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

GROUNDWATER FLUX
the riverbed piezometer sampling (Chapter 7, Tables 7.1 and 7.3). The results indicate lower
quality groundwater discharging through the riverbed across the Birmingham aquifer
(measured in the piezometers) compared with the gauging station estimate that is
representative of groundwater across the entire catchment. The TDS content of the surface
water at Water Orton is higher than the groundwater (809 mgl -1 ), and rose from 884 mgl -1 to
965 mgl -1 over the period 21/1/98 to 14/8/98. This implies that on the catchment scale the
groundwater is diluting the surface water.
During high baseflow conditions (21/1 to 19/2/98) a 30-day recession period (Figure 8.1b)
showed a fall in the daily minimum mass flux from 3.44 kgs -1 to 2.44 kgs -1 and a drop in
discharge from 3.89 m 3 s -1 to 2.25 m 3 s -1 . The TDS in the groundwater was calculated as 610
mgl -1 , lower than the estimate of 809 mgl -1 over the full range in baseflow conditions. This
difference may be related to variation in the STW discharges or it may indicate that the
average TDS content of the groundwater increases with a decrease in baseflow. The answer to
why the TDS content of the groundwater should change is not straightforward and is
dependent on mass flux from a complex system. One explanation is that groundwater under
high baseflow conditions may contain a greater proportion of recent recharge than under low
flow conditions when the proportion of groundwater with longer residence times (and higher
TDS) increases. Groundwater modelling (Section 6.2) indicated that the permeable alluvial
flood plain deposits are an important control on the groundwater system and provide a rapid
flow path for the shallow groundwater (comprising a large component of recent recharge) to
the river. Computer modelling of contaminant flux through the aquifer and flood plain
deposits incorporating recharge is required in order to understand the system better.
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