Natural Attenuation - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 998

heterogeneity. Frequently, a complete redox sequence from methanogenic (near the
source) to aerobic at the plume fringe can be found. If at all, aerobic conditions can
only be found at the plume front. Based on the mapped redox environments and the
CVOC distribution, it may be derived in which zones biodegradation (either reduc-
tive or oxidative) can occur and in which zones CVOC degradation is not likely.
With regard to possible contributions of the organic material to the supply of
H 2 (electron donor) required for the reductive dechlorination of CVOCs, it is help-
ful to analyze for anthropogenic contaminants as well as for dissolved organic
carbon (DOC). Both naturally occurring organic carbon (e.g. peat or organic
carbon rich fluidal sediments) or organic carbon from anthropogenic contamina-
tions (e.g. petroleum hydrocarbons) can act as an electron donor to produce H 2 .
However the microbial degradability of organic carbon depends on the type of DOC.
Investigations on the degradability of the DOC have been performed (Rectanus et al.
2005 ), but these do not consider anaerobic degradation. Furthermore, numerous
thermodynamic parameters determine which part of the DOC ends up in methano-
genesis and which part may be used for reductive dehalogenation (Ballapragada
et al. 1997 ). Hence, at present the degree of dechlorination cannot be predicted via
characterization of the DOC.
Due to the high sensitivity of the redox environments on the degradation pro-
cesses of CVOCs, factors or measures influencing the redox environments might
have significant effects on the CVOC degradation. This might be - among other
things - hydraulic alteration of the flow characteristics, either anthropogenically
(e.g. by Pump and Treat measures) or naturally (e.g. due to river or sea water level
variations) as well as source remediation measures.
Pump & Treat might interrupt the supply of CVOCs into the plume as well as
the input of organic non-chlorinated material, altering the redox and degradation
regime down-gradient and within the vicinity of the hydraulic measure. If positive
or negative effects on the CVOC degradation prevail, depends among other things
on the design of the Pump & Treat measure (positioning relative to the source and
plume, pumping rate etc.) as well as on the composition of the uncontaminated
groundwater (e.g. presence of DOC) up-gradient of the source and laterally next
to the pumping well and on the prevalent CVOC contaminants (e.g. high- or low-
chlorinated CVOCs).
Also naturally occurring alterations of the groundwater flow characteristics, e.g.
due to seasonal or event driven variations of surface water levels, may cause tem-
poral lateral deflection of groundwater flow leading to a meandering of the plume.
In case the plume front as well as lateral areas show aerobic conditions, the stagger-
ing will lead to a spatially more extensive aerobic zone at the plume fringes. Hence
the aerobic bio-reactive area is substantially increased, thus conditions for aerobic
CVOC oxidation improve but reductive dechlorination is hampered.
Besides groundwater flow variations, also remediation of other contaminations
which serve as electron donors (e.g. BTEX) has substantial impact on the CVOC
degradation, as necessary reaction partners are eliminated. Thus, remediation e.g.
of an adjacent petroleum hydrocarbon contamination, should be aligned with the
remediation of the CVOC contamination.