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Role of soil physical and chemical characteristics and landscape factors in defining soil behaviour under long term wastewater dispersal

Dawes, Les A. (2006) Role of soil physical and chemical characteristics and landscape factors in defining soil behaviour under long term wastewater dispersal. PhD by Publication, Queensland University of Technology.

Abstract

The use of on-site wastewater treatment systems for the treatment and dispersal of domestic effluent is common in urban fringe areas which are not serviced by centralised wastewater collection systems. However, due to inappropriate siting and inadequate evaluation of soil characteristics, the failure of these systems has become a common scenario. The current standards and guidelines adopted by many local authorities for assessing suitable site and soil conditions for on-site dispersal areas are coming under increasing scrutiny due to the public health and environmental impacts caused by poorly performing systems, in particular septic tank-soil adsorption systems. In order to achieve sustainable on-site wastewater treatment with minimal impacts on the environment and public health, more appropriate means of assessment of long term performance of on-site dispersal areas are required.

The research described in the thesis details the investigations undertaken for the development of robust assessment criteria for on-site dispersal area siting and design and assessment of the long term performance of soil dispersal areas. The research undertaken focused on three key research areas; (i) assessment of site and soil suitability for providing adequate treatment and dispersal of domestic wastewater; (ii) understanding sorption, purification and transport processes influencing retention and release of pollutants and the natural controls governing these processes and (iii) the development of assessment criteria for long term behaviour of soils under effluent dispersal.

The research conducted was multidisciplinary in nature, with detailed investigations of the physical and chemical processes involved in on-site wastewater treatment and dispersal. This involved extensive field investigations, sampling and monitoring, laboratory and soil column testing and detailed data analysis across the fields of soil science, groundwater quality, subsurface hydrology, chemical contamination, and contaminant fate and transport processes. The interactions between these different disciplines can be complex which resulted in substantial amounts of data being generated from the numerous field and laboratory investigations and sampling undertaken. In order to understand the complex relationships that can occur, multivariate statistical techniques were utilised. The use of these techniques was extremely beneficial. These techniques not only allowed not only the respective relationships between investigated parameters to be identified, but also adequate decisions based on the correlations were able to be formulated. This allowed a more appropriate assessment of the influential factors, and the prediction of ongoing changes to soil properties due to effluent disposal.

The primary outcomes for this research were disseminated through a series of peer reviewed scientific papers centred on these key disciplines. The assessment of site and soil suitability was achieved through extensive soil sampling throughout the study areas and detailed laboratory testing and data analysis. The study identified and investigated the role of influential site and soil characteristics in the treatment performance of subsurface effluent dispersal areas. The extent of effluent travel and the ability of the soil to remove pollutants contained in the effluent by adsorption and/or nutrient uptake were investigated. A framework for assessing the renovation ability of the major soil groups located throughout Southeast Queensland was also developed. The outcomes provide a more rigorous scientific basis for assessing the ability of soil and evaluating site factors to develop more reliable methods for siting effluent dispersal areas. The resulting assessment criteria developed was compared with soil column studies to determine the robustness and validity of the outcomes. This allowed refinement of the assessment criteria in developing a more reliable approach to predicting long term behaviour of soils under sewage effluent dispersal. Multivariate techniques assisted in characterising appropriate soils and to determine their long-term suitability for effluent treatment and dispersal.

The assessment criteria developed included physical, chemical and sub-surface hydrological properties of a site and soil which can be used to predict suitability for long term effluent treatment and dispersal. These include:

 Moderate to slow drainage (permeability) to assist the movement of effluent (percolation) through the soil profile and allow adequate time for treatment and dispersal to occur. With longer percolation times, the opportunity for exchange and transport processes increase.

 Significant soil cation exchange capacity and dominance of exchangeable Ca2+ or exchangeable Mg2+ over exchangeable Na+. Although a soil dominated by Mg2+ is found to promote dispersion of soil particles to some extent, its impact is far less than that of Na+. A stable soil would have a Ca: Mg ratio > 0.5.

 Low exchangeable Na+ content to maintain soil stability.

 Minimum depth of 400mm of potentially unsaturated soil before encountering a restrictive horizon, to permit adequate purification to take place.

 Clay type with Illite and mixed mineralogy soils being the most sensitive to Na+. In general, significant increases in ESP occur in soils with 30 to 40% clay and in the presence of illite clay. Small amounts of smectite clays enhance treatment potential of a soil.

The research outcomes have significantly contributed to the knowledge base on best practice in on-site dispersal area siting and design. The developed predictive site and soil suitability assessment criteria allows more appropriate evaluation of site and soil characteristics for providing long term effluent renovation. This is generally not done in the current assessment techniques for on-site dispersal areas. The processes and techniques used in the site and soil suitability assessment, although based on the common soil types typical of South East Queensland, can be implemented in other regions, provided appropriate soil information is collected or available.

The predictive assessment criteria have been developed at a generic level, allowing easy implementation into most assessment processes. This gives the framework the flexibility to be developed for other areas specifically targeting the most influential on-site dispersal area siting and design factors, and assessment of long term performance under wastewater application.

Impact and interest:

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ID Code: 16297
Item Type: QUT Thesis (PhD by Publication)
Supervisor: Goonetilleke, Sobana& Cox, Malcolm
Keywords: on-site wastewater treatment systems, soil dispersal areas, soil chemistry, multivariate analysis, physical and chemical soil parameters
Department: Faculty of Built Environment and Engineering
Institution: Queensland University of Technology
Copyright Owner: Copyright Lesley Dawes
Deposited On: 03 Dec 2008 14:00
Last Modified: 29 Oct 2011 05:45

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