Leaching of Contaminants to Groundwater - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 787

18.2.4 The Evapotranspiration Process ..........................

799

18.2.5 Penman-Monteith Equation for Evapotranspiration .............. 801

18.2.6 FAO56 Reference Evapotranspiration ...................... 802

18.2.7 Root Water Uptake .................................. 803

18.2.8 Application: Numerical Simulations of Variably Saturated Flow
in a Soil Profile .................................... 804

18.3 Contaminant Transport .................................... 808

18.3.1 Transport Processes ................................. 809

18.3.2 Advection-Dispersion Equations ......................... 813

18.3.3 Nonequilibrium Transport .............................

817

18.3.4 Stochastic Models .................................. 821

18.3.5 Multicomponent Reactive Solute Transport ...................

826

18.3.6 Multiphase Flow and Transport .......................... 827

18.4 Analytical Models ....................................... 828

18.4.1 Analytical Approaches ............................... 828

18.4.2 Existing Models ................................... 828

18.5 Numerical Models .......................................

829

18.5.1 Numerical Approaches ............................... 829

18.5.2 Existing Models ...................................

831

18.6 Concluding Remarks ..................................... 839

References ................................................ 841

18.1 Introduction

Many contaminants are finding their way into the subsurface environment, either
intentionally applied, for example in agricultural operations, or unintentionally
released from leaking industrial and municipal waste disposal sites, or from other
sources (Šimunek and Van Genuchten 2006 ). A broad range of fertilizers, pesti-
cides and fumigants are now routinely applied to agricultural lands, thus making
agriculture one of the most important sources for non-point source contamination.
The same is true for salts and toxic trace elements, which are often an unintended
consequence of irrigation in arid and semi-arid regions. Agriculture also increas-
ingly uses a variety of pharmaceuticals and hormones in animal production many
of which, along with pathogenic microorganisms, are being released to the environ-
ment through animal waste. Similar problems arise with point-source contamination
from industrial and municipal waste disposal sites, leaking underground storage
tanks, chemicals spills, nuclear waste repositories, and mine tailings, among other
sources.
Mathematical models should be critical components of any effort to under-
stand and predict site-specific subsurface water flow and contaminant transport
processes. For example, models can be helpful tools for designing, testing and
implementing soil, water and crop management practices that minimize soil and
water contamination. Models are equally needed for designing or remediating indus-
trial waste disposal sites and landfills, or for long-term stewardship of nuclear waste