Environmental Engineering Reference
In-Depth Information
Except this general pragmatic approach as first estimate of toxicity to micro-
organisms, the effect of toxicity can be rather complex. For example the co-
metabolic aerobic conversion of 1,1 cis-DCE will lead to formation of toxic
intermediates and therefore bioremediation will cease (Oldenhuis et al. 1991 ).
Availability
In addition to the above factors, the bioavailability of contaminants plays an
important role, especially in attaining low residual concentrations in treated soils.
For soil remediation most of the factors above can be manipulated to maximize
conversion or removal rates. However, as limited availability is believed to be the
result of a long aging procedure in which especially the soil organic matter, but also
the surface area of especially small sized particles like clay particles play a role,
the limited availability seems to be a principle obstacle for In Situ remediation to
negligible concentrations.
From a risk perspective only the available fraction of contaminants impose a risk
to the soil ecosystem (see Chapter 16 by Hodson et al., this topic). However, from a
remediation perspective, only the available fraction can be removed or remediated.
The combination of these perspectives is leading to new approaches in legislation
in which residual concentrations after soil remediation are accepted as end points,
as long as the availability of the contaminants with time is negligible. On the other
hand, most legislations are still based on concentrations rather than on mass fluxes
that are transported from the soil to the surrounding water in which they pose a risk
for uptake in the food chain. At present several techniques are described to deter-
mine the available fraction of Hydrophobic Organic Compounds, which helps to
obtain insight in the potential risk of a contaminant (Cornelissen et al. 1998 ; Cuypers
et al. 2001 ;Reidetal. 2000 ). However only recently insight was obtained under
what conditions contaminants will be released from soils and sediments (Smit 2009 ;
Smit et al. 2008 ). These release studies showed that turbulent conditions like flood-
ing and transfer of sediment into depots will temporarily lead to high contaminant
fluxes to the environment.
21.2.2 In Situ Technologies
For In Situ technologies one can discriminate between source, path and receptor
oriented technologies (Fig. 21.3 ). These different measures are often linked to dif-
ferent parts of legal instruments such as in Europe the Groundwater Directive,
the Water Framework Directive, and the Soil Thematic Strategy (Chapman et al.
2008 ).
21.2.2.1 Source Oriented In Situ Technologies
Physical and chemical technologies are especially well suited to treat sources with
pure products of contamination, as bioremediation can only function in an aqueous
environment and contaminant concentrations should be below toxic concentrations.
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