Contaminant Fate and Reactive Transport in Groundwater - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 863

In groundwater systems the biodegradation potential not only depends on the rel-
ative energy yields of the different reactions, but also on the availability of organic
substrates, electron acceptors, and active microbial populations. Hence, not all pos-
sible processes are necessarily occurring in a given system. Most pristine shallow
groundwater systems are oligotrophic (organic-poor) and hence may remain aero-
bic, if the dissolved oxygen is not consumed (Chapelle 2001 ). For aquifers which
become contaminated with oxidizable organic contaminants, or which have natu-
rally high concentrations of dissolved organic matter in their recharge waters or solid
matrix, dissolved oxygen may become depleted, thereby allowing the emergence of
anaerobic processes. In contaminated aquifers, the differences in the distribution of
microbial activity lead to physical separation (spatial and temporal) of the zones
in which particular TEAPs dominate. Thus, the sequential utilization of electron
acceptors by microorganisms gives rise to a characteristic redox zonation, fre-
quently observed down-gradient of contaminated sites (Baedecker and Back 1979 ;
Christensen et al. 2000 ;Brunetal. 2002 ; Chapelle et al. 1995 ; Rolle et al. 2008a ).
The spatial distribution of the TEAPs allows to recognize two different patterns
of biodegradation in contaminant plumes: fringe and core processes. Highly bioac-
tive zones develop at the plume fringe, where the oxidizable organic contaminants
are brought into contact with electron acceptors (e.g. dissolved oxygen, nitrate and
sulfate) through diffusion/dispersion controlled mixing processes. Core processes,
such as manganese reduction, iron reduction and methanogenesis, take place inside
the contaminant plume.
The identification of different redox zones (as shown in the previous example for
the oxidation of toluene) is of fundamental importance to assess the overall potential
for degradation of most organic contaminants in groundwater. In fact, attenuation
of specific organic contaminants is strongly influenced by redox zonation. This
concept is valid not only for contaminants degrading preferentially (and/or most
rapidly) in an oxidizing environment (e.g. hydrocarbons such as BTEX and PAHs),
but also for contaminants (e.g. chlorinated solvents) that are typically reduced under
anaerobic conditions (Wiedemeier et al. 1999 ). For the latter group of contami-
nants, the prevailing redox environment, determined by the competition for electron
donors among different members of the microbial community, strongly influence
the efficiency of degradation.
19.3 Contaminant Transport Models
Mathematical models for contaminant transport have become increasingly impor-
tant tools to assess the risk due to contaminant migration and to support the design
of Risk Management solutions, such as groundwater remediation. They provide a
rational framework to rigorously incorporate a wide variety of processes, either
physical or biogeochemical. Therefore, they offer an ideal playground to integrate
and test process knowledge gained from theoretical, experimental (laboratory) and
field investigations.