Environmental Engineering Reference
In-Depth Information
emerging more and more. At present they are used, for example, for the detection
of Dehalococcoides ethenogenes , the only microorganism known to be able to
completely mineralize CVOCs.
Despite the huge efforts spent in understanding the processes, the existing indi-
vidual national protocols are not yet harmonized (Rügner et al. 2006 ). The most
challenging questions are whether MNA can be used as a stand-alone approach,
whether source removal should be mandatory and whether MNA is effective enough
to reduce contaminant concentrations below the target levels within a manageable
time frame, which is usually 30 years. This understanding ignores many of the
inevitable processes. Most sources have a lifetime larger than human lifetimes, even
for such comparable moderate soluble contaminants like (mono)aromatics, it might
take hundreds of years until sources comprising small NAPL-blobs are dissolved
(Eberhardt and Grathwohl 2002 ). In case the source is removed by the common
applied technique, Dig and Dump, the soil exchange must be complete. In case
of incomplete removal of NAPLs, it needs only 10 cm of residual phase being
passed by the natural groundwater flow to increase the contaminant concentration
by dissolving again to values comparable to its maximum solubility.
Although research on NA started with overwhelming enthusiasm, maybe even
longing for remediation activities becoming dispensable, today the scientific and
regulatory communities have a realistic understanding about the possibilities and
limitations of NA. This results in the fact that MNA (provided it is effective at the
site of concern) is considered as one possible approach in the area of Brownfield
management. Only in a small number of cases (e.g., where the source cannot be
removed or treated because the respective area is overbuilt or not accessible due to
other reasons) MNA is accepted as a stand alone approach. In most cases it is used
as an additional step after active remediation techniques have failed to reach the per-
missible contaminant levels in the predicted time frame and prolongation is uneco-
nomical and disproportional. MNA as a stand alone approach might be regarded
as an option for e.g. mega sites which are not remediable within an appropriate
temporal and financial frame applying conventional remediation technologies.
Using MNA in the described context seems to be quite reasonable, because it
appreciates on the one hand that groundwater is an important source of drinking
water in many regions of the world and should be managed in a sustainable way, to
guarantee water resources for future generations. On the other hand, this approach
also considers natural and technical constraints such as the impossibility at many
sites of cleaning up to the permissible contaminant levels with active remediation
techniques.
Considering that in many cases Dig and Dump procedures do not remove the
contamination completely, the discussion came up as to what extent source removal
is necessary and MNA is effective enough to do the rest of the job (contaminant
removal). This must still be concordant with the results of a Risk Assessment: the
contamination must be diminished to an extent that after source removal, sensitive
receptors are no longer affected. This approach, however, is in some conflict with
the groundwater precaution policy. Hence, in the field, where MNA is applicable or
not there is still some discussion needed.
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