RBF_Cover (for eps) - National Water Research Institute

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When the hydraulic conductivity of the bed and bank are reduced to moderate values (see sets
A-A and B-B in Ray et al., 2002), there is a significant attenuation of atrazine at the collector well
caisson (Figure 5). It is clear that the transient effects call for the use of complex three-dimensional
models to account for the spatial and temporal variability of streamlines. Based on hydraulic head
measurements and water-quality observations, the calibration of flow and transport models is
essential for an accurate description and prediction of the fate of individual or multiple chemicals.
The current version of the standard MT3DMS code (Zheng and Wang, 1999) accounts for the
transport of multiple species and a small range of basic reactions; however, for cases where
chemical species are assumed to interact in a more complex manner, MT3DMS-based packages
such as RT3D (Clement, 1997) or PHT3D (Prommer et al., 2003a), which account for a greater
variety of biogeochemical processes, are available.
Atrazine Concentration, µg/L
4
3
2
1
0
River water
Caisson (set B-B)
Caisson (set A-A)
Multi-Species and Multi-Component Transport Modeling
No reaction – filtrate
With reaction – filtrate
River water
0 20 40 60 80 100
Days Since Start of Simulation
Figure 5. Atrazine transport from the river to the aquifer and the pumping well for a case
with low permeability riverbed/bank (after Ray et al., 2002).
In those scenarios where the reaction progress of a dissolved chemical strongly depends on
the concentration of one or more other dissolved species, reactive multi-species models can
typically provide a better process description. This is particularly important if models are used in a
predictive mode. Furthermore, if water-quality changes are additionally affected by water-sediment
interactions, such as mineral dissolution/precipitation and/or ion-exchange reactions, a reactive
multi-component transport model might need to be applied to explain specific field observations.
Multi-component models typically use an extensive reaction database. Until recently, those
databases were (in most cases) confined to the definition of thermodynamic equilibrium reactions,
which made those models only applicable to systems where (all) reactions proceed relatively fast in
relation to groundwater flow velocity (local equilibrium assumption); however, in most of the
recently published models, a wide range of different kinetic reactions and processes can be defined.
Models with those capabilities can be used to study complex process interactions that lead to non-