Natural Attenuation - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 986

• How far will the contaminant plume migrate from the source zone, i.e., which

receptors are likely to be affected?

• How long will the plume persist, until a certain acceptable 'clean-up' level is

achieved?
Answers to these key questions will require answers to many sub-questions,
which are all highly site-specific.
According to the US EPA ( 1999 ) several indicators or lines of evidence have
been identified that can be used to prove that natural processes are effective. At least
the first two of the following lines of evidence are normally required to document
NA efficiency (U.S. EPA 2009 ):

• Historical trends indicate a decrease in contaminant concentrations and/or mass

over time and revealing a plume whose spatial extension is stable or shrinking
over time. A stable or shrinking plume indicates that biodegradation and other
mass flux reducing processes, e.g. volatilization and sorption, are removing dis-
solved contaminants from the groundwater at an equal or greater rate than the
source is adding them to the plume.

• Contaminant concentration and mass decrease with distance as well as chemi-

cal indicators: Biodegradation of contaminants is directly related to changes in
groundwater chemistry such as the biological consumption of natural levels of
electron acceptors (like dissolved oxygen, nitrate, manganese (IV), iron (III), and
sulphate) and the formation of by-products such as dissolved iron (II), manganese
(II), and methane. These geochemical redox indicators can be used to estimate the
site-specific potential for contaminants to be mineralized by biodegradation.

• Laboratory microcosm studies. These studies can be used to investigate aquifer

conditions under more controlled conditions and to demonstrate that native bac-
teria can biodegrade contaminants of concern. In microcosms, degradation rates
can be measured, allowing a comparison of relative degradability for different
contaminants. Generally, this technique is recommended when one of the first
two lines of evidence is inconclusive.

Besides this 'Three Lines of Evidence'-approach several additional NA protocols
exist to address the above mentioned questions (NRC 2000 ; Rice et al. 1995 ;USEPA
1994 , 1999 , 2005 ; Wiedemeier and Chapelle 1998 ; Wiedemeier et al. 1995 ). In the
following, the comprehensive protocol developed within KORA, a six year priority
research program on NA and MNA ( http://www.natural-attenuation.de ) funded by
the German Ministry of Education and Research, is briefly presented. More detail is
found in (Michels et al. 2008 ) and (Wabbels and Teutsch 2008 ). This approach for
investigating NA and implementing MNA is divided into the following main steps:

I. Checking the prerequisites for MNA.
II. Site investigations to prove the effectiveness of NA.
III. Prediction and decision concerning MNA.
IV. Monitoring and site closure.