The impact of wastewater irrigation on soils in the ACT
Date
2011
Authors
Puttyfoot, Bronwyn Louise Yvonne
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Degrading catchments and water shortages are the most immediate and arguably the biggest environmental issues affecting the world today. In recent years, wastewater irrigation has gained popular support as a substitute to irrigation of potable water. The use of wastewater for irrigation is considered beneficial recycling of a limited resource; in this case freshwater within Australia. Although it has economic, environmental and social benefits, it is sometimes questioned as an environmentally sound method, because wastewater irrigation has been shown to change the chemical, physical and biological properties of soil. Soil is an excellent medium for the removal of some contaminants in wastewater. However, there is a lack of in-depth knowledge and the need for further research into the influence of site conditions and climate which may inhibit the performance of the wastewater irrigation scheme. This research is focused on determining the impacts of wastewater irrigation on the chemical and physical properties of the soils at selected sites within the Australian Capital Territory. Specifically, this study of wastewater irrigation of urban greenspaces sets out to (a) determine the differences in chemical and physical properties of the irrigated soils to nearby control sites for a single time slice, and (b) evaluate any changes that occurred over a six-year monitoring period. Temporal data analysis for a six-year monitoring period of Southwell Park and North Canberra Water Reuse Scheme revealed some broadscale trends in the chemical and physical properties of wastewater-irrigated soils. Generally, the chemical properties pH, electrical conductivity (EC), total dissolved solids (TDS), and nutrients have increased and the physical property hydraulic conductivity has decreased over the monitoring period. Non-wastewater-irrigated (control) soils were used to provide background concentrations to assess the extent of change as a result of wastewater irrigation. This study found that soil pH, total phosphorus (TP), exchangeable sodium percentage (ESP) and sodium significantly increased from application of wastewater. Soil hydraulic conductivity and permeability were significantly reduced by the application of wastewater. The application of wastewater to land increased soil pH, EC, chloride, organic carbon, total nitrogen (TN), TP, nitrate, total oxidised nitrogen (NOx), bulk density, ESP, sodium, calcium and sodium adsorption ratio (SAR) and reduced the levels of nitrite, hydraulic conductivity, soil permeability, magnesium, aluminium and P sorption. Ammonia, cation exchange capacity (CEC) and potassium showed no variation between the sites irrigated with wastewater and the control sites. This research found that responses to the soil chemical and physical properties occurred, some of which could be tentatively attributed to wastewater irrigation, but other factors were also contributing. The influence of climate, contrasting ratios of wastewater to potable water used for irrigation, and other management strategies were important factors that emerged during this research. These factors not only represented important constraints but also clarified the specific role that wastewater irrigation played in defining soil characteristics. Climate was a dominant factor in defining soil chemical and physical properties. Under high evaporation, if there were going to be negative impacts imposed by wastewater irrigation, then this is when they would be most pronounced. The responses under wastewater irrigation are independent to the ratio of wastewater to potable water used for irrigation. The responses observed at the Belconnen Magpies Golf Course, which uses 100% wastewater for irrigation, were also observed at the other study sites. Because wastewater irrigation is not the dominant factor controlling soil properties, additional land-management strategies are playing a crucial role. Those strategies occurring pene-contemporaneously with wastewater irrigation include: the use of sites for recreation with consequent impacts on soil compaction and hydraulic conductivity, and the application of a range of agri-chemicals to optimise soil fertility and plant growth. Overall, it can be concluded that the wastewater irrigation is not the dominant factor controlling the soil chemical and physical properties. Although the soils in the study area have experienced physical and chemical changes following wastewater irrigation, the soils have so far retained most of their functions. Land application of wastewater at the study sites has been shown to cause no negative effects on the environment. Therefore, the wastewater irrigation on soils within the Australian Capital Territory is sustainable and does not adversely affect the chemical and physical properties of the soils.
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