A hydrological study of on-site soil absorption systems

The use of on-site soil absorption systems (OSASs) to dispose of domestic wastewater is common in rural and semi-rural areas in Australia as well as in many other developed and developing countries. A need exists for an improved method of evaluating OSAS performance and predicting potential impacts of OSASs on public health and environmental quality. This paper focuses on the hydrological prediction of saturated and unsaturated moisture flow patterns of wastewater dispersed into soil adjacent to OSAS disposal trenches. It presents field and laboratory measurements obtained during an extensive investigation conducted at eight OSAS sites on predominantly silty soil in Victoria, Australia. A numerical moisture flow model was also employed to predict how the hydrological performance of OSAS might be affected by a variety of site specific conditions including soil properties, climatic data, and hydraulic loading rates. The model was fine-tuned using both published data and data from the eight study sites. The predictions show that over 50%, and in some instances over 80%, of water received by an OSAS is lost by way of evapotranspiration. This highlights the importance of evapotranspiration in determining soil absorption capacity and moisture flow patterns around an OSAS, something that is not accounted for in existing OSAS design guidelines.