Natural Attenuation - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 991

Attenuation processes. These methods address the topics as outlined above in
Section 22.2.2 to first delineate the contaminant source zone and plume development
and then to show qualitatively and quantitatively that Natural Attenuation processes
take place.
Most important qualitative hints on active NA processes are the first two out of
the 'Three Lines of Evidence' (Wiedemeier et al. 1995 ), i.e., the proof of plume
stability or shrinkage over time and the contaminant and reaction partner reduction
versus distance accompanied by increasing daughter contaminant concentrations.
As quantitative methods to prove NA processes, several NA protocols call for
a proof of contaminant mass flow rate reduction at several control planes consec-
utively down-gradient of the source. This can be realized either by the so called
'groundwater fence' method (e.g., King et al. 1999 ) or by integral pumping tests
(e.g., Bockelmann et al. 2000 ).
Further quantitative methods to obtain evidence on degradation activities are the
determination of degradation rates from field data, where applicable by using a
tracer correction term according to (Wiedemeier et al. 1999 ), or the contaminant-
specific isotope analyses to quantify via contaminant fractionation the share of
degradation that has occurred (e.g., Hunkeler et al. 2001 ).
The sole possibility to quantify all NA processes and their interactions and to
predict future plume development is by employing reactive transport modelling.
Various reactive transport models exist that have incorporated NA processes
at different levels of detail. An example for comprehensive reactive transport
modelling considering various degradation reactions can be found, for example, in
Miles et al. 2008 .
22.3 Natural Attenuation at Petroleum Hydrocarbon
Contaminated Sites
22.3.1 Characteristics of Petroleum Hydrocarbon Mixtures
Distillation of crude oil leads to various petroleum hydrocarbons products,
exhibiting a large range of different contaminant compositions and thus different
physico-chemical behaviour. Gasoline is the lightest distillate (40-175 ◦ C), fol-
lowed by kerosene (150-280 ◦ C), Diesel (160-390 ◦ C) and lube oil (300-525 ◦ C).
Petroleum hydrocarbon products consist mainly of aliphatics, which can be divided
into alkanes, alkenes and cyclo-alkanes, and aromatics, i.e., mono- and polycyclic
aromatics. Aliphatics constitute with 40-50 weight percentage the main contam-
inant group, whereas aromatics have shares of 22-35 weight percentage (Potter and
Simmons 1998 ). Additionally, additives and blending agents are added to petroleum
fuel products to improve their desired characteristics. Due to its environmental
relevance, the best known additive is methyl-tert-butylether (MTBE), used as
anti-knock agent in gasoline.