Transport of Reactive Contaminants - Contaminant Geochemistry - page 272

in some pores, as a coating on pore walls and as microdroplets trapped within pore
spaces by interfacial surface tension. This NAPL fraction is referred to as the
residual saturation. The presence of this fraction may contribute volatilized phases
and dissolved components during subsequent water infiltration and redistribution.
In addition, if the residual fraction is hydrophobic, its presence further affects
subsequent water and contaminant movement. The flow and distribution of the
liquid phases are controlled by the movements of the interfaces between the
nonwetting and wetting phases that result from changes in pressure and saturation
(Blunt and Scher 1995 ; Reeves and Celia 1996 ; Cheng et al. 2004 ).
Quantification of NAPL transport usually is considered by using advection-
dispersion equations for each of the water and NAPL phases and defining relative
permeabilities (as noted previously). Alternatively, if the emphasis is on con-
tamination of water from a dissolving NAPL source (e.g., from residual NAPL in
the vadose zone or NAPL pools and localized leaks), then transport equations
containing a simple source term, such as Eq. ( 11.7 ), can be considered.
The previously mentioned approaches are useful when quantification at the
average (''effective'') continuum (''macroscale'') level is appropriate. However,
pore-scale network models are more appropriately used to examine fluid distri-
bution within pores and clusters of pores (recall Sect. 9.1 ) . NAPL migration
through the vadose zone undergoes significant fingering (unstable infiltration), and
because it is a nonwetting liquid relative to the solid phase (and immiscible in
water), NAPL displacement of water is in effect a drainage process, as discussed in
Sect. 9.1 . Pore-scale information such as NAPL geometry or interfacial configu-
ration (or the interfacial area between the nonwetting and wetting liquids) is
important (Miller et al. 1990 ; Powers et al. 1992 ; Cheng et al. 2004 ; Ovdat and
Berkowitz 2006 ). Knowledge of this interfacial area is particularly useful for
quantifying processes such as contaminant adsorption, dissolution, volatilization,
and enhanced oil recovery (Reeves and Celia 1996 ; Saripalli et al. 1997 ; Johns and
Gladden 1999 ; Schaefer et al. 2000 ; Jain et al. 2003 ). NAPL migration at the pore
scale is illustrated in Fig. 11.4 .
At the pore level, fluids are conveniently characterized by three main param-
eters. The viscosity ratio is given by M = l d /l r , where l d and l r are the viscosities
of the displacing and resident fluids, respectively, while the capillary number is
defined as Ca = ql d /c, where q is the specific discharge of the displacing fluid and
c is the interfacial tension. The bond number, Bo = gr 2 Dq/c, where g is the
gravitational acceleration, r is the characteristic pore radius, and Dq is the fluid
density difference, is then introduced to account for buoyancy-gravity forces. The
usual convention is that Bo [ 0 (dense fluid displaced by light fluid) stabilizes the
interface, whereas Bo \ 0 (light fluid displaced by dense fluid) destabilizes it.
Lenormand et al. ( 1988 ) pioneered experiments on immiscible displacements in
two-dimensional, artificial porous media, using horizontal models with negligible
inertial and gravity forces and developed a numerical model to describe fluid
displacement in such porous media, accounting for capillary and viscous forces.
Comparison between the experiments and numerical simulations led Lenormand