Leaching of Contaminants to Groundwater - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 826

transport (i.e., advection-dispersion equations for each component) plus a set of non-
linear algebraic and ordinary differential equations describing the equilibrium and
kinetic reactions, respectively. Each contaminant and/or biological reaction must be
represented by corresponding algebraic or ordinary differential equations depend-
ing upon the rate of the reaction. Since the reaction of one species depends upon the
concentration of many other species, the final sets of equations are tightly coupled.
For complex geochemical systems, consisting of many components and multidi-
mensional transport, numerical solution of these coupled equations is challenging
(Šimunek and Valocchi 2002 ). As an alternative, more general models have recently
been developed that more loosely couple transport and chemistry using a variety
of sequential iterative or non-iterative operator-splitting approaches (e.g., Bell and
Binning 2004 ; Jacques and Šimunek 2005 ; Jacques et al. 2006 ). Models based on
these various approaches are further discussed in Section 18.5.2.2 .
18.3.6 Multiphase Flow and Transport
While the transport of contaminants in variably saturated media generally involves
two phases (i.e., the liquid phase and soil gas, with advection in the gaseous phase
often being neglected), many contamination problems also increasingly involve
nonaqueous phase liquids (NAPLs) that are often only slightly miscible with water.
Nonaqueous phase liquids may consist of single organic compounds such as many
industrial solvents, or of a mixture of organic compounds such as gasoline and diesel
fuel. Some of these compounds can be denser than water (commonly referred to as
dense nonaqueous phase liquids, or DNAPLs) or lighter than water ( light nonaque-
ous phase liquids, or LNAPLs). Their fate and dynamics in the subsurface is affected
by a multitude of compound-specific flow and multicomponent transport processes,
including interphase mass transfer and exchange (also with the solid phase).
Multiphase fluid flow models generally require flow equations for each fluid
phase (water, air, NAPL). Two-phase air-water systems hence could be modeled also
using separate equations for air and water. This shows that the standard Richards
equation (Eq. ( 18.11 )) is a simplification of a more complete multiphase (air-
water) approach in that the air phase is assumed to have a negligible effect on
variably-saturated flow, and that the air pressure varies only little in space and
time. This assumption appears adequate for most variably-saturated flow problems.
Similar assumptions, however, are generally not possible when NAPLs are present.
Mathematical descriptions of multiphase flow and transport in general hence require
separate flow equations for each of the three fluid phases, mass transport equations
for all organic components (including those associated with the solid phase), and
appropriate equations to account for interphase mass transfer processes. We refer
readers to reviews by Abriola et al. ( 1999 ) and Rathfelder et al. ( 2000 ) for discus-
sions of the complexities involved in modeling systems subject to multiphase flow,
multicomponent transport and interphase mass transfer. A useful overview of a vari-
ety of experimental approaches for measuring the physical and hydraulic properties
of multi-fluid systems is given by Lenhard et al. ( 2002 ).