Infiltration, Throughflow & Salinity
Abstract
Much of the debate over the potential effects of large-scale urban development on shalederived soils in western Sydney has invoked the prospect of a rising water table bringing salt-laden groundwater to the surface. This is certainly a common situation in other parts of Australia, where enhanced infiltration following land clearing brings diluted hypersaline groundwater from unconfined aquifers to within capillary reach of the surface. However this paper will argue that while a rising water table could be troublesome in some parts of western Sydney, the more general mechanism involves salt transfer in throughflow within the soil profile. This shallow-moving subsurface water never reaches the water table and hence cannot correctly be called groundwater.
Furthermore, though groundwater in shale bedrock aquifers typically contains 10,000-30,000mg/L total dissolved salts (TDS), much of the near-surface salt is stored within the B-horizon of the soil profile, well above the water table in most places. The rate of flushing of this stored salt is at present not known, although this may greatly affect the feasibility of large-scale irrigation using recycled water with salt contents in the order of 500-600mg/L TDS.
The paper also briefly discusses groundwater and salinity in relation to the lining or not of artificial wetlands. Although saline groundwater is characteristic of the fractured shale bedrock, the normally very low permeability (hydraulic conductivity) and extremely small storage capacity of the shale greatly limit its potential damage.
Author
Greg McNally