Environmental Engineering Reference
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processes which lead - without human intervention - to a reduction of mass,
toxicity, mobility, volume or concentration of a contaminant in soil and groundwater.
22.1.4 Political and Practical Acceptance
At present, Monitored Natural Attenuation is widely recognized and used as an
alternative to or in combination with active technologies. Especially, the cost-
effectiveness of MNA is appreciated at the first glance. However, in some countries
(e.g. Germany) comprehensive site investigations exceeding those for typical active
remediation measures are required to fulfil regulatory demands (see also Section
22.2.2 ). Together with a well-founded prediction by reactive transport modelling
and a long lasting monitoring program, MNA might become as expensive as active
remediation measures.
Despite this uncertainty concerning the costs, using natural processes for contam-
inated site management has some additional serious drawbacks. Firstly, the release
of contaminants out of a NAPL source into soil and groundwater due to dissolution
is a long lasting process under natural conditions. Time scales for NAPL dissolu-
tion range from tens of years for residual NAPL blobs to hundreds to thousands
of years for NAPL pools (see Grathwohl 1998 ). When released to groundwater,
the degradation of contaminants is usually very fast, as long as reaction partners
(i.e. electron acceptors or donors) are present. After consumption of reaction part-
ners the delivery of additional reaction partners occurs via diffusion across the
fringes of the contaminant plume, which is a slow process. As a consequence of
the long lasting contaminant release and the slow delivery of reaction partners that
act as a limiting factor for biodegradation, contaminated sites (more precisely: the
groundwater) are often not suitable for their purpose for many decades. During this
period, control measures might be necessary to ensure that no unexpected risks
for human health and the ecosystem may arise. Another major problem is the fact
that the time span at which natural processes achieve the maximum cleanup level
(MCL) is very difficult to predict as it is directly related to the source lifetime. Both
the physico-chemical processes which are responsible for the contaminant release
and transport and the biological degradation processes are difficult to quantify.
Moreover, the extent to which processes such as competing degradation of differ-
ent contaminants or bioavailability of different forms of electron acceptors in the
mineral phase (e.g. FeIII or MnIV) influence plume development, are often largely
unknown.
Another problem that may arise when implementing MNA is that some degra-
dation interim metabolites are more harmful than the original contaminants, e.g.,
the carcinogenic and mobile vinyl chloride as a degradation metabolite of chlori-
nated solvent such as perchloroethene (PCE). As a consequence, MNA can only be
accepted in case the degradation does not end with the accumulation of metabolites.
An interesting political dilemma is related to dilution of contaminants, caused by
migration of contaminants in soil and groundwater. On the one hand, dilution results
in lower concentrations, which is often the major goal of Risk Management. On the
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