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of the complexity, we provide graphs in Figure 5, that reveal that the concentration pattern
may be completely different from what we are used to, and that the transport process is
quite irregular in space.
Fig. 5. Computationally determined map of concentrations of an electronacceptor (EA) such
as oxygen or nitrate with scaled concentrations ranging from 0 to 1. The EA enters the
aquifer from the left and moves towards the right, and is used by micro-organisms to
degrade initially resident gasoline that contaminates the aquifer. Computationally, this
situation is quite demanding to solve, and a streamline dependent spatial discretization was
needed to suppress numerical dispersion (Janssen et al., 2006).
Figure 6 gives a representation of the behaviour of Figure 5, in terms of concentration
distributions in the longitudinal direction. This result implies that the three linked
concentration curves for initially resident degradable contaminant G (gasoline, or kerosene,
for instance), an incoming (from the left) electron acceptor (EA such as oxygen) C, and
resident, growing and decaying bacterial mass M, show patterns that for each time are
directly related with each other. What is complicated is that the electron acceptor
concentration distribution for all three times is different, and that between time 1 and 2 the
injected electron acceptor seems to withdraw to the left again. This seemingly illogical result
is purely the consequence of the increasing microbial mass when both substrate (G) and e.g.
oxygen are present, as then, the consumption of G suddenly increases and its concentration
decreases. As Figure 5 reveals, spatial variability of the hydraulic conductivity may strongly
affect the transport of electron acceptor and the related degradation of resident contaminant.
Erratic patterns may develop, that are difficult to communicate.
Whereas the complexity of the transport problem is profound, the strength of identifying the
right robust features may inspire to equally profound simplifications. Considering the
downward transport of a degrading contaminant towards the groundwater level, the mass
balance equation for linearly adsorption and 1 st order degradation reads:
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