Groundwater-Related Risk Assessment - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 769

hypotheses and using an iterative approach to modify the model in the light of new
data or improved understanding, the practitioner reaches the best representation of
the system being studied. Secondly, models support decision-making. It is generally
recognized, however, that models are not perfect nor ever will be, in representing
complex systems such as the subsurface environment.
Basically, contaminant transport includes four types of processes; these are water
flow, migration of contaminants within the water phase, physico-chemical interac-
tion of contaminants in the pore water or groundwater with the soil material, and
degradation. For many decades, at least since the famous French scientist Henry
Darcy formulated his simple groundwater flow equation in the mid of the 19th
century (Darcy 1856 ), engineers have been able to estimate the water flow veloc-
ity. Today, many equations and a wide variety of models exist to calculate water
flow and contaminant transport.
Generally, the calculation of saturated groundwater flow is equivalent to calculat-
ing electrical current: the flow is proportional to the hydraulic gradient (as compared
with the gradient in voltage) and inversely proportional to the hydraulic resistance
(as compared with electrical resistance). Groundwater flow in the unsaturated zone
(pore water), however, is far from linear. This makes the calculation of contaminant
transport in the water-unsaturated upper soil layer much more complex than in the
saturated groundwater zone. However, analytical solutions for contaminant trans-
port in the water-unsaturated upper soil layer, like those for the saturated zone, exist
for a whole series of initial and boundary conditions.
17.4.2.2 Numerical Models
Since the early 1970s, groundwater engineers have used computers to overcome the
difficult problem of non-linearity. Today, numerous numerical models for estimat-
ing the groundwater flow and contaminant transport in both the saturated and the
unsaturated upper soil layer are available for every type of contaminated ground-
water problem. Such a numerical model calculates the water flow and contaminant
transport between concrete soil layers in a series of concrete time steps, while the
calculations in each time step build on the results from the former time step. To this
end, for one-dimensional numerical models, for example, a contaminated site soil
profile must be subdivided into hypothetical concrete layers with fixed thickness
(often a few centimetres), or, in case of more complex models into concrete layers
with different thicknesses. Time steps can vary from seconds to days, depending on
the complexity of the transport problem. An example of the use of numerical models
is given in Chang et al. ( 2001 ), who used a numerical model to calculate copper and
cadmium transport in a lateritic silty-clay soil column.
Some numerical models include the possibility of assessing the transport of mul-
tiple contaminants. An example of such a model is given in Mayer et al. ( 2002 ), who
calculated the transport of organic contaminants in an unconfined aquifer overlaid
by unsaturated sediments and of acid mine drainage in the unsaturated zone of a
tailings impoundment at the Nickel Rim Mine Site near Sudbury, Ontario, Canada,
and the subsequent reactive transport in the saturated portion of the tailings.