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University of Melbourne (2011)

Management of Sodic soils under reclaimed water irrigation

Muyen, Zahida

Titre : Management of Sodic soils under reclaimed water irrigation

Auteur : Muyen, Zahida

Grade : Doctor of Philosophy (PhD) 2011

Université de soutenance : University of Melbourne

Land application of raw sewage or partially treated wastewater has been practiced widely around the world over the past few decades. But regulatory authorities, concerned about water quality, environment and public health, are restricting the uncontrolled and direct disposal of these wastes into water bodies. However, current management practices related to wastewater and/or reclaimed water irrigation are mainly targeted at volumetric application rates and controlling nutrient levels and pathogens. There has been limited effort in understanding the long-term consequences on soil physical properties caused by cations, particularly sodium (Na), which is dominant in many of the wastewaters and reclaimed waters applied to land. Sodium is also the dominant cation in soil profiles of Australia and many other parts of the world. Leaching these soil profiles by irrigation water or rain leads to the formation of sodic soils. Once formed, irrigating these already salt-affected soils also results in the build-up of Na salts, leading to the formation of saline-sodic soils. This is also true in the choice of irrigation water quality, frequency and overall management. Consequently, irrigation management in these soil profiles is intrinsically linked to the management of soil sodicity. With increasing percentage of arable land being salt-affected or increasing percentage of salt-affected soils having to be brought under cultivation, proper management of these soils are vitally important in order to maintain agricultural productivity. The Western Treatment Plant (WTP) at Werribee, Victoria, Australia, had been employed to treat sewage/wastewater for over a hundred years (since the 1890s). Partially treated sewage/wastewater was used to irrigate pastures and to graze cattle and sheep. In 2003 the application of wastewater ceased and reclaimed water irrigation commenced. In response to climate change and drought induced reclaimed water shortage further land and water use changes are proposed. These are likely to consist of an increasing amount of rotational cropping and a decline in animal production. Of key interest is the fate of soil organic matter (SOM) and its role in these changes and associated sodicity impacts. As land and water use at WTP has been managed and monitored for 100 years it provides a useful study site to ascertain the amount and value of OM in this landscape. For example, the primary production system supporting direct land application of wastewater resulted in the formation of a thick sludge/Organic Matter (OM) mat. An estimate of the contribution of wastewater to this mat gives an idea as to how significant the impact of reclaimed water irrigation practice at WTP would be. With the change in irrigation water quality at WTP, the aim of this study was to investigate the changes it would bring to the soil properties of these salt-affected soils. One key change was the reduction in soil organic carbon (OC) contents since the cessation of wastewater irrigation. This reduction had led to the search for an alternative OM source. As a result the possibility of using wet and dry biosolids had been explored together with the impact of different quality irrigation water. Controlled laboratory experiments had been carried out with samples of top-and subsoils collected from designated parts of the WTP. The laboratory experiments comprised of 36 weeks of actual treatment (in terms of irrigation and biosolids application) and analysis. At the end of this study it was concluded that irrigating these salt-affected soils with reclaimed water, while depending on rain fed irrigation during certain months, was still a good option for these soils as long as there are OM inputs to these soils to compensate for the previous input of OM coming from wastewater irrigation. The dosage for such inputs had also been identified. Given that the on-site biosolids are to be used, the volume of generated biosolids and possible application rates had also been checked to determine the feasibility of the suggestions made. Since heavy metal concentrations in biosolids are an important concern, these application rates had been checked against other application rates with similar heavy metal concentrations in other studies in Australia. However, the current study did not measure phosphorus and/or any metals for the application rates suggested and further study can be carried out before finalising such application rates for the WTP soils. Some of the trends in soil properties could not be established beyond reasonable doubt or with significant statistical validity. Running field experiments and for longer periods would be a possibility which could not be done in this particular study due to limited resources.

Subjects : Western Treatment Plant (Werribee, Vic.) ; Sodic soils ; management - reclaimed water.


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