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

GROUNDWATER FLOW MODELLING
and gravel formations with a combined thickness of 8 m, overlay 85 to 90 m of the less
permeable bedrock sandstone. The model was run in steady state with bedrock permeability
remaining constant (Kx 2 md -1 ) and the overlying deposits with Kx values of 3 and 5 md -1
(Figure 6.9a) and Kx 5 and 10 md -1 (Figure 6.9b). The highest flow rates are seen to occur
within the top high conductivity layers with flow through the bedrock aquifer slower and
decreasing with depth. Velocities through the drift range from 0.18 to 0.4 md -1 while average
velocities through the deep aquifer are 50% is derived from the superficial deposits
whereas the majority of flow from the eastern boundary is derived from the bedrock
sandstone. Flow is seen to occur in two directions across the western boundary, with shallow
groundwater flowing towards the river and deeper groundwater (below 40 m.a.o.d) flowing
away towards the flood plain.
These results may be considered in relation to contaminant discharge to the river. The
groundwater quality of the bedrock aquifer on the eastern bank will provide the greatest
contribution to the total discharge quality. Shallow groundwater will allow rapid transit of
contaminants to the river and will provide higher proportions of the total discharge to the river
with higher hydraulic conductivities of the gravel. Higher values of conductivity in the gravel
will increase the contribution from the western bank. Contaminants present in older deeper
groundwater derived from more distant sources will tend to discharge through the centre of
the channel and contaminants in the shallow groundwater from sources more local to the river
will tend to discharge closer to the channel sides. Contaminant travel velocities may differ by
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