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

GROUNDWATER FLOW TO THE RIVER TAME
of channel with a typical dry weather discharge of 3.8 m 3 d -1 . There is no conclusive evidence
that discharge to the river channel is higher as it flows across the Birmingham Aquifer than
when it passes above the coal measures or the mudstone. The baseflow discharge directly to
the river channel overlying the aquifer for 1999 was 8.86 x10 6 m 3 yr -1 and the total
groundwater discharge to surface water from the aquifer may be as high as 3.38x10 7 m 3 yr -1 .
The analytical methods of estimating groundwater discharge, based on spot measurements of
hydraulic conductivity, head and temperature gradient, show spatial variability in the
discharge from the sandstone aquifer in the range 0.06 to 10.7 m 3 d -1 from individual lateral
profiles. Average discharges to the river were 0.76 m 3 d -1 per metre of channel (calculated
from the radial flow method) and 1.8 m 3 d -1 per metre of channel (calculated using the Darcy
method). These estimates are considerably lower than the 3.8 m 3 d -1 per metre of channel
estimated for typical summer baseflow by direct measurement of discharge accretion in the
river. A similar occurrence has been noted in other studies (Cey et al.,1998, Harvey et al.,
1993) with estimates of Darcy flow amounting to 19-25% of the measured baseflow
accretion. This may indicate an unknown geological control or a shortfall in the data coverage
in relation to what is required to estimate flow in a highly heterogeneous system. The direct
measurement of baseflow accretion is considered to provide the best estimate for the
groundwater discharge. However, it must be regarded as an upper limit and it is reliant on
knowledge of all the additional sources of discharge to the river which is problematic to
obtain in an urban setting.
The Darcy flow estimates are very site specific to individual points within the riverbed and
give a wide range in specific discharge values from –0.01 to +2.53 md -1 . A high sample
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