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

WATER QUALITY INTERACTIONS
value of Koc a mean retardation factor of 58 may be calculated for the riverbed , ranging from
5 to 465 depending on the fraction of organic carbon. Estimates of linear discharge velocities
from the field measurements and modelling indicate maximum velocities >5 md -1 (effective
porosity 0.2), most likely through the channel sides, with discharge through the central
riverbed ~50 cmd -1 . The mean residence time for PCE across a vertical 2m section of riverbed
may be as high as 232 days, with extremes ranging from 2 to 1860 days depending on
groundwater velocity and fOC content. This has implications regarding whether biodegradation
has time to occur within the riverbed prior to contaminant discharge.
The plume detected at Profile 8 discharges across a 4 m section of the riverbed (Figure 7.25 a
and b) showing a similar distribution to the elevated levels of fluoride, sulphate and nitrate.
The main components of the plume are TCE and 1,1-TCA with minor amounts of 1,2-DCE,
1,1-DCE, 1,1-DCA, TCM PCE, VC, and bromodichloride. TCE and TCA have similar lateral
and vertical (Figure 7.26 a) distributions within the channel and show a considerable temporal
variation in concentration, dropping by half between the summer and winter levels. This fall
in concentration was also detected in the inorganic quality data with fluoride, sulphate and
nitrate following a similar trend between summer and autumn in both 2000 and 2001 (Figure
7.8). This may be a seasonal trend related to the changes in the recharge rate and groundwater
levels. These may affect the level of dilution within the plume and groundwater flow rates,
and flow direction through the source zone. However, the contaminant travel time from the
source zone to the river is unknown, making it difficult to relate the sampling time directly to
any temporal variability in the source. Another explanation for the temporal variability may
be that the direction of plume migration changes the high concentrations of the plume centre
being sampled in the summer, but the lower concentration fringes of the plume being sampled
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