Geoscience Reference
In-Depth Information
Chapter 10
Transport of Passive Contaminants
A tremendous amount of research has been devoted to quantifying and modeling
transport processes in the vadose zone, with readily available scientific literature
(journals and textbooks) extending over the last half century. Modeling is used to
quantify the dynamic redistribution of chemicals along the near-surface and dee-
per-subsurface profile, which often also is subject to reactive chemical processes
including sorption, dissolution or precipitation, and volatilization.
There are fundamental differences in the conceptual approaches to quantification
and model application between water flow and contaminant transport situations.
While bulk and effective parameters can be very useful for quantifying water flow—
especially at the field scale, where heterogeneity effects can be extreme and extend
to geological features with controlling lengths from meters to kilometers—treating
contamination in terms of ''averages'' is less meaningful. To illustrate this argument,
consider that, in terms of water flow, the average (vertical, downward) volumetric
flow in a partially saturated soil column can be sufficient to estimate water recharge
to the water table. However, in terms of contaminant migration, there usually is a
complex interaction of mechanisms as contaminants are transported by advection,
diffusion, and dispersion (see
Sect. 10.1
) and as contaminants interact geochemi-
cally with the solid matrix (and possibly with other dissolved constituents). Together
even define an ''average arrival time'' of a contaminant to the water table. In terms of
water quality, the arrival of only a few percent of the total amount of released
contaminant to the water table is sufficient to make the water unpotable; in this
context, average contaminant arrival time is of less relevance.
In this chapter, we examine the various mechanisms that influence chemical
redistribution in the subsurface and the means to quantify these mechanisms. The
same basic principles can be applied to both saturated and partially saturated
porous media; in the latter case, the volumetric water content (and, if relevant,
volatilization of NAPL constituents into the air phase) must be taken into account.
Also, such treatments must assume that the partially saturated zone is subject to an
''equilibrium'' (steady state) flow pattern; otherwise, for example, under periods of
heavy infiltration, the volumetric water content is both highly space and time
dependent. When dealing with contaminant transport associated with unstable
