Contaminant Flow In Groundwater In Hawkesbury Sandstone – Experience From Major Basement Excavations

The geology and hydrogeology of the Hawkesbury Sandstone is well documented in relation to the construction of deep excavations and water resources. Less well known is the impact of structure/defects and bedding in relation to contaminant migration into excavations.

The Triassic Hawkesbury Sandstone is a quartz sandstone, cut by many igneous dykes and characterised by dominant NNE and ESE trending fault/joint zones. The sandstone can be divided into sheet, massive and mudstone facies (Hebert, 1983). In vertical section, the sheet (or cross bedded) and massive facies make up 95% of the formation (Pells, 1998). Tammetta and Hewitt (2004) indicate the hydraulic conductivity of the sandstone is related to defect characteristics, which is influenced by depth and in situ stress and ranges from a mean of 0.1 m/day near surface to 0.002 m/day at 50 m. Many studies also document the changes in vertical and horizontal permeability of the sandstone (AGL, 2013).

With urban renewal, many old industrial sites are being redeveloped for residential purposes. The developments are often multi-storey with multiple basement car parking levels. Investigations of groundwater contamination at these sites often requires installation of monitoring wells to below the basement depth to assess the potential risk to human health and the environment.

The migration of contaminants particularly hydrocarbon compounds can migrate into the defects in the unsaturated zone and may form light non aqueous phase liquids (LNAPLs such as petrol) in the fractures or dense non aqueous phase liquids (DNAPLs such as chlorinated solvents or coal tar) that sink below the water table.

Contamination assessments in Hawkesbury Sandstone should identify the various facies and target particular bedding planes and defects for groundwater sampling. Identifying the presence of major joint sets in vertical boreholes can be more problematic and may result in a poor understanding of the vertical migration. Often this can only be fully appreciated during excavation. In many areas, the water level measured, also represents a phreatic surface due to the location of the site within a catchment.

This paper provides examples showing how contaminant migration is driven by the vertical permeability in sub-vertical defects/joints and by the horizontal bedding planes between the various sedimentary facies in the sandstone.