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

GROUNDWATER FLUX
changes in concentration could also be due to systematic error - if, for example, the sample
points were regularly visited but at times corresponding to different stages in the daily
sewage discharge cycle as it propagated down river. The dry-weather data show a fall in mean
concentration at Perry Barr for Cd, Cu(total), Cr, Pb and Zn showing that, despite the increased
dilution caused by wet weather, the mass flux is greater, because wet weather inputs and
increased flow velocities carry more suspended material. The concentrations of NH4,
Cu(dissolved), Ni and total hardness increase because of less dilution from run-off. Similar trends
are observed between the Saltley composite and dry weather averages but the change is not as
great.
The discharge used to calculate the mass flux was derived using mean values for baseflow
discharge at Bescot plus known mean discharges. In addition to this the average baseflow
increment of 3.6 m 3 d -1 per metre of channel was added over the 10.3 km from Bescot to Perry
Barr and the further 7.4 km to Saltley. For comparative purposes, readings from the 22/5/01
discharge measurements were also used to calculate the mass flux. Both sets of discharge
values used show a similar increase of 17-22% but the mass flux is much lower for the
22/5/01 and it is unknown which, if either, is more appropriate. The mass flux reveals a
complex story that is not immediately obvious from consideration of the concentration data
alone. Some of the determinands using the 22/5/01 discharge data, Cd and Cu(dissolved) and
Cu(total), exhibit an overall reduction in mass flux which may be due to sorption or settling of
particulates. NH4 shows an increase of between 2 and 21 kgd -1 even though some losses will
have occurred by the ready oxidation of NH4 to NO3. The most significant increases in metal
mass flux are for Zn (2.5-5 kgd -1 , Ni 0.5-2 kgd -1 and Cr 0.5-0.8 kgd -1 ). It is not known
whether the additional mass flux is in dissolved form but it is not unreasonable to assume that
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