Leaching of Contaminants to Groundwater - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 789

Fig. 18.1 Schematic of water fluxes and various hydrologic components in the vadose zone
(Šimunek and Van Genuchten 2006)
available for simulating vadose zone flow and transport processes (Sections 18.4.2
and 18.5.2 ). Several sections of this text are adapted from a recent topic chapter by
Šimunek and Van Genuchten ( 2006 ).
18.2 Variably Saturated Water Flow
In this Section we briefly present the equations governing unsaturated water flow
in the subsurface. Traditionally, descriptions of variably saturated flow in soils are
based on the Richards ( 1931 ) equation, which combines the Darcy-Buckingham
equation for the fluid flux with a mass balance equation. The Richards equation typi-
cally predicts a uniform flow process in the vadose zone, although possibly modified
macroscopically by assuming the presence of spatially variable soil hydraulic prop-
erties (e.g., as dictated by the presence of different soil horizons, but possibly also
varying laterally). Unfortunately, the vadose zone can be extremely heterogeneous
at a range of scales, from the microscopic (e.g., pore scale) to the macroscopic (e.g.,
field or larger scale). Some of these heterogeneities can lead to a preferential flow
process that macroscopically is very difficult to capture with the standard Richards
equation. One obvious example of preferential flow is the rapid movement of water