Natural Attenuation - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 981

bacterial culture. Since organisms are ubiquitous and work for free, most of these
experiments were triggered by the cost efficiency aspect. Since the mid 1980s, and
certainly since the mid 1990s, regulators and environmental engineers are increas-
ingly relying upon MNA as an alternative 'remediation' approach. This ended in
the US in the so-called OSWER directive (office of solid waste and emergency
response) published by the US EPA in 1999 , where the usage of MNA was regu-
lated. At the same time several technical protocols were published (AFCEE 2000 ;
Wiedemeier and Chapelle 1998 ; Wiedemeier et al. 1995 ;USEPA 1998 ). In Europe,
the idea of Natural Attenuation was taken up very early in various countries (Rügner
et al. 2006 ). In Germany, a huge research and development program was launched
in 2002 by the Ministry for Education and Research (BMBF) on the technical and
legal application of MNA in Germany. As a result, seven contaminant-specific tech-
nical guidelines as well as a recommended course of action were produced (see
www.natural-attenuation.de ).
22.1.3 Definition
Many institutions have created their own definitions of Natural Attenuation. Most
of them came up with nearly identical definitions. Basically, Natural Attenuation
is an approach that relies on natural processes to attenuate contamination in soil
and groundwater. The U.S. Environmental Protection Agency (OSWER) defines
MNA as the 'reliance on Natural Attenuation processes (within the context of a
carefully controlled and monitored site cleanup approach) to achieve site-specific
remediation objectives within a time frame that is reasonable compared to that
offered by other more active methods' (US EPA 1999 ). According to this definition
the NA processes include in situ occurring physical, chemical, or biological pro-
cesses, e.g., biodegradation, dispersion, dilution, sorption, volatilization, radioactive
decay and chemical or biological stabilisation, transformation, or destruction of
contaminants. Under favourable conditions these processes act without human
intervention.
The American Society for Testing and Materials (ASTM) defines Natural
Attenuation as the 'reduction in mass or concentration of a contaminant in ground-
water over time or distance from the source of constituents of concern due to
naturally occurring physical, chemical, and biological processes, such as; biodegra-
dation, dispersion, dilution, adsorption, and volatilization' (ASTM 2004 ). The
ASTM-definition did not contain guidance to the time MNA should be applied.
The U.S. Air Force Centre for Environmental Excellence defines NA as the
processes resulting 'from the integration of several subsurface attenuation mech-
anisms that are classified as either destructive or non-destructive. Biodegradation is
the most important destructive attenuation mechanism. Non-destructive attenuation
mechanisms include sorption, dispersion, dilution from recharge, and volatilization'
(Wiedemeier et al. 1999 ).
In Germany, the official wording is based on the OSWER directive (LABO
2009 ). Natural Attenuation processes are physical, chemical and biological