Natural Attenuation - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 1008

vs. high molecular PAH) and is on the one hand substance specific and on the other
hand it might be influenced by the multi-component mixture itself. The biodegra-
dation rate under in-situ conditions is very low for a lot of tar oil contaminants. For
some substances the degradability especially under anaerobic conditions is even not
known.
Nevertheless, for most of the sites the plume length does not exceed a few
hundred meters even if the sites were closed decades ago. This fact indicates the
potential of the autochthonous microorganisms for degrading these contaminants
and/or for effective sorption processes taking place. Additionally, knowledge about
the in-situ processes increased considerably. For identification and quantification
of the Natural Attenuation processes the strategy and the methods described in
Sections 22.2.2 and 22.2.3 are recommended.
In several case studies the potential of Natural Attenuation processes was inves-
tigated in detail and implemented (e.g. Neuhauser et al. 2009 ; Rügner et al. 2004 ;
Werner et al. 2008a ). For the acceptance of Monitored Natural Attenuation at tar oil
contaminated sites, an intensive discussion with the local authorities is necessary.
Long lifetimes of tar oil sources and the uncertainties in the degradation processes
for PAHs and NSO-HET are the outstanding challenges.
22.6 Conclusions and Outlook
In recent years, substantial work has been undertaken in the field of research
and development, preparation of technical guidelines, and legal implementation
of MNA, mainly in the USA, but also in various European nations. This work
and especially the comprehensive six year priority research program performed
in Germany until the end of 2008 and involving more than 1000 researchers,
have contributed to the nowadays highly improved understanding of physical,
chemical, and biological processes that contribute to the transport and degradation
of contaminants in soil and groundwater. In this context, it was important to realize
that some of the most important processes are restricted to only small spaces within
the huge mass of contaminated soil and groundwater. Usually the inner parts of
contaminant plumes are depleted of mobile electron acceptors and therefore the
biodegradation predominantly takes place in the fringe of the plume body, apart
from degradation processes using immobile electron acceptors as Fe(III)/Mn(IV)
or methanogeneses, respectively.
Besides an increase of process understanding, the progress was also accompa-
nied by intensified development of methods, needed to investigate NA processes
and especially to derive quantitative and not only qualitative data from site samples.
Whereas in the past it was quite easy to determine biodegradation rates in labo-
ratory experiments, today this is for many contaminants also possible in the field,
e.g., applying the contaminant specific isotope analysis (CSIA). The method can be
used to determine degradation rates for productive degradable carbon contaminants
(e.g., (mono)aromatics) as well as for the degradation of chlorinated hydrocarbons
which are usually used as electron acceptors. Furthermore, DNA-based methods are