Natural Attenuation - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 987

I. Checking the prerequisites for MNA implies the answers to the following
questions:
(1) Are the general conditions concerning actual and future use of the site and
surroundings including its groundwater suitable to implement MNA?
(2) Are the existing contaminants degradable, in principal?
Furthermore, a first spatial delineation of the NA reaction zone, i.e., the source
zone and the contaminant plume, has to be performed, including a rough char-
acterization of the geochemical and hydraulic situation. Also, a first conceptual
site model including the hydraulic and geochemical data has to be developed.
This first step results in a primary evaluation of the NA potential at the site.
II. Site investigations to prove the effectiveness of NA .Aimsofthisstepare:
(1) Identification and quantification of NA processes.
(2) Proof of decreasing contaminant mass flow rates.
(3) Generation of data basis for NA prediction.
This implies the application of various methods, which depend in part on
the site and/or contaminants present. Table 22.1 and Section 22.2.3 summa-
rize methods that have been proven suitable at various test sites (Wabbels and
Teutsch 2008 , Peter et al. 2006 ).
III. Prediction and decision concerning MNA include the following steps:
(1) Set up of a flow and reactive transport model to simulate future source and
plume development.
(2) Proof of proportionality: only if active remediation options are not propor-
tional, i.e., if they are too expensive in relation to their efficiency, MNA
might become a remediation alternative. Thus, active remediation options
have to be considered and compared to MNA.
(3) Design and implementation of a MNA concept with definition of targets
and fall-back criteria.
IV. Monitoring and site closure comprise the following aspect:
(1) Monitoring with data assembly for conservation of evidence.
This target performance comparison can eventually lead to an end of mon-
itoring and a site closure, or to an abortion of MNA and a return to active
remediation measures.
22.2.3 Methods to Prove Monitored Natural Attenuation
As mentioned above in Section 22.2.2 several lines of evidence may lead to the
proof of Natural Attenuation, entailing a wide portfolio of site and laboratory inves-
tigations, often accompanied by modelling. In Table 22.1 investigation methods
are summarized that have been shown suitable at several sites contaminated with
petroleum hydrocarbons or other contaminants to identify and quantify Natural