Leaching of Contaminants to Groundwater - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 839

transport problems during one-dimensional steady-state flow, to sophisticated
numerical models for addressing multi-dimensional variably-saturated flow and
contaminant transport problems at the field scale.
One may expect that unsaturated zone flow and transport models will be used
increasingly as tools for higher tier Groundwater-related Risk Assessments (see
Chapter 17 by Swartjes and Grima, this topic). Moreover, these models may sup-
port the development of cost-effective, yet technically sound strategies for resource
management, contamination remediation, and/or prevention. Improved understand-
ing of the key underlying processes, continued advances in numerical methods,
and the introduction of more and more powerful computers make such simula-
tions increasingly practical for many field-scale problems. Models can be helpful
tools also 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 industrial waste disposal sites and landfills, for pre-
dicting contaminant transport from mining wastes, or for long-term stewardship of
nuclear waste repositories. A major challenge is to make the models as realistic as
possible for such complex applications. Yet another challenge is to obtain input data
appropriate for the time and spatial scale under consideration, accounting for spatial
variability and possibly even time-dependent parameters. Several approaches were
discussed that permit generation of input data from other more basic (soil) data, such
as pedotransfer functions. These efforts must continue to also include (bio)chemical
parameters.
Continued progress in subsurface flow and transport modeling requires equal
advances in both numerical techniques as well as the underlying science. Addressing
preferential flow phenomena, and the related problems of subsurface heterogene-
ity, including the stochastic nature of boundary conditions (precipitation and/or
evapotranspiration), will continue to pose formidable challenges. The same is true
for improving multicomponent geochemical transport modeling for the vadose zone.
For example, numerical algorithms and databases for multicomponent transport
models must be extended to higher temperatures and ionic strengths, complex
contaminant mixtures (including especially mixed organic and inorganic wastes),
multiphase flow, redox disequilibria for low-temperature systems, and coupled
physico-chemical systems to account for possible changes in the water retention
and hydraulic conductivity functions. Better integration is also needed between
variably-saturated zone and existing larger-scale surface numerical models, which
in turn requires further research on such issues as spatial and temporal scaling of
hydrological, chemical and biological processes and properties, linking constitutive
(soil hydraulic) relationships to measurements scales, preferential flow, and issues
of parameter and model uncertainty.
Many scientific questions related to colloid and colloid-facilitated transport are
also still largely unresolved. This is an area of research where our understanding lags
far behind current numerical capabilities. Much work is needed to better understand
the processes of filtration, straining, size exclusion, colloid-colloid interactions,
mobilization of colloids and microorganisms; accumulation at air-water interfaces,
interactions between microorganisms and contaminants (including biodegradation),