Non-aqueous Phase Liquid Migration Research Articles

SIMILITUDE REQUIREMENTS FOR MODELLING NAPL MOVEMENT WITH A GEOTECHNICAL CENTRIFUGE.

Keywords MASSACHUSETTS TECHNOLOGY INSTITUTE, MIT, EDINBURGH UNIVERSITY CENTRIFUGES, GEOTECHNICAL, MOVEMENT, MODELLING, NON AQUEOUS, PHASE, LIQUIDS, NAPL, TRANSPORT, SIMILITUDE, MODELS, GEOTECHNICS, MEDIA, POROUS, SCALING, STUDIES, SUBSURFACE, UNDERGROUND, WETTING, MIGRATION, INSTABILITY, BEHAVIOUR, GROUNDWATER, POLLUTION, REQUIREMENTS, RELATIONSHIPS... Show All

A generalized growth model for simulating initial migration of dense non‐aqueous phase liquids

Modeling the migration of spilled non–aqueous phase liquids (NAPLs) is currently difficult because the physics governing their movement is complex and knowledge of the local geology is always incomplete. NAPL movement is subject to buoyancy, capillary, and viscous forces in addition to being directed by the particular structural and hydraulic properties of the porous medium. Consideration of buoyancy forces suggests that the flow regime diagram of Lenormand et al. [1988] can be expanded to a third dimension. We develop a generalized growth model, based on invasion percolation, that captures the essential physics of initial NAPL migration but is simple enough computationally that simulations can be conducted much faster than by using continuum simulation models that attempt to capture all details of the physics. In comparison with available experimental data, our model realistically simulates movement of a NAPL for a wide range of values of Bond and capillary numbers, in any kind of porous medium. Because this approach allows much faster and simpler simulation than other approaches, a higher spatial resolution and/or the use of Monte Carlo methods to reduce the effect of geological uncertainty becomes a realistic possibility.

Mobile NAPL Recovery: Conceptual, Field, and Mathematical Considerations

AbstractRecovery of mobile Nonaqueous Phase Liquids (NAPLs), referred to as “oil recovery,” is one of the most common remedial technologies currently being implemented at sites where NAPLs have been released. The rationale for oil recovery typically includes resource recovery, mitigation of further NAPL migration, and compliance with regulatory mandates for source reduction. Efficient oil recovery can be achieved by optimizing conditions within the oil flow path. This concept is referred to as flow path management. Building on this concept, a waterflood oil recovery technique utilizing dual recovery and parallel delivery drain lines has been developed for recovery of creosote‐based wood‐treating oil, a Dense Nonaqueous Phase Liquid (DNAPL). Full‐scale application of this technique at a contaminated site has yielded 1.5 million gallons of DNAPL. Furthermore, an operational endpoint of 95 percent recovery of the mobile oil is being achieved. Building on the concept of flow path management and the observed performance of the waterflood oil recovery system, a first‐order analytical solution for DNAPL flow to a drain line has been derived and validated using field data. This solution leads to a set of useful design equations and further insight into the factors that control oil recovery.

Movement of liquid contaminants in partially saturated frozen granular soils

Understanding the migration of nonaqueous phase liquids in frozen subsurface soils is becoming increasingly important in permafrost regions and in temperate zones where frozen subsurface barriers have been proposed to confine contaminants. Tests were performed on 32 specimens of gravelly sands from the Hanford, Washington reservation to determine the relationship between degree of ice saturation and intrinsic permeability. Decane, a representative nonaqueous phase liquid, was employed as the permeant, and was infiltrated through the frozen specimens at −10°C. In addition to pure water, a NaCl brine and a mixture of water and decane were utilized as the pore liquids. For all specimens the intrinsic permeability correlated linearly with the ice saturation, varying from approximately 2.7× 10 −7 cm 2 at 0% saturation to negligible values at nearly 100% saturation. The different pore liquids did not affect the correlation significantly. Bentonite was added to some of the specimens prior to freezing, reducing the intrinsic permeability to negligible levels.

The role of field trials in development and feasibility assessment of surfactant‐enhanced aquifer remediation

The development of surfactants for aquifer remediation involved simultaneous research at laboratory and field scale to expedite technology development. The results of field trials provided direction for laboratory research by identifying the critical parameters that limit performance while the results of laboratory research helped design and interpret the field trials. The combined field and laboratory work has identified the factors to be considered in a feasibility assessment of surfactant‐enhanced remediation. The principal requirements are adequate hydraulic conductivity (> 10−6 m/s), the ability to attain hydraulic control and ability to control downward dissolved nonaqueous phase liquid migration. Field trials have demonstrated that surfactants can rapidly remove mass; however, because heterogeneities cause tailing in contamination removal, restoration to drinking water standards may not be achieved. Optimization of surfactant solution composition can increase the rate of mass removal and hence minimize cost. The fraction of mass removal, the level of contamination remaining and the cost of remediation using surfactants are all dependent upon a site's hydrogeology. Thus both minimization of cost and maximization of nonaqueous phase liquid removal requires detailed site characterization as well as optimization of surfactant solution composition. Field trials provide an essential component both in technology development and detailed feasibility studies.

A composite numerical model for assessing subsurface transport of oily wastes and chemical constituents

Subsurface fate and transport models are utilized to predict concentrations of chemicals leaching from wastes into downgradient receptor wells. The contaminant concentrations in groundwater provide a measure of the risk to human health and the environment. The level of potential risk is currently used by the U.S. Environmental Protection Agency to determine whether management of the wastes should conform to hazardous waste management standards. It is important that the transport and fate of contaminants is simulated realistically. Most models in common use are inappropriate for simulating the migration of wastes containing significant fractions of nonaqueous-phase liquids (NAPLs). The migration of NAPL and its dissolved constituents may not be reliably predicted using conventional aqueous-phase transport simulations. To overcome this deficiency, an efficient and robust regulatory assessment model incorporating multiphase flow and transport in the unsaturated and saturated zones of the subsurface environment has been developed. The proposed composite model takes into account all of the major transport processes including infiltration and ambient flow of NAPL, entrapment of residual NAPL, adsorption, volatilization, degradation, dissolution of chemical constituents, and transport by advection and hydrodynamic dispersion. Conceptually, the subsurface is treated as a composite unsaturated zone-saturated zone system. The composite simulator consists of three major interconnected computational modules representing the following components of the migration pathway: (1) vertical multiphase flow and transport in the unsaturated zone; (2) areal movement of the free-product lens in the saturated zone with vertical equilibrium; and (3) three-dimensional aqueous-phase transport of dissolved chemicals in ambient groundwater. Such a composite model configuration promotes computational efficiency and robustness (desirable for regulatory assessment applications). Two examples are presented to demonstrate the model verification and a site application. Simulation results obtained using the composite modeling approach are compared with a rigorous numerical solution and field observations of crude oil saturations and plume concentrations of total dissolved organic carbon at a spill site in Minnesota, U.S.A. These comparisons demonstrate the ability of the present model to provide realistic depiction of field-scale situations.

The use of upward gradients to arrest downward dense, nonaqueous phase liquid (DNAPL) migration in the presence of solubilizing surfactants

This paper presents a mathematical analysis and the results of a series of laboratory experiments investigating in situ dissolution of dense, nonaqueous phase liquid (DNAPL) pools from porous media using solubilizing surfactants under upward-flowing conditions. Surfactant flood experiments were conducted in a one-dimensional glass column packed with natural sand. Perchloroethylene (PCE) was emplaced in the column as a pool perched above a coarse–fine sand interface to reproduce conditions occurring when DNAPL pools on a low permeability layer in the subsurface. Aqueous surfactant solution was directed vertically upward to counteract downward pool mobilization resulting from reduction of the PCE-water interfacial tension in the presence of surfactant. Experiments were conducted at overall hydraulic gradients across the sand pack of 0.0, 0.1, 0.2, 0.3, and 0.5. The surfactant flood solution consisted of a 1:1 blend of two commercial surfactants, Witconol NP-100 and Rexophos 25/97, in a 2% aqueous solution. Surfactant flooding was conducted until all visible traces of PCE were removed from the column. Subsequent water flooding was continued until effluent PCE concentrations exiting the column dropped below 5 ppm. In the zero gradient experiment, the PCE pool invaded the fine sand layer immediately upon contact with the surfactant because of a lowering of interfacial tension. PCE entry into the fine sand did not occur in the 0.3 and 0.5 gradient runs. Minor amounts of PCE mobilization occurred in the late stages of the 0.1 and 0.2 gradient runs and is attributed to a lowering of interfacial tension with time upon exposure to surfactant. Key words : solubilizing surfactant, groundwater remediation, DNAPL, tetrachloroethylene.