Leaching of Contaminants to Groundwater - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 838

increase in T Cd,aq . At a depth of 3 cm, about 0.65 and up to 3.5 percent of the aque-
ous cadmium was in the form of CdCl + during winter and summer, respectively.
Other cadmium complexes of the form CdCl n (2 − n) were present in much smaller
concentrations. Similar to the pH changes, changing water contents and upward
fluxes both affected the amount of cadmium in the liquid phase.
Our results above indicate that atmospheric boundary conditions can have a sig-
nificant effect on the amount and transport of cadmium in a soil profile, and on its
bioavailability since uptake processes by plants and soil micro-organisms are often
concentration-dependent. Passive root uptake of contaminants together with water
increases with increasing contaminant concentrations. Similarly, active uptake as
described with Monod or Michealis-Menten kinetics will increase with increas-
ing contaminant concentrations. Moreover, the high heavy metal concentrations
occurred during the summer months with the highest (micro)biological activity. In
addition, cadmium speciation may also play a role in uptake. For example, Smolders
and McLaughlin ( 1996 ) observed more cadmium uptake by chard ( Beta vulgaris var.
cicla ) when Cl concentrations increased while the Cd 2+ activity was kept constant,
likely due to phytoavailability of CdCl + and other CdCl n 2-n -species.
The example illustrates that simulators such as HP1 are potentially attractive
tools for studying reactive transport processes in the vadose zone during transient
variably-saturated flow. Geochemical conditions are an important factor since they
determine the speciation (both in the liquid phase and on the solid phase) of the
elements involved, and thus their mobility and bioavailability. Small variations in
prevailing geochemical conditions may alter significantly the speciation and mobil-
ity of heavy metals or other constituents. Soil systems, moreover, are subject to
large transient variations since they are open to the atmosphere. Hence, changes
in the composition of rain water or atmospheric deposition, among other external
factors, can materially alter the geochemical conditions in a soil profile.
For the time being, only a limited number of test cases and experimental data
exist for the unsaturated zone to assess the full capabilities of a reactive transport
code (see also Davis et al. 2004 ). One elaborate recent application is a study by
Gonçalves et al. ( 2006 ), who successfully applied the chemistry-specific major ion
geochemistry module of HYDRUS-1D (version 3.0, Šimunek et al. 2005 ) to a 4-year
experimental data set involving water flow and contaminant transport in lysimeters
irrigated with waters of different quality and subjected to atmospheric conditions.
Additional studies of this type should provide more credibility to the use of coupled
hydrogeochemical models for addressing flow and reactive transport problems in
the vadose zone.
18.6 Concluding Remarks
This chapter demonstrates the abundance of models and modeling approaches that
are currently available for simulating variably-saturated water flow and contaminant
transport at various levels of approximation and for different applications. Models
range from relatively simple analytical approaches for analyzing contaminant