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

WATER QUALITY INTERACTIONS
attenuation will decrease contaminant concentrations but this is partially dependent on travel
times, and the least amount of attenuation will occur for contaminants with sources close to
the river. The high concentration plumes detected in this study may have sources within 100
m of the river where the protection afforded by the unsaturated zone is least.
Mixing of groundwater from different origins may occur in the focussed discharge zone
around the river, and this would cause dilution of contaminant concentrations. In some
instances, a substantial variation was observed in the composition of waters discharging from
opposite banks, and this together with further variations in concentrations observed across the
riverbed may reflect the culmination of other water types from a variety of flow paths. The
vertical extent of the active groundwater system is unknown but modelling indicates that the
deeper groundwater does not discharge to the river. The older groundwater from depth would
be expected to discharge through the middle of the channel and younger, shallower
groundwater, through the riverbanks, but the chemical data set is insufficient to confirm this.
The similarities in water quality between the shallow and the riverbed piezometers are greater
than between the abstraction wells and the riverbed piezometers, implying that the majority of
groundwater discharging to the river is of relatively shallow origin.
Interaction between groundwater and surface waters was observed in multilevel piezometers
using the concentration gradient between high levels of chloride in the surface water and
usually lower levels in the groundwater. The maximum observed depth of surface water
penetration into the riverbed was