Laboratory Investigation of Residual Liquid Organics from Spills, Leaks, and the Disposal of Hazardous Wastes in Groundwater (EPA/600/6-90/004)
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Organic liquids that are essentially immiscible with water migrate through the subsurface under the influence of capillary, viscous, and buoyancy forces. These liquids originate from the improper disposal of hazardous wastes, and the spills and leaks of petroleum hydrocarbons and solvents. The laboratory studies described in this report examined this migration, with a primary focus on the behavior of the residual organic liquid saturation, referring to that portion of the organic liquid that is ‘trapped’ by capillary forces in the soil matrix. Residual organic saturation often constitutes the major volume of the organic liquid pollution, and acts as a continual source of dissolved or vapor phase organics.
Four experimental methods were employed. First, quantitative displacement experiments using short soil columns were performed to relate the magnitude of residual organic liquid saturation to fluid properties, the soil, and the number of fluid phases present. Second, additional quantitative displacement experiments using a long soil column were performed to relate the mobilization of residual organic liquid saturation in the saturated zone to wetting fluid flow rates. Third, pore and blob casts were produced by a technique in which an organic liquid was solidified in place within a soil column at the conclusion of a displacement experiment, allowing the distribution of fluid phases within the pore space to be observed. The columns were sectioned and examined under optical and scanning electron microscopes. Photomicrographs of these sections show the location of the organic phase within the porous soil matrix under a variety of conditions. Fourth, etched glass micromodels were used to visually observe dynamic multi-phase displacement processes in pore networks. Fluid movement was recorded on film and video tape.
Findings were that the spatial distribution and saturation of organic liquid within the porous media depends on a variety of factors, including: (1) the fluid properties of interfacial tension, viscosity, and density; (2) the soil structure and heterogeneity; (3) the number of fluid phases present; and (4) the fluid flow rates. Photomicrographs on a pore scale show that the residual organic liquid appears as blobs, films, rings, and wedges of microscopic size, depending on these factors. The size, shape, and spatial distribution of these blobs, films, rings and wedges affects the dissolution of organic liquid into the water phase, volatilization into the air phase, and the adsorption and biodegradation of organic components. These four processes are of concern in the prediction of pollution migration and the design of aauifer remediation schemes.
Large amounts of residual organic are trapped as isolated blobs in the saturated zone. Smaller amounts are ‘trapped’ as interconnected films, rings and wedges in the vadose zone, where the movement and distribution of organic liquids is much more complex. Residual saturations are very sensitive to soil textural heterogeneities. Even minor amounts of clay in an otherwise sandy soil may play a significant role. In the saturated zone residual saturations are largely independent of fluid properties. The rate of initial invasion of a non-wetting organic liquid may influence ‘irreducible water saturations’, and subsequent residual organic liquid saturations.The term ‘irreducible saturation’ is misleading, because the water phase is still interconnected by a water film and can be drained. In the vadose zone, the residual organic liquid saturation, which is interconnected by a similar film, may also be drained and is probably sensitive to the air flow rates in vacuum extraction and similar remedial schemes. The organic liquid film becomes a population of non-connected coalesced lenses floating at the water-air interface, when non-spreading organic liquids are involved. Residual saturation in the saturated zone can by mobilized by increasing groundwater velocities or reducing interfacial tensions with surfactants. The former is impractical while the later is potentially feasible, at least for partial mobilization.