Geoscience Reference
In-Depth Information
Chapter 11
Transport of Reactive Contaminants
The previous chapter focused on the physical mechanisms (advection, diffusion,
and dispersion) and the physical characteristics of the subsurface (heterogeneity)
that control the dynamics of contaminant transport from the land surface to the
water table. In addition, contaminants are subject to a range of chemical inter-
actions with other (dissolved) chemical species present in the subsurface, colloids,
and the porous matrix itself. The previous sections of this topic dealt with such
reactions, including sorption (by various types of bonding), decay, degradation,
complexation, precipitation, dissolution, and volatilization as well as interactions
(and transport) with migrating colloids. These reactions thus influence—and are
influenced by—advective, diffusive, and dispersive transport mechanisms. To
include the effects of these reactions in quantifying the dynamics of contaminant
transport, additional terms can be included in the transport equations surveyed in
terms that account for chemical species loss from or entry to the aqueous solution.
It should be emphasized that, to date, the ability to quantify the complex
chemical reaction phenomena that occur in the subsurface and also integrate the
variability in flow behavior caused by natural heterogeneity and fluctuating
boundary (land surface) conditions remains very limited. As a consequence,
developing and improving the predictive capabilities of models are the areas of
active research.
11.1 Contaminant Sorption
Sorption of contaminants can be included in the advection-dispersion equation by
introducing a retardation factor:
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