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.