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

CONCLUSIONS
surface-water quality by diluting sewage effluent and industrial pipe discharges. The impact
of the groundwater on the river was found to be scale dependent. High concentration
contaminant plumes were observed within the riverbed which may have a localised impact on
the ecology, but the large dilution within the river limited the impact on the overall surface-
water quality. There is some evidence that natural attenuation within the aquifer and/or the
riverbed limits the quantity of organic and inorganic contaminants entering the river by the
groundwater pathway.
Objective 2. To investigate the physical and chemical processes controlling contaminant flux
across the groundwater/surface water interface.
Surface water was observed to penetrate the riverbed and mix with the groundwater in the
hyporheic zone to depths of up to 60 cm . Rapid changes in pH, Eh and dissolved oxygen
content occurred across the groundwater/surface water interface leading to precipitation or
sorption of some inorganic contaminants, probably to iron oxyhydroxide grain coatings.
Zonation of some processes occurred within the interface over a limited (tens of centimetres)
vertical extent. Dissolved oxygen levels were generally lower in the riverbed than the
groundwater or surface water, probably due to microbial activity, and there was evidence of
nitrate and sulphate reduction in some localities from both the groundwater and the surface
water. There was no conclusive proof of the biodegradation of organic contaminants such as
chlorinated solvents within the riverbed, but the presence of daughter compounds indicated
that biodegradation of PCE/TCE had occurred at least somewhere in the groundwater system.
Natural attenuation within the riverbed does not occur in all situations and may be limited by
rapid rates of groundwater discharge through the more permeable bed sediments.
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