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

GROUNDWATER FLOW MODELLING
40% was related to a high value of Kx (15 md -1 ) in the alluvial gravel and a rise of 35% due to
an elevated value of Kx (5 md -1 ) in the sandstone. The effect of an impermeable mudstone
layer within the sandstone was simulated by reducing the unit thickness to 10 m. This caused
a 48% reduction in groundwater discharge to the river and a 25 cm drop in the modelled head
for piezometer P10.
The sensitivity analyses suggests that sufficient groundwater discharge to the river could be
supplied under conditions of reduced conductivity or thickness in the sandstone and increased
conductivity of the gravel. However the average head modelled for piezometer P10 fell by 20
cm under low Kx conditions in the sandstone. It fell by 25 cm when the sandstone thickness
was reduced to 10 m and also fell by 17 cm under high Kx conditions in the gravel. This
indicates that a combination of a high permeability gravel unit and a thin and/or low
permeability sandstone unit is unlikely to represent the actual field conditions. Of most
significance in controlling groundwater discharge under steady-state flow conditions is the
transmissivity (T) of the sandstone aquifer. FAT3D results indicate a minimum T of 200 m 2 d -
1 for the sandstone. Therefore, flow must be occurring through a significant thickness of the
aquifer.
The steady-state analytical model was used to derive an estimate of the saturated thickness of
the aquifer based on field data. The initial value of saturated thickness derived was 1.07 m
which is inconsistent with geological information that indicates sandstone to depths of greater
than 100 m. A sensitivity analyses was undertaken (Figure 6.10) to investigate the impact of
varying the groundwater head values to the likely maximum and minimum values. The
calculated saturated thickness was most sensitive to variations in the value of the river
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