Geotechnical Challenges for On-site Wastewater Management in the Hunter Region

A large number and growing proportion of both single lot residential and larger scale new developments in the Hunter Region are not serviced by a conventional reticulated sewerage system. In such cases, wastewater treatment, its possible reuse and final disposal, is on-site, where the effluent is generated. A range of geotechnical factors including the site geology, geomorphology, soils, availability and performance of geo and geosynthetic materials, along with climatic factors, have a bearing on the selection, design, sizing and performance of an on-site wastewater system which will perform adequately and meet regulatory requirements. Geotechnical skills in site and soil assessment are fundamental to and necessary for good on-site wastewater system design and to ensure that the environmental impacts associated with on-site wastewater management are minimised. The Hunter Region displays a number of challenging geological settings and soil types for wastewater management. These include perched and shallow water tables, sensitive aquifers, floodplains and coastal lake and estuary catchments. Soils include sodic, dispersive and duplex soils and high permeability sandy soils with limited capacities for wastewater assimilation. Hydraulic and nutrient loading capacities of some of the region’s soils are limiting and present a challenge to designers. An understanding of transport and assimilation of nutrients and pathogens through permeable materials is significant in understanding the potential contribution of on-site wastewater management systems to surface and groundwater contamination and the protection of those sensitive receiving bodies by appropriate design. This paper reviews the geotechnical aspects of on-site wastewater management in the Hunter Region and illustrates, with a number of case studies, both the problems commonly encountered and their possible solutions.