Engineers Australia

Shale, salinity and groundwater in Western Sydney

Greg McNally

Abstract

The Wianamatta shales, though best thought of as leaky aquicludes, do include scattered zones of fracture porosity within the weathered shale and soil profile, and also at depth in the unaltered shale bedrock. The water within these fractures is generally saline, typically in the range 5,000-50,000 mg/L. The bulk permeability, like the salinity, is extremely variable, typically 10-7 to 10-9 m/s (1-0.01Lu) in fresh shale and 10-6 to 10-9 m/s in the weathered regolith. Although the shales have no value as sources of groundwater, they do present a hydrogeological nuisance. They are associated with surface salting in parts of western Sydney and eastwards to Lakemba. Salt rises with groundwater along semi-permanent watercourses such as South Creek and Second Ponds Creek, which are the lowest parts of their landscapes. Deep weathering zones and enhanced fracturing along lineaments may be significant in bringing this saline groundwater into the creek beds. However another important process in land salinisation is considered to be throughflow parallel to the ground surface and well above the water table. Much salt is believed to be stored within the soil B-horizon, which may contain 30-50 tonnes per hectare; it is added to by infiltration and depleted by throughflow and by deeper percolation to the water table. The probable source of this salt is not seawater trapped in the pores of the shale, which is non-marine, but rather windblown aerosols. These annually deposit between 20 and 200 kg of salt per hectare in western Sydney, which largely accumulates in the clay subsoil below the root zone.