Leaching of Contaminants to Groundwater - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 788

repositories. A large number of specialized numerical models now exist to sim-
ulate the different processes at various levels of approximation and for different
applications.
Increasing attention is being paid recently to the unsaturated or vadose zone
where much of the subsurface contamination originates, passes through, or can
be eliminated before it contaminates groundwater, surface and subsurface water
resources. Sources of contamination often can be more easily remediated in the
vadose zone, before contaminants reach the underlying groundwater. The focus of
this chapter thus will be on conceptual and mathematical descriptions of water flow
and especially transport processes in the predominantly unsaturated or variably-
saturated vadose zone. The vadose zone is defined here as the zone between the land
surface and the permanent (seasonal) groundwater table. The vadose zone is usu-
ally only partially saturated, although saturated regions may exist, such as perched
water above a low-permeable fine-textured (clay) layer or a saturated zone behind
the infiltration front during or after a high-intensity rainfall event.
Since the transport of contaminants is closely linked with the water flux in soils
and rocks making up the vadose zone, any quantitative analysis of contaminant
transport must first evaluate water fluxes into and through the vadose zone. Water
typically enters the vadose zone in the form of rainfall or irrigation (Fig. 18.1 ), or
by means of industrial and municipal spills. Some of the rainfall or irrigation water
may be intercepted on the leaves of vegetation. If the rainfall or irrigation inten-
sity is larger than the infiltration capacity of the soil, water will be removed by
surface runoff, or will accumulate at the soil surface until it evaporates back to the
atmosphere or infiltrates into the soil. Some of the water that infiltrates into the soil
profile may be taken up by plant roots and eventually returned to the atmosphere by
plant transpiration. The processes of evaporation and transpiration are often com-
bined into the single process of evapotranspiration. Only water that is not returned
to the atmosphere by evapotranspiration may percolate to the deeper vadose zone
and eventually reach the groundwater table. If the water table is close enough to
the soil surface, the process of capillary rise may move water from the groundwater
table through the capillary fringe towards the root zone and the soil surface.
Because of the close linkage between water flow and contaminant transport, we
will first briefly focus on the physics and mathematical description of water flow
in the vadose zone (Section 18.2 ). An overview is given of the governing equa-
tions for water flow, while a comprehensive example is used to illustrate water
content and pressure head distributions in single- and multi-layered soil profiles
following steady-state infiltration (Section 18.2.8 ). This is followed by a discussion
of the equations governing contaminant transport (Section 18.3 ) where we review
the standard equilibrium formulations for contaminant transport (Section 18.3.2 )as
well as alternative non-equilibrium models (Section 18.3.3 ). We also briefly discuss
possible formulations for colloid-facilitated transport (Section 18.3.3.3 ), stochastic
approaches for contaminant transport (Section 18.3.4 ), and multicomponent geo-
chemical transport (Section 18.3.5 ). This is followed by a discussion of analytical
(Section 18.4 ) and numerical (Section 18.5 ) approaches for solving the govern-
ing flow and/or transport equations, and an overview of computer models currently