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

REVIEW
The physical and chemical properties of the contaminant determine its mobility and toxic
effect. A contaminant may move through the subsurface as a pure liquid or gas phase, as a
dissolved phase, in particulate form or attached to colloids. Soluble compounds may be
transported readily within the groundwater, and attain high levels of concentration. Less
soluble compounds will occur in low concentrations but may provide a long-term source of
contamination. Advective transport of dissolved phase contaminants within the groundwater
is seen as the primary mechanism by which subsurface contaminants may impact upon
surface water systems.
The sources of contaminated groundwater may be spatially restricted point sources such as an
industrial spill or waste dump, or more diffuse sources such as arise from the widespread
application of agricultural fertilisers and pesticides. Point sources tend to give rise to narrow
plumes (Rivett et al., 2001) which migrate with the groundwater flow and may eventually
discharge to the surface water (Figure 2.3). In the USA more than 75% of the contaminated
land categorised under the government’s ‘superfund’ sites lie within 0.5 miles of a surface
water body and more than half had an impact on surface water in some way (Environmental
Protection Agency (U.S.), 2000).
The initial contaminant concentration in the groundwater will depend on the mass and
distribution of the contaminant in the source area, the rate of groundwater flow and the
physical-chemical-biological processes controlling contaminant dissolution (Fetter, 1999).
Contaminants derived from the land surface may take a considerable time to enter the
groundwater if a large unsaturated zone is present. Groundwater contaminant concentrations
in the source area may vary with time or may give rise to discrete pulse-type inputs.
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