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Surface water salinity
Salinity is a major water quality issue for 24 of the 74 assessed basins (32% of basins). Key areas are the South-West Coast, the South-East Coast and Southern Murray-Darling Drainage Divisions. Land clearing has been a key component of increasing salinity in catchments. High levels of salinity occur in catchments where a large proportion of land has been cleared. For example, 56% of the Frankland River catchment has been cleared and has a high level of salinity at 2,760 mg/L total soluble salts (TSS) (Government of Western Australia 1998). The upper limit for drinking water quality is 800 EC units (less than 1,500 mg/L TSS).
Rivers supply much of Australia's water for crop irrigation. Increased salinity in our ground and surface water will worsen the irrigation salinity problem that many farmers already face. Some environmental impacts of increasing salinisation in our freshwater systems are the change in freshwater habitats and loss of diversity of aquatic life and fringing vegetation.
The Murray-Darling Basin Commission Salinity Audit has predicted that salinity levels in the rivers of the Murray-Darling basin will rise over the next 100 years, with a number of rivers predicted to have salinity levels that exceed the World Health Organization (WHO) standards for drinking water. There is considerable time lag between land use changes and mobilisation of salt loads into rivers and the landscape (MBDC 1999).
Effluent released into inland rivers
The ability of inland waters to maintain environmental values is increasingly threatened by the steady growth in population, urbanisation and the use of catchments for recreational and commercial purposes (ARMCANZ and ANZECC 1997). Effluent released into inland waters contains nutrients, toxic substances, pathogens and dissolved solids. Sewage discharged into inland waters is usually treated to secondary level (removal of 85-95% of biodegradable material by biological oxidation). Some areas have additional sewage treatment by nutrient reduction (tertiary treatment).
Some of the important sources of polluting nutrients are from fertilisers for broad-acre applications; nutrient-rich run-off from rural industries; urban development and wastewater (Government of Western Australia 1998). An estimated 1.1 million tonnes of phosphate fertiliser and 880 thousand tonnes of nitrogen fertiliser were consumed in 1998-99 (ABARE 1999). Nutrients are a major water quality issue in 43 of the 70 assessed river basins in Australia (NLWRA 2001). Areas that exceeded major surface water quality nutrient guidelines are common in the more intensively developed basins of the North-East Coast, Murray-Darling, South-East Coast and South-West Coast Drainage Divisions. The wide range of soil types and vegetation influence the natural nutrient status of surface water. Water quality guidelines are tailored to reflect this variation. Victoria has more river basins that exceed their State nutrient quality guidelines than any other State or Territory (table 14.18). This largely reflects the intensive nutrient monitoring and greater coverage that occurs in Victoria compared to the other States and Territories (NLWRA 2001). The ACT did not record any major exceedances in relation to total phosphorus guidelines at any of the monitoring stations. This could be partly due to the nutrient removal process at the sewage treatment plant, which removes 98% of phosphorus before the effluent is returned to the river system (Smith 1998).
14.18 MAJOR EXCEEDANCES OF NUTRIENT GUIDELINES, By State(a)
The addition of nutrients to freshwater rivers can increase the likelihood of algal blooms, although it is dependent on the nature of the receiving waters and climatic conditions. The cost of algal blooms in the late 1990s was estimated at $180-240m per year (LWRRDC 1999).
Ground water salinity and sustainability
Ground water is a vital water resource in Australia. Ground water underlies 60% of Australia (5,226,440 square kilometres). Around 70% of Australia's readily accessible ground water resources are suitable for human consumption and crop irrigation (less than 1,500 mg/L of total soluble salts (TSS)) (graph 14.19). Ground water flows can substantially contribute to the risk of areas developing salinity. The time it takes for agricultural development to contribute to salinity problems is also influenced by ground water flows (NLWRA 2000).
Clearing of native vegetation and prolonged irrigation have caused ground water levels to rise and naturally occurring salt loads to come to the surface. This affects plant growth and results in excess salt loads to rivers via loose top soil and direct seepage into river systems (MDBC 1999).
The largest store of salt has accumulated in the sedimentary Murray Groundwater Basin, with ground water stores often approaching sea water salinity (35,000 mg/L TSS). The Murray-Darling has a limited sub-surface outlet to the sea. This has resulted in a massive store of salt that has continued to be mobilised through agricultural and land development practices (MDBC 1999).