Alexandria Landfill, located in inner Sydney, is now a closed solid waste landfill, which formerly operated as a quarry (Ralford Pit) and brickworks (St Peters Brick works). The landfill is at the proposed location for the construction of the St Peters Interchange (SPI), which forms part of the WestConnex New M5 Motorway. The landfill continually generates leachate that is collected via an underground herring bone drainage network beneath the landfill. The leachate is subsequently treated at an on-site batch reactor leachate treatment plant and then discharged to sewer via a trade waste agreement. A major objective of landfill rehabilitation is to reduce leachate generation to minimise on-going maintenance and reduce the inflow of groundwater from the Botany Sands aquifer.
Development of a hydrogeological conceptual model was based on the collation of 117 boreholes and monitoring wells in the landfill and immediate surrounds. Leachate is generated from rainfall infiltration, stormwater infiltration and groundwater ingress from the Ashfield Shale and Botany Sands aquifers at the average rate of 180 kL/day. The extent of the Botany Sands was mapped from the borelogs to assist in calculating groundwater inflow to the landfill from the Botany Sands. A water balance approach confirmed groundwater from the Botany Sands as a major contributor to leachate generation. Historically a groundwater interception scheme pumped water from the Botany Sands at two locations, discharging off-site to reduce leachate generation. However this management approach was not supported by the regulatory authorities nor a sustainable long term mitigation measure and an alternative approach to reduce leachate generation was therefore required.
The solution to reduce groundwater inflow from the Botany Sands into the landfill was demonstrated to be the construction of a one kilometre long cut-off wall founded in the underlying shale and clay. The cut-off wall is also designed to capture any localised areas of groundwater contamination across the SPI site, where it will be directed to the landfill and treated prior to discharge. Leachate generation after the construction of SPI will be reduced by the construction of a landfill cap and directing rainfall and stormwater infiltration into a water collection and management system. Rehabilitation of the Alexandra Landfill for public infrastructure and park land is an excellent example of solving an environmental problem and re-using an otherwise abandoned asset located at the edge of the Sydney city fringe.
Wednesday, January 4
10:30 AM – 12:30 PM