Correlation of soil data with treatment performance of subsurface effluent disposal systems

Abstract

The paper discusses research undertaken to contribute to providing long-term sustainable solutions for on-site sewage treatment based on soil physico-chemical evaluation. The study was conducted within the unsewered area in Logan City Council, Southeast Queensland. The unsewered areas were classified into four different zones based on the future development in the region and environmental sensitivity. Forty eight sampling sites were identified and samples taken from the A and B horizons. The selected soil samples were analysed for a range of physico-chemical characteristics such as pH, electrical conductivity, chloride ion, total nitrogen, total phosphorus, orthophosphate, cation exchange capacity (CEC), individual cations, particle size distribution, organic matter content and mineralogical composition. The soils at the different sites recorded pH levels ranging from moderately acidic to acidic. In the case of CEC, 50% of the sites reported values between 0 and 4 meq/100g, about 30% of the sites reported values from 10 to 47 meq/100g, and the remainder had values between 60 to 90 meq/100g. The data analysis showed that based on location and intended land use, about 70% of the sites investigated had inadequate capability to renovate effluent discharged to the subsurface area from a conventional septic tank. In areas which have a very low cation exchange capacity and low organic matter content, the dominant clay has a low capacity to adsorb the wastewater pollutants. It can be surmised that the use of septic tank/soil absorption systems based on conventional and typical designs would not be feasible. These sites would require extra care in designing and locating of the subsurface on-site disposal system or the use of alternatively designed systems.