Natural Attenuation - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 997

requires an increasingly reductive redox potential. This might be an important rea-
son why, in many CVOC plumes, biodegradation stalls at cDCE and/or VC. Hence,
aerobic degradation of the low chlorinated metabolites (Hanson and Brusseau 1994 ;
Ve r c e e t a l . 2000 , 2001 ) in an aerobic plume fringe is regarded as an important
degradation pathway. Whether or not also anaerobic oxidative degradation in less
reducing environments such as the plume front plays a significant role at real field
sites is not yet answered and has been addressed in various laboratory experiments
(Bradley and Chapelle 1996 , 1997 ; Bradley et al. 1998 ; Chapelle and Bradley 2003 ).
Substantial concentrations of the end products of reductive CVOC degrada-
tion (ethene and ethane) - i.e. complete degradation of PCE or TCE - may be
found exclusively in areas where elevated concentrations of dissolved organic
carbon (DOC) are still present and a strongly reducing environment prevails.
Down-gradient of these areas, ethene and ethane concentrations usually drop below
the detection limit, suggesting that the non-chlorinated end products are rapidly
mineralized to CO 2 in a more oxidizing environment.
Given the above mentioned facts that CVOCs are fairly soluble, less degradable
than e.g. petroleum hydrocarbons and have a low tendency to adsorb to the soil
matrix, causes CVOC contaminations usually to form spatially expanded plumes
with a length up to several kilometres.
22.4.2 Evaluation of Natural Attenuation Potential and Challenges
at Chlorinated Hydrocarbon Contaminated Sites
Basic to application of MNA is a detailed understanding of the localization of free
phase (source) and the 3-dimensional spreading of the CVOC plume in the ground-
water. Hence, innovative investigation technologies and especially a depth-related
groundwater sampling are essential at DNAPL contaminated sites (Held 2007 ).
Regularly (e.g. annual) mapping of the CVOC plume on the basis of analytical
results from groundwater monitoring wells is an important tool to describe and mon-
itor spatial changes of the plume extension over time (i.e., to determine whether
the plume extension is stable or not). Besides the metabolites cDCE and VC, the
end products of reductive dechlorination, i.e., ethene and ethane, also have to be
routinely analyzed and mapped.
Concomitant with the investigation of CVOC concentrations, the parameters
indicating the biogeochemical environment should be analyzed. These redox indi-
cators (see Section 22.2.3 ) comprise dissolved oxygen, nitrate, dissolved iron and
manganese, sulphate and methane as well as the total and/or dissolved organic car-
bon (TOC/DOC) content. Apart from conventional analyses of these parameters,
redox sensitive textile tapes (RST) may be used which, after incubation, show spe-
cific changes in colour due to the redox environment to which they were exposed.
The RST may show that the vertical extents of the transition zones are a few
centimetres only. Due to the fact that during biodegradation the microorganisms
consume the electron acceptors in the sequence given above, different redox zones
form within the aquifer. In theory, these redox environments are in the form of
interleaved blisters. In reality, also other forms may be observed due to subsurface