a stainless-steel column. Finer-grain sand zones and porous ceramic cubes that contained residual TCE were emplaced within a courser sand matrix. Control experiments were conducted with homogeneous systems. Effluent samples were collected throughout the course of each experiment. A mathematical model was used to simulate transport. The results show that effluent concentration profiles are much more asymptotic for the systems wherein immiscible-liquid is entrapped within lower permeability zones.
H31A-0365 0800h
Characterizing Mass Transfer of a Complex Non-aqueous Phase Liquid
A series of laboratory experiments were conducted with a multiple-component, non-aqueous phase liquid (NAPL) collected from the Picillo Farm Superfund Site. Physical property analysis and compositional analysis were performed to provide initial information about the NAPL. Batch experiments were conducted to evaluate equilibrium phase-partitioning behavior. Two sets of air-stripping column studies were conducted to examine the elution behavior, mass-transfer dynamics, and mass recovery of five selected target compounds present in the NAPL mixture. Initial elution behavior of all target components appeared to be ideal, as the initial vapor-phase concentrations were similar to vapor-phase concentrations measured for the batch equilibrium experiment and those estimated using Raoult's law based calculations incorporating NAPL composition data. Air-stripping of columns containing a pool of NAPL and no porous media revealed that removal of 1,2-DCB appeared to be limited by the molecular diffusion of the compound to the NAPL-air interface. Air-stripping of NAPL distributed relatively uniformly as a residual phase within a sandy porous medium exhibited ideal behavior.
H31A-0366 0800h
Experimental Study of Acoustically Enhanced Multicomponent DNAPL Ganglia Dissolution
The impact of acoustic pressure waves on multicomponent nonaqueous phase liquid (NAPL) ganglia dissolution in water-saturated columns packed with glass beads was investigated. Laboratory data from dissolution experiments with two and three component NAPL mixtures suggested that acoustic waves significantly enhance ganglia dissolution due to the imposed oscillatory interstitial water velocity. The dissolution enhancement was shown to be directly proportional to the acoustic wave frequency. Furthermore, it was demonstrated that the greatest dissolution enhancement in the presence of acoustic waves is associated with the component of the NAPL mixture having the smallest equilibrium aqueous solubility. Finally, square shaped acoustic waves were shown to lead to greater NAPL dissolution enhancement compared to sinusoidal and triangular acoustic waves. The results of this study suggested that aquifer remediation using acoustic waves is a promising method particularly for aquifers contaminated with NAPLs containing components with very low equilibrium aqueous solubilities.
H31A-0367 0800h
Plume Development and Mass Flux Following Surfactant-Based Treatment of Heterogeneous PCE-DNAPL Source Zones
The zones of contamination at typical chlorinated solvent-contaminated sites can be divided into two regions: a source zone in which free-phase contaminants (dense nonaqueous phase liquids, or DNAPLs) are present, and a solute plume containing only dissolved-phase contaminants. Currently, pump-and-treat is the most common method for achieving dissolved-phase plume containment; however, it is widely recognized that this approach is generally ineffective for source zone mass removal. As a result, a number of innovative technologies for in situ DNAPL source zone treatment have been developed, but mass removal using these methods is often incomplete. In addition, the effects of partial source zone mass removal on subsequent dissolved-phase plume development and contaminant flux remain poorly understood. To address these issues laboratory-scale experiments were conducted in a two dimensional (2-D) aquifer cell having overall dimensions of 150 cm (length) by 48 cm (height) by 1.4 cm (internal thickness) and containing both source zone and down-gradient "plume" regions. The aquifer cells were packed under water-saturated conditions with Accusand (either 20/30 sieve size or a mixture of 50% 20/30 and 50% 40/50 sieve sizes). Within the source zone, three layers of F-70 Ottawa sand lenses were emplaced to mimic heterogeneous regions of lower permeability media. Following tetrachloroethene (PCE) release and redistribution in the source zone, a solubilizing surfactant solution containing 4% Tween 80 was used to achieve sequential PCE mass removals ranging from 30% to 80%. At the conclusion of each surfactant flood, down-gradient contaminant concentrations and mass fluxes were monitored at a hydraulic gradient of 1x10$^{-3}$. The PCE-DNAPL distributions in the source zone were quantified using light transmission prior to and following each surfactant flood. PCE-DNAPL distribution was expressed in terms of a ganglia to pool ratio (G:P), for which the volume of PCE above residual saturation ({\it S$_{r}$} = 11%) was considered to be "pooled". Results from three aquifer cell experiments are reported here; the first two cells contained highly-pooled source zones having an initial G:P values of 0.26:1 (80% pooled) and 0.50:1 (70% pooled), while the third contained a moderately-pooled source zone with a G:P value of 1.6:1 (40% pooled). For the first highly-pooled cell, flux-averaged effluent PCE concentrations cell decreased from 150 mg/L to 70 mg/L after 45% PCE mass removal, with a subsequent reduction to 5 mg/L following 80% PCE mass removal. Similar behavior was observed for the second highly pooled cell, with effluent PCE concentrations decreasing from 100 mg/L to 50 mg/L following 60% PCE mass removal and subsequently to 20 mg/L following 80% PCE mass removal. Effluent PCE concentrations in the moderately pooled aquifer cell were not statistically different from the initial value of 100 mg/L after 40% PCE mass removal, but subsequently decreased to approximately 30 mg/L following 75% PCE mass removal. Differences in post-treatment plume development and mass flux between the experiments were attributed to (a) preferential removal of PCE mass that existed as entrapped ganglia and were readily solubilized during the surfactant flood, and (b) the persistence of DNAPL pools which accounted for 40 to 80% of the initial PCE mass. Results obtained from these studies provide direct experimental evidence of the potential impacts of DNAPL source zone architecture and partial mass removal on plume development and reductions in contaminant mass flux.
H31A-0368 0800h
Upscaled Mass Transfer Correlations for Estimating Mass Discharge From DNAPL Source Zones: Comparisons to Field-Scale Numerical Simulations
In recent years, multiphase, multicomponent numerical simulators have been used in the literature to enhance our understanding of the potential distribution and persistence of dense non-aqueous phase liquids (DNAPLs), such as tetrachloroethene (PCE), in plume source regions. Incorporation of hysteretic capillary pressure - saturation relationships, nonuniform flow fields, and rate-limited mass transfer between phases in these simulators facilitates prediction of realistic, spatially variable DNAPL saturation distributions, and enables quantification of down-gradient aqueous-phase contaminant concentrations over the life of a source zone. Application of such numerical models, however, often requires extensive user training and a large amount of site-specific information. In contrast, upscaled mass transfer correlations have been used with simplified one-dimensional analytical solutions to approximate the temporal evolution of a source zone and resulting contaminant mass discharge. This work compares predictions of simplified methodologies for estimating the mass discharge from a PCE-DNAPL source zone with those generated using a three dimensional multiphase, multicomponent simulator. In the numerical simulator, local-scale mass transfer and mixing due to a nonuniform flow field are modeled directly. In the simplified approach these effects are consolidated into an upscaled mass transfer coefficient. Preliminary results for saturation distributions dominated by low saturation ganglia indicate that, for moderate levels of DNAPL mass removal (less than 80 percent), predictions of mass discharge using an upscaled mass transfer correlation are generally within 5 percent of those computed using the numerical simulator. However, as ganglia dissolve and the source-zone architecture becomes dominated by DNAPL pools (typical of higher mass removal conditions), the simplified methodology over-predicts mass discharge by a factor of 2 or more, resulting in the under-prediction of source longevity. These effects become more exaggerated as the fraction of pools in the initial saturation distribution increases. Thus, understanding the limitations of the upscaled modeling approach at high levels of DNAPL mass removal will be critical to the accurate estimation of long-term mass discharge. Simulations suggest that late-time predictions of contaminant mass discharge may be improved by incorporating additional information about the source zone into the upscaled mass transfer correlation; however, more work will be needed to confirm this relationship.
H31A-0369 0800h
Effects of Contaminated Site Age on Dissolution Dynamics
This work presents a streamtube-based analytical approach to evaluate reduction in groundwater contaminant flux resulting from partial mass reduction in a nonaqueous phase liquid (NAPL) source zone. The reduction in contaminant flux, Rj, discharged from the source zone is a remediation performance metric that has a direct effect on the fundamental drivers of remediation: protection of human health risks and the environment. Spatial variability is described within a Lagrangian framework where aquifer hydrodynamic heterogeneities are characterized using nonreactive travel time distributions, while NAPL spatial distribution heterogeneity can be similarly described using reactive travel time distributions. The combined statistics of these distributions are used to evaluate the relationship between reduction in contaminant mass, Rm, and Rj. A portion of the contaminant mass in the source zone is assumed to be removed via in-situ flushing remediation, with the initial and final conditions defined as steady-state natural-gradient groundwater flow through the contaminant source zone. The combined effect of aquifer and NAPL heterogeneities are shown to be captured in a single parameter, reactive travel time variability, that was determined to be the most important factor controlling the relationship between Rm and Rj. Increased values of the following parameters are shown to result in more favorable contaminant elution dynamics (i.e., greater flux reduction for a given reduction in mass): aquifer hydrodynamic heterogeneity, NAPL source zone heterogeneity, positive correlation between travel time and NAPL content, and time since the contamination event. Less favorable elution behavior is shown to result from negative correlations between travel time and NAPL content and rate-limited dissolution. The specific emphasis of this presentation is on the effects of the length of time that has elapsed since the contamination event (site age) on the dissolution dynamics.