--> Observing Transport and Fate of Petroleum Hydrocarbons in Soils and in Ground Water Using Flow Visualization Techniques, by Stephen H. Conrad, John L. Wilson, William Mason, and William Peplinski; #91024 (1989)

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Observing Transport and Fate of Petroleum Hydrocarbons in Soils and in Ground Water Using Flow Visualization Techniques

Stephen H. Conrad, John L. Wilson, William Mason, William Peplinski

Many petroleum hydrocarbons and organic liquid pollutants are largely immiscible with water and therefore travel through the subsurface as a separate liquid phase. Usually released at or near the surface, this organic phase initially moves downward through the vadose zone under the force of gravity. As the organic phase moves, a portion of its volume is immobilized by capillary forces. The remainder passes on, and if the volume of the organic phase is large enough, eventually reaches the water table. At the water table, the organic phase spreads laterally along the water table if it is less dense than water, or it continues to move downward if it is more dense than water. In both cases, the organic phase migrates down gradient with the ambient groundwater flow. At the leading edge of plume of organic liquid contaminants, organic liquid displaces water as it advances through the aquifer. At the trailing edge of the plume, the organic liquid becomes displaced by water. At the trailing edge, the saturation of the organic phase and its permeability become reduced until it becomes discontinuous and immobile within the pore space. Eventually, the entire volume of an organic liquid spill may become immobilized by this process.

The process of organic liquid advance into the subsurface, followed, in turn, by the displacement and trapping of the organic phase may be observed using flow visualization techniques. In this study, two approaches were used: (1) multi-phase displacement experiments were performed within glass micromodels, and (2) the organic phase was solidified in place at the conclusion of multi-phase displacement experiments conducted in soil columns. Both techniques allow the distribution of the organic phase within the pore space to be observed under two-phase (saturated zone) and three-phase (vadose zone) conditions.

Micromodels are two-dimensional physical models of a pore space network, created by etching a pattern onto two glass plates that are then fused together. The advantage of performing multi-phase flow experiments using micromodels is that they give us the ability to actually see fluids displace one another in both a bulk sense and in individual pores. Photographs of the entire model allow examination of the bulk displacement processes, and photomicrographs taken through an optical microscope permit observation of details on a pore level. Etched glass micromodels provide an excellent method with which to study the mechanisms controlling the transport and capillary trapping of organic liquids because the make-up of the pore network can be closely controlled.

Organic phase polymerization, the other visualization approach used in this study, is a technique in which the organic liquid is solidified in place within a soil. In the two-phase procedure, styrene monomer containing a polymerizing agent and a fluorescent dye is used as the organic phase when performing a water/organic-displacement experiment in a soil column. At the conclusion of the experiment, the column is heated in an oven to polymerize the styrene. The polymerized styrene is rigid and chemically resistant. Following polymerization, the water phase is removed and replaced by a dyed expoxy resin. The solid core of soil, solidified styrene (the organic phase), and hardened expoxy resin (the water phase) can be cut in sections to show the organic liquid phase in relation to the so l and the water phase. (Three-phase experiments use styrene and two expoxy resins to represent the fluid phases.) The sections are photographed under an optical microscope. Although polymerization gives only a "snapshot" of the displacement process, it offers the advantage of allowing us to see organic liquid in its "natural habitat" (i.e., within a soil) as compared to that observed in two-dimensional micromodels. Sometimes, instead of replacing the water with expoxy resin, the solid matrix of the soil column is dissolved with hydrofluoric acid, leaving only the hardened organic liquid. The solidified organic phase may then be observed under a scanning electron microscope and photographed.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.