Natural Attenuation - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 980

processes than a technology and its efficiency depends on the contaminants and on
individual site-specific conditions.
Since early appraisals proved the power of natural degradation to contribute to
soil and groundwater remediation, methods to stimulate these processes have been
developed. Especially, technologies were tested and used to stimulate biodegrada-
tion, e.g., by increasing the number of soil and groundwater organisms ( bioaugmen-
tation ) or more frequently by increasing the activity of organisms ( biostimulation ),
i.e. by overcoming limiting factors as, for example, electron acceptors or donor
shortage. Today, this in situ remediation technology is also known as ' Enhanced
Natural Attenuation' (ENA) . It is generally acknowledged that, in cases where
the efficiency of Natural Attenuation has been shown, monitoring is necessary to
demonstrate that Natural Attenuation works in a sustainable manner. Therefore, the
term Monitored Natural Attenuation ( MNA ) is used.
Monitored Natural Attenuation is primarily applied at sites that have been con-
taminated by readily degradable contaminants, i.e. organic contaminants such as
volatile organic contaminants (VOCs), semi-volatile organic contaminants (sVOCs),
including especially fuel hydrocarbons such as gasoline, jet fuel or diesel fuel. To a
lesser extent it has even been used for less degradable contaminants such as chlo-
rinated hydrocarbons, methyl-tert-butylether (MTBE), polychlorinated biphenyls
(PCBs) and explosives (Pennington et al. 1999 ). But also for metal and metalloids,
permanent immobilization within a biogeochemical active zone might be consid-
ered as an attenuation process. The observed immobilization is caused by a change
in the valence state of the metal/metalloid. The change in the valence state may not
only reduce the solubility and therefore the mobility of the inorganic contaminant,
but also its toxicity. Chromium, for example, is reduced in anaerobic environments
from the highly mobile and toxic Cr(VI) form to the low soluble and low toxic
Cr(III) form (Laitine 2006 ).
22.1.2 History
In fact, before Monitored Natural Attenuation emerged as an alternative to active
remediation measures, Natural Attenuation processes had always taken place.
From the moment that contaminants enter soil and groundwater, they are spread
and diluted into the environment, immobilised and degraded by microorganisms.
Already in the early half of the last century it was recognized that organic contami-
nants are degraded in groundwater. The realization that anthropogenic contaminants
may enter and pollute the groundwater started in the 1970s. With regard to con-
taminated site management, the possibilities of NA were also recognized at an
early stage. In the 1980s, several experiments that focused on the degradation of
organic contaminants by organisms were performed in North America and Europe.
This new phenomenon was illustrated, for example, by Wilson and Wilson ( 1985 ),
who demonstrated the aerobic degradation of trichloroethene in an unsaturated soil
column, while in Europe, e.g . ,Valoetal.( 1985 ) studied the requirements and
conditions for pentachlorophenol (PCP) biodegradation in Finish soils by a mixed