H31B-0370 0800h
Effects of Micro-Heterogeneities on Laboratory-Scale Pc-Kr-S Relationships
Commonly, capillary pressure-saturation-relative permeability (Pc-S-Kr) relationships are obtained by means of laboratory experiments carried out on soil samples that are up to 10-12 cm long. In obtaining these relationships, it is implicitly assumed that the soil sample is homogeneous. However, it is well known that even at such scales, some micro-heterogeneities may exist. These heterogeneous regions will have distinct multiphase flow properties and will affect the saturation and distribution of wetting and non-wetting phases within the soil sample. This, in turn, may affect the measured two-phase flow relationships. In the present work, numerical simulations have been carried out to investigate how the variations in type, quantity, and distribution of sub-sample scale heterogeneities affect Pc-S-Kr relationships for dense non-aqueous phase liquid (DNAPL) and water flow. Fourteen combinations of sand types and heterogeneous patterns have been defined. These include binary combinations of coarse sand imbedded in fine sand and vice versa. The domain size is chosen such that it represents typical laboratory samples used in the measurements of Pc-S-Kr curves. Upscaled drainage and imbibition Pc-S-Kr relationships for various heterogeneity patterns have been obtained and compared in order to determine the relative significance of the heterogeneity patterns. The results show that for micro-heterogeneities of the type used in this work, the upscaled Pc-S curve mainly follows the corresponding curve for the background sand. Irreducible water saturation (in drainage) and residual DNAPL saturation (in imbibition) are affected by the presence and intensity of heterogeneities.
H31B-0371 0800h
Movement and Remediation of a Volatile, Multicomponent DNAPL in a Variably-Saturated, Heterogeneous Porous Medium
An intermediate-scale flow cell experiment was conducted to study the behavior of a multicomponent DNAPL at structural interfaces and subsequent remediation using two different forms of the soil vapor extraction (SVE) technique. The flow cell (100-cm long, 5-cm wide, and 80 cm high), was packed under saturated conditions with sloped layers of Hanford silt and coarse sand, embedded in a matrix of a medium-grained laboratory sand. After packing, the water table was lowered to 2 cm above the bottom of the flow cell to establish variably saturated conditions. A finite amount of a volatile multicomponent DNAPL, mimicking the organic liquid disposed at the Hanford Site, was then injected from a small source zone. The infiltration and redistribution processes were visually recorded. In addition, a dual-energy gamma radiation system was used to determine DNAPL and water saturation at more than 1000 locations. Results indicate that lateral spreading of the DNAPL is greatly enhanced by the heterogeneities. The silt layers, by virtue of their substantial non-wetting fluid entry pressures and high water saturations, completely diverted the DNAPL laterally. The relatively dry coarse-sand layers forced some of the DNAPL to move laterally but also allowed some infiltration.
H31B-0372 0800h
Effect of soil moisture dynamics on the DNAPL spill zone architecture in heterogeneous porous media
The water saturation, infiltration events, and soil permeability in the vadose zone determine the amount, location, and form of NAPL that requires remediation. Lenhard et al. (2004) recently proposed a new permeability-liquid saturation-capillary pressure (k-S-P) model that considers three NAPL forms: free, residual, and trapped. In this new constitutive model, both trapped and residual NAPL saturations depend on water saturation and saturation path history. Both free and residual NAPL have direct access to the pore gas, but the latter is held immobile by capillary forces. Trapped NAPL is surrounded by water, consequently, it does not have direct access to pore gas and it is immobile. The form and location of NAPL are likely to impact the time scales of cleanup. We used the three-phase flow simulator (STOMP), which includes this new constitutive model, to distribute NAPL in heterogeneous porous media for different NAPL and water loading histories. A 2-D vertical cross-section with layered heterogeneity was assumed, and simulations were performed for two scenarios of high and low water infiltration rates. For the first scenario, the form and location of NAPL were strongly influenced by the high water recharge rates. Water saturation increased as the water infiltration front swept downward. Since the advancing front of NAPL migrated slower than that of water, NAPL occupied only large pore spaces due to high water saturation. As a result, both trapped and residual NAPL saturations were low. The effect of different heterogeneous patterns of permeability and NAPL spill event scenarios on NAPL distribution was overwhelmed by high water recharge rates. For the low infiltration rate scenario, the distribution of water content prior to a NAPL spill event had a significant impact on NAPL migration and distribution due to the formation of residual and trapped NAPL. For initially low water saturation cases, the NAPL front was followed by a water infiltration front, which results in the formation of trapped NAPL. For initially moderate water saturation cases, trapped NAPL saturation was very low and residual NAPL saturation was relatively high. For all cases simulated, use of the new constitutive model that allows the formation of residual and trapped NAPL increased the amount of NAPL retained in the vadose zone. Although total NAPL saturation in the vadose zone was similar, the trapped NAPL saturation was different depending on the water infiltration scenarios. These results indicate that soil permeability, NAPL spill scenario, and infiltration events must all be simultaneously accounted for to describe the NAPL distribution and to predict the efficiency of remediation actions.
H31B-0373 0800h
Significance of Diffused Zone Mass Flux Plume in Determining the Longevity of Solute Plumes Emanating From Heterogeneous DNAPL Source Zones: Intermediate-Scale Experimental Investigations
Two primary mechanisms contribute to mass flux emission from source zones where DNAPLs are entrapped. The first is the result of the soluble constituents of the DNAPL dissolving into the flowing groundwater through mass transfer that occurs at DNAPL-water interfaces in residual zones and pools. The second is a result of rebounding of dissolved mass that has diffused into the low permeability zones of the aquifer during active DNAPL dissolution. The heterogeneity of the aquifer and the DNAPL entrapment architecture that determine the significance of the rebounding diffused mass contributing to source zone emission will affect the longevity of the solute plume that produces downstream risk. The goal of this research study is to develop decision tools that could be used to manage source zones where conditions dictate for the plume to persist after the DNAPL source is depleted. These decision tools are based on numerical models that simulate the fundamental processes of emission from a source zone as determined by dissolution of the entrapped sources, diffusion of mass from the source and the plume and the rebound from the low permeability zones. The results from an experimental study conducted in intermediate-scale test tanks designed to obtain an insight into this process and generate data to validate the numerical models are presented. A series of experiments were conducted in two-dimensional tanks with dimensions 4.9 m x 1.2 m x 0.06 m. The soil-packing configurations were designed in each experiment to represent different conditions where mass from dissolved plume diffuses into low permeability zones. The three conditions that were studied represented: (1) a layered system containing a high and a low permeability zones, (2) a low permeability mound embedded in a high permeability matrix and (3) an inclined low permeability layer in high permeability formation. In each case, a known volume of DNAPL was placed in the source zone and the mass depletion during dissolution was monitored using an X-ray attenuation system. The variation of mass emission as a function of time was monitored by sampling the effluent. At the end of the experiment, the low permeability zones were cored and analyzed to determine the spatial distribution of diffused mass. The experimental observations and the preliminary results of the model validation study are presented.
http://cesep.mines.edu/people/rodriguez.htm
H31B-0374 0800h
Microscale Modeling Studies of Chemical Transport From Petroleum Contaminant Sources
Thirty years after the earlier studies of groundwater pollution by petroleum hydrocarbons, the dominant mechanisms and the uncertainties that arise in real field behaviors of nonaqueous phase liquid (NAPL) are still largely unresolved. The relevant literature has emphasized numerical modeling of multiphase flow using continuum approaches. These approaches do not consider chemical component transport following NAPL release into the subsurface and at the later time, both of which are of interest in the cleanup of contaminated sites. In current studies of natural attenuation, the release of chemicals tended to be overestimated by general continuum-concentration-gradient approaches. In this work, we propose using the Lattice Boltzmann method to simulate the dissolution of chemicals out of the NAPL source to quantify the release of dissolved chemicals into the water. The findings from these simulations would add insight to the multiphase flow and partitioning of contaminants from contaminated sources.
H31B-0375 0800h
DNAPL Mobility in Heterogeneous Porous Media: Sensitivity of Migration Times to Source Characteristics and Release Location Parameters
This study examined the factors the influence the time required for a release of dense nonaqueous phase liquid (DNAPL) to cease migrating through heterogeneous porous media below the watertable. Using numerical simulation, the temporal and spatial sensitivity of DNAPL migration was evaluated for four DNAPL source characteristics - nonwetting fluid type (i.e., density and viscosity), interfacial tension (IFT), source strength, and volume released - and for three release location parameters - mean permeability, porosity, and hydraulic gradient. The study was conducted using the multiphase code DNAPL3D whose constitutive relationships were developed, and validated, for DNAPL migration in both space and time. All numerical simulations employed a single correlated random permeability field and identical boundary and source conditions to the base case, except for systematic variation of the parameter under investigation. It was found that all of the parameters examined had a significant spatial effect on the final DNAPL migration pattern, either on the overall volume of subsurface invaded (e.g., direct correlation to volume released) or on the amount of lateral spreading (e.g., direct correlation to IFT). However, only two of the parameters were found to have a significant effect on the time required to achieve the final, stable distribution of DNAPL pools and residual. Migration rates were very sensitive to DNAPL type, with predicted cessation times ranging from 30 days for the high mobility fluid tetrachloroethylene (PCE) to over 1000 years for the low mobility fluid coal tar. These simulations reveal that while density primarily influences the spatial extent of penetration and viscosity primarily influences penetration rate, the two effects are not independent due to interactions with site-specific heterogeneity. In addition, the mean permeability of the heterogeneous domain was found to be significant, with increases in mean k corresponding to decreases in both cessation time and degree of lateral spreading.
H31B-0376 0800h
Aspects of Numerical Simulation of DNAPLs in Fractured Bedrock
The distribution of dense non-aqueous phase liquids (DNAPLs) is notoriously difficult to predict in heterogeneous geologies. This single factor has serious implications for predicting DNAPL source zone architecture, assessing the risk to groundwater from the dissolution of the non-aqueous phase, and targeting the source zone with a specific remediation technology. When DNAPL enters a fractured bedrock the fluid will follow a complex path based predominantly on the heterogeneous distribution of fractures in the rock mass where fluid migration is controlled by both large- and small-scale processes. At the large-scale, connectivity of the fracture network defines whether a potential pathway is present. Whereas at the small-scale, fracture aperture variability and pore size distribution of the matrix define the distribution of the DNAPL. Simulating DNAPL migration in heterogeneous geologies demands numerical models which reconstruct the complex fracture network architecture and at the same time couple the transport of the fluid phase through interconnected fractures. Discrete fracture network models (DFNMs) should incorporate the 3-D characteristics of natural systems. DFNMs may be deterministic or stochastic. Stochastic models are applied where an exact description of fractured rock mass is unachievable. However, few DFNMs are available that incorporate 3-D non-orthogonal fracture geometries and, furthermore, field measurements are often not available to adequately validate the fracture simulations. DNAPL models may be classified into dynamic fully compositional simulators which are based on a detailed description of two-phase flow physics and mass transfer terms, or static models which are limited to simpler invasion percolation concepts. Problems of discretisation and convergence limit the application of coupled multiphase compositional simulators, which are, at present, constrained to realisations of orthogonal fracture geometries. Furthermore, the computational time to run fully compositional simulators in dense fracture networks often limits application to a limited number of model realisations and therefore precludes application to stochastic methods. Investigators of DNAPL release to fractured bedrock are, therefore, restricted to resolving representative descriptions of either the geology or the non-aqueous phase, but not both. A recent study is described where a coupled stochastic DFNM and invasion percolation model is applied to conceptualise the fate of DNAPLs in heterogeneous geologies. This approach is used, at a fundamental level, to explore the effect of variability in natural fracture networks in estimating the distribution of DNAPLs in fractured rock. The implications of this study are that prediction of DNAPL migration is not adequately described by deterministic approaches, that investigators of fractured rock sites should expect to work with a range of values, and that the range may be large. Discussion includes an assessment of the uncertainties associated with applying numerical simulation at real DNAPL release sites in heterogeneous geologies and recommendations are presented to address future research needs.
H31B-0377 0800h
Remediation of Dense Non-Aqueous Phase Liquids Using Biostimulation and Bioaugmentation
Launch Complex 34 (LC-34) at the Kennedy Space Center is the site of historic releases of trichloroethene (TCE) which is present in the subsurface as a dense, non-aqueous phase liquid (DNAPL). Under intrinsic condition at LC-34, TCE biodegrades via anaerobic reductive dechlorination resulting in the accumulation of cis-1,2-dichloroethene (cis-1,2-DCE), suggesting that complete biodegradation to ethene is limited by either: 1) the absence of dehalorespiring microorganisms or 2) the absence of soluble organic carbon substrates for microbial metabolism and growth. Anaerobic reductive dechlorination, the most common biodegradation process for chlorinated solvents, is a well-understood degradation mechanism for TCE and less-chlorinated alkenes (i.e., cis-1,2-DCE and vinyl chloride) that can result in complete dechlorination to ethene, a non-toxic degradation product. Typically, enhanced bioremediation treatment involves the amendment of groundwater with an organic substrate (e.g., alcohols, organic acids) as an electron donor (biostimulation) although at sites where the activity of microorganisms capable of mediating complete dechlorination is low the addition of the microbial inoculum containing the requisite dechlorinating microorganisms (bioaugmentation) may be necessary. In May 2002, an enhanced bioremediation pilot study of the performance of biostimulation and bioaugmentation was initiated at LC-34 with the concurrent collection of performance data by the USEPA Superfund Innovative Technology Evaluation (SITE) program for the purpose of technology validation. The primary purpose of this study was to determine if ethanol amendment and the addition of microorganisms capable of mediating complete conversion of TCE to ethene would increase the rate of VOC degradation, thereby minimizing the migration of chloroethenes in groundwater and increasing the rate of DNAPL dissolution. An indigenous Dehalococcoides microorganism present in groundwater was demonstrated to be capable of TCE biodegradation, resulting in complete conversion to ethene. The activity of the indigenous dechlorinating microorganisms was significantly enhanced by the addition of ethanol. Surprisingly, ethanol addition did not stimulate methanogenesis, likely as a result of the relatively high chloroethene concentrations remaining in groundwater (primarily cis-1,2-DCE and vinyl chloride) and/or the absence of an indigenous methanogenic population. Biostimulation resulted in rapid biodegradation of TCE to cis-DCE and VC, with limited conversion to ethene (concentrations as high as 5 mg/L). Additional conversion past VC to ethene was observed following bioaugmentation. Ongoing efforts are underway to further evaluate the impacts of bioaugmentation on both dechlorinating activity and microbial population diversity.
H31B-0378 0800h
Pre and Post Treatment Characterization of DNAPL Source Zone Architecture in Heterogeneous Aquifers Using Mass Flux and Tracer Data
Dense non-aqueous phase liquid (DNAPL) in source zones generates continuous mass flux long after the initial spill. Dissolved concentration observed in monitoring wells downstream of a DNAPL source zone alone provides very little information on the entrapment architecture that is needed to design effective remediation schemes. A method was developed to analyze measured vertical distribution of mass flux using a modified mass transfer model based on MODFLOW and RT3D and inverse modeling code PEST to determine DNAPL entrapment architecture as well as the hydrodynamically accessible DNAPL mass. This paper presents the validation of this method using experiments conducted in an intermediate-scale test tank. A 1.2 m high and 4.5 m long two-dimensional tank was packed with five test sands to represent a spatially correlated random field. The heterogeneity is characterized using the variance, correlation length and the anisotropy ratios in the vertical and horizontal directions. After wet-packing the tank, pressure measurements were made at 45 locations and a conservative tracer study was conducted to calibrate the flow and transport parameters of the test aquifer. A test DNAPL was spilled to create an entrapment zone. A tracer study using partitioning tracers was conducted for comparative purposes and to identify its limitations in characterizing source zones with complex entrapment architecture containing residual zones as well as pools. The DNAPL distribution was measured using a scanning gamma attenuation system. The solute mass flux emanating from the source zone and the tracer concentrations were monitored at multilevel observation ports placed downstream of the source zone at various distances. A surfactant flood was implemented to remove the entrapped DNAPL by enhancing dissolution. Flow by-passing as controlled by the heterogeneity of the aquifer as well as DNAPL entrapment will not produce complete mass removal, thus creating a different entrapment architecture that needed to be characterized to determine the treatment effectiveness. The same pre-treatment source zone characterization was implemented to determine the post-treatment state of the source zone. This comprehensive data set was used to validate the inverse modeling based characterization technique. The conclusions that are derived from this validations study are presented.