The impact of urban groundwater upon surface water - eTheses ...

WATER QUALITY INTERACTIONS
the bias resulting from multilevel samples collected within the same plume. These ratios
suggest the abstraction wells are close to the DNAPL source where, despite the occurrence of
biodegradation (as evidenced by some high levels of cis 1,2-DCE), concentrations of TCE
remain high due to continued dissolution from the pure phase DNAPL. Further away from the
source, the TCE/DCE ratio drops as TCE is removed by biodegradation. This shows that
natural attenuation does occur within the aquifer but not at sufficient levels to remove all the
VOCs prior to discharge to the river.
The low concentration of PCE in groundwater discharging from the southern bank (Figure
7.24) indicates limited sources and/or biodegradation of the parent compound to TCE. The
low TCE/DCE ratio (0.01) at Profile 1 shows that nearly complete biodegradation of the
original TCE has occurred, with concentrations of DCE at 16 μgl -1 and TCE at 0.17 μgl -1 . By
contrast, the high ratio and concentrations of TCE at Profiles 8 and 9 imply rapid transit times
from a nearby source. However, as anaerobic biodegradation for dechlorination requires a
specific set of physical-chemical conditions there is no straightforward relationship between
biodegradation and travel time from the source. Previous work (Lorah et al., 1999, and Fetter,
1999) indicates that hydrogenolysis occurs under anaerobic reducing (sulphate reducing to
methanogenic) conditions which are not the general conditions throughout the aquifer. It may
be that high levels of microbial activity close to the source, perhaps related to the degradation
of mono-aromatic (BTEX) compounds, consumes the oxygen producing localised anaerobic
zones. Reduction may occur in these zones before the plume travels further from the source
and mixes with more oxygenated groundwater containing less dissolved carbon, at which time
dechlorination ceases.
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