Recent hydrogeological investigations for the WestConnex motorway tunnelling programs beneath Sydney have improved our knowledge of the Triassic sediments of the southern Sydney Basin. Previously during resource investigations in western Sydney, triggered by the Millennium Drought, pump tests were conducted to assess the capacity of the Hawkesbury Sandstone and calculate aquifer parameters. During the WestConnex investigations the hydraulic conductivity of the Hawkesbury Sandstone and Ashfield Shale was measured in‑situ by packer testing and in the laboratory using a permeameter. Porosity was also measured in the laboratory from core samples collected during the drilling programs. These hydraulic parameters have been used to assist in developing three-dimensional flow models to assess potential impacts of the drained WestConnex tunnels on the environment.
The Hawkesbury Sandstone and Ashfield Shale are dual porosity aquifers where groundwater is transmitted primarily though secondary structural features, rather than the primary porosity or interconnected void space between grains of the rock matrix. Packer test data analysis confirmed the hydraulic conductivity tends to decrease with depth due to increasing over burden pressures causing a decrease in sub‑horizontal defect apertures with depth.
Sydney Basin sediments exhibit hydraulic conductivity anisotropy where the horizontal (Kh) hydraulic conductivity is typically greater than the vertical (Kv) due to well-developed horizontal bedding planes and laminations and less well developed vertical defects. In this investigation the Kh:Kv ratio has been estimated using real data, rather than relying on model calibration. Hydraulic conductivity measurements by in-situ (packer) and ex-situ (permeameter) tests has allowed the measurement of Kh and Kv respectively. Average packer test results (harmonic mean) for the Hawkesbury Sandstone (Kh = 0.011 m/day) and laboratory (Kv = 0.00028) confirms Kh is greater than Kv by approximately two orders of magnitude. Porosity results within the Hawkesbury Sandstone range from 11.3 to 19.2% and 5.6% for the Ashfield Shale.