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

WATER QUALITY INTERACTIONS
The groundwater/surface water interface provides an environment rich in dissolved organic
carbon and other nutrients suitable for sustained microbial activity, which may lead to
reducing conditions conducive to biodegradation. Previous work (Conant, 2000) has recorded
dramatic reductions in PCE concentration (from 3700 μgl -1 to < 50 μgl -1 ) in a plume over a
15 cm vertical interval of riverbed in areas of low to moderate groundwater discharge rates.
Biodegradation reactions themselves often occur rapidly, but time may be required for
microbial activity to consume oxygen and develop a suitable anaerobic and reducing
environment before the groundwater discharges to the river. Reducing conditions have been
recorded at several locations within the Tame riverbed but often the oxygen content is too
high and sulphate reducing conditions have not developed. Flow velocities are thought to be
generally rapid (>50 cmd -1 ) through the sand and gravel of the riverbed with a limited
residence time over which degradation may occur. However, considerable heterogeneity in
flow exists both across the channel and along the reach and it is likely that low flow zones
exist that are conducive to biodegradation. Modelling indicates that the highest flow rates
occur immediately adjacent to, and through, the riverbank. Flow from these areas is derived
from local shallow groundwater that flows at relatively high velocity through the drift
deposits. The chance for biodegradation to occur in plumes derived from sources local to the
river therefore appears to be considerably lower than for sources that are more distant.
Sorption of the hydrophobic contaminants onto organic matter will retard transport rates and
will increase the residence time during which microbial action can occur. The results of the
fOC analyses performed on 15 riverbed core samples gave an organic carbon content ranging
from 0.08% to 9.18% with a mean of 1.12% (median 0.37%). These values are similar to
those found by analyses (Shepherd, 2002) of the flood plain alluvium (mean 1.2%, n=2) and
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