NAPL Migration

The problems associated with LNAPLs are well documented in the literature, ranging from small releases where just enough LNAPL is present to be a nuisance, to pools ranging up to millions of barrels of LNAPL and encompassing hundreds of acres in lateral extent. Subsurface migration of LNAPL (and DNAPL) are affected by several mechanisms depending upon the vapor pressure of the liquid, the density of the liquid, the solubility of the liquid (how much dissolves in water at equilibrium), and the polar nature of the NAPL. 

Typical Oil Retention Capacities for Kerosene in Unsaturated Soils 

coarse sand Gravel, coarse sand Coarse sand, medium sand Medium sand, fine sand Fine sand, silt 

NAPL will migrate from the liquid phase into the vapor phase until the vapor pressure is reached for that liquid. NAPL will move from the liquid phase into the water phase until the solubility is reached. Also, NAPL will move from the gas phase into any water that is not saturated with respect to that NAPL. Because hydraulic conductivities can be so low under highly unsaturated conditions, the gas phase may move much more rapidly than either of the liquid phases, and NAPLs can be transported to wetter zones where the NAPL can then move from the gas phase to a previously uncontaminated water phase. To understand and model these multiphase systems, the characteristic behavior and the diffusion coefficients for each phase must be known for each sediment or type of porous media, leading to an incredible amount of information, much of which is at present lacking. 

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In partially saturated media, the moisture content affects NAPL migration significantly. Abriola and Pinder (1985) suggested that the flux of a phase p (NAPL, water, or air), J, can be written as... 

Retention in Porous Media As NAPL migrates through the subsurface, some of it becomes entrapped according to its retention capacity. In addition, NAPL constituents may become redistributed in the gaseous phase. 

For aquifers with sufficient permeability and controlled NAPL migration, extraction using DeNAPLs can be accomplished in months. The DeNAPLs process can also increase the solubility of NAPLs by one or more orders of magnitude, depending on the chemical or mixture of chemicals in question. 

Nonaqueous phase Hquids (NAPLs) present special problems for soil and ground water cleanup. Contaminant transport through ground water depends in part on the water solubiHty of the compound. Because NAPLs cling to subsurface particles and are slow to dissolve in ground water, they hinder cleanups and prolong cleanup times. Dense nonaqueous phase Hquids (DNAPLs) migrate downward in the aquifer and can coUect in pools or pockets of the substmcture. Examples of DNAPLs are the common solvents tetrachloroethylene (PCE) and trichloroethylene (TCE) which were used extensively at many faciHties before the extent of subsurface contamination problems was realized. 

Liquid phase migration and retention — NAPL held suspended by the water table or capillary fringe or perched above low permeability zones (water wet soil) in the unsaturated zone. 

Migration of free-phase NAPLs in the subsurface is governed by numerous properties including density, viscosity, surface tension, interfacial tension, immisci-bility, capillary pressure, wettability, saturation, residual saturation, relative permeability, solubility, and volatilization. The two most important factors that control their flow behavior are density and viscosity. 

Free-phase liquid occurring in pore spaces, but trapped between aquifer grains which are water-wet, and not able to migrate because the NAPL... 

NAPL that is attenuated to the surface of soil grains (oil-wet grains), and retained too tightly to migrate. 

Surtek s surfactant remediation is limited by the same factors that affect any pump-and-treat technology. Performance may be reduced in areas with low hydraulic conductivity or high soil heterogeneity. Incorrect formulation and application of the Surtek method can also make NAPLs more mobile, thereby increasing their potential to migrate to previously uncontaminated areas. In addition, no complete process for the treatment of fiuid extracted by the process has been identified. 

The concentration of a dissolved NAPL in groundwater is governed mainly by interface mass-transfer processes that often are slow and rate-limited [7,8]. There is a relatively large body of available literature on the migration of NAPLs and dissolution of residual blobs [6,9-22], and pools [5,23-34]. Furthermore, empirical correlations useful for convenient estimation of NAPL dissolution... 

Lee KY, Chrysikopoulos CV (1995) Numerical modeling of three-dimensional contaminant migration from dissolution of multicomponent NAPL pools in saturated porous media. Environ Geol 26 157-165... 

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