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

6.12 Conclusions
GROUNDWATER FLOW MODELLING
Particle tracking indicates that in some cases groundwater contaminant plumes within a
meander may flow away from the river beneath the flood plain before discharging to the river
further downstream. This may considerably increase contaminant travel times and the chance
for natural attenuation to occur. Flow beneath the flood plane is primarily horizontal to within
approximately 100 m of the river when the vertical component becomes increasingly
important, as flows with a variety of origins converge and discharge to the river. The younger
groundwater discharges through the sides of the channel and the older groundwater through
the centre. The modelling indicated the existence of a flow divide between shallow
groundwater that discharges directly to the river and deeper groundwater that may flow over a
considerable distance before discharging to the river further downstream.
Groundwater flows to the river occur through a complex geology comprising made ground,
alluvial gravel, weathered and unweathered sandstone and the riverbed sediments. In
agreement with the field data, the modelling indicated that the riverbed sediments did not
significantly restrict groundwater discharge to the river. Borehole evidence (Figure 5.25a)
indicates that on the edges of the flood plain the gravel receives groundwater flow from the
underlying sandstone. The FAT3D model indicates that flow velocities through the alluvial
gravels (