Natural Attenuation - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 999

For evaluation of NA at a chlorinated hydrocarbon contaminated site, the step-
wise approach as presented in Section 22.2.2 can be applied, using the methods
summarized in Section 22.2.3 . Another important proof concerning the sustainabil-
ity of NA at a CVOC contaminated site relates to the question whether the DOC
source is as long lasting as the CVOC source. Sustainability of the biodegradation
processes is only provided when the longevity of the electron donor source is at
least within the same range as the longevity of the CVOC source. If the source
of the electron donors is geogene and not due to anthropogenic spills, it may be
assumed that surplus electron donors are available. A more detailed evaluation of
this phenomenon in given by Chapelle et al. ( 2007 ). In case of an anthropogenic
DOC source, the lifetime and source emission for both sources (DOC and CVOC)
have to be estimated. Analytical and/or numerical multiphase models are important
tools for these predictions.
22.4.3 Enhanced Natural Attenuation
To improve the conditions for reductive CVOC degradation one frequently used
ENA measure is to increase the supply of organic matter. Especially degradation of
the parental products (TCE, PCE) is stimulated, however also cDCE and VC degra-
dation might be increased by addition of organic material. For this purpose a variety
of substrates have been tested or designed. Low cost substrates like molasses or lac-
tate or so-called slow release contaminants like emulsified vegetable oil seem to be
promising. In any case the application of these substrates is not trivial. Special tech-
nical protocols have been developed providing guidelines to optimize the organic
matter supply (Suthersan and Payne 2005 ; Suthersan et al. 2002 ). Such injections of
organic material may enhance CVOC degradation substantially.
22.5 Natural Attenuation at Tar Oil Contaminated Sites
22.5.1 Introduction
Tar oils are mixtures of hundreds to thousands of different organic contami-
nants. The main components are polycyclic aromatic hydrocarbons (PAH), NSO-
heterocyclic contaminants (NSO-HET), mono aromatic contaminants like benzene,
toluene, xylene, and ethylbenzene (BTEX) and short chained alkyl phenols (SCAP).
These organic liquids were produced in high quantities as a by-product of town
gas production (Collin and Höke 2005 ). Today coke processing is still a source for
tar oil. The composition as well as the properties of tar oil is mainly influenced
by the raw material (e.g., coal or lignite) and process parameters (e.g., treatment
temperature during production).
During the production of town gas, large amounts of tar were deposed at the
sites, causing severe environmental problems due to their toxic and carcinogenic
properties. By moving into the subsurface, the tar posed a high risk to soil and
groundwater contamination.