Assessing changes in soil physical and chemical properties under long term effluent disposal

Abstract

On-site wastewater treatment systems aim to assimilate domestic effluent into the environment. Unfortunately failure of such systems is common and inadequate effluent treatment can have serious environmental implications. The capacity of a particular soil to treat wastewater will change over time. The physical properties influence the rate of effluent movement through the soil and its chemical properties dictate the ability to renovate effluent. A research project was undertaken to determine the role of physical and chemical soil properties in the treatment performance of subsurface effluent disposal areas. Monitoring changes in these properties will permit improved prediction of the treatment potential of a soil. The changes within soil properties of the disposal area due to effluent application were found to be directly related to the subsurface drainage characteristics including permeability, clay content and clay type. The major controlling soil physical and chemical attributes were found to be moderate drainage, significant soil cation exchange capacity and dominance of exchangeable Ca or exchangeable Mg over exchangeable Na, low exchangeable Na, clay type and a minimum depth of 0.4m of potentially unsaturated soil before encountering a restrictive horizon. An in-depth knowledge of the local soil characteristics and associated soil hydrology is needed for better prediction of long term behaviour of subsurface effluent disposal systems. The study confirmed that both the physical properties and chemistry of the soil can be valuable predictive tools for evaluating the long term operation of sewage effluent disposal systems.