Coastal river and estuary pollution
Pollution is a longstanding pressure on coastal rivers and estuaries, particularly in areas of urbanisation, industrialisation, mining and agriculture. There remains a legacy of pollution associated with early European history in Australia (Wolanski 2014), and both legacy chemical contamination and contemporary inputs to poorly flushed systems are among the most significant pressures on coastal waterways. Common contaminants found in coastal rivers and estuaries include excess metals, nutrients and organic matter (see Nutrient pollution), and industrial chemicals, pesticides, herbicides, terrigenous sediments and debris (see Marine debris).
Ecological consequences of pollution in coastal rivers and estuaries include loss or change of biodiversity, habitat, ecosystem function and ecological processes (Johnston & Roberts 2009, Johnston et al. 2015). Bioaccumulation of toxicants in the food chain can also affect vertebrate population viability and human health (Hamilton et al. 2016). Most pollution results from historical legacies, ongoing diffuse sources (e.g. agricultural run-off and storm water) and waste management. There is a need for greater understanding of the bioavailability and toxicity of toxicants in both sediments and the water column, and for the development of sensitive biomonitoring tools such as those achieved for estuaries in south-eastern Australia (Edge et al. 2014).
New classes of contaminants such as plastics, cosmetic products and therapeutics are growing concerns (Galloway & Lewis 2016). There is little knowledge about the chemical compounds that many of these emerging contaminants degrade into once in sea water or sediment, and we do not know the full suite of their potential impacts. Detecting new contaminants and forming national-scale assessments are hindered by lack of national-scale data agencies, standardised monitoring programs, and the communication and accessibility of data.
Nationally, pollution pressure on many estuaries is moderate to strong, but both pollution levels and data availability vary greatly among locations. Existing data for most jurisdictions focus on modified estuaries, leaving knowledge gaps regarding effects of pollution in relatively unmodified estuaries (Hallett et al. 2016b, 2016d). In New South Wales, the condition of heavily developed coastal waterways is poor, and nutrients, metals, pesticides and other contaminants are found at high levels in some estuaries (e.g. Sydney Harbour, Port Kembla Harbour; Dafforn et al. 2012). Metal enrichment of sediments was found to be related to population density across 38 central New South Wales rivers and estuaries (Birch et al. 2015a). Between 1999 and 2010, sediment metal concentrations in Sydney Harbour declined in large sections of the upper and central estuary, but slightly increased in the lower estuary following urban and industrial shifts (Birch et al. 2013). Pesticide distributions in Sydney Harbour are linked to stormwater inputs (Birch et al. 2015b).
For estuaries in Queensland and northern New South Wales, major pollution sources are a combination of agriculture, mining, industry and urban land use. Several Queensland and New South Wales rivers and estuaries are moderately to strongly affected by sediments and pesticides. Agriculturally modified estuaries continue to suffer water quality issues, including major fish die-offs because of organic enrichment (see Nutrient pollution), and drainage of floodplains to cause acid sulfate soil run-off. Report card systems are in place for south-east Queensland, Gladstone Harbour, the Great Barrier Reef and Mackay, but monitoring has revealed little improvement in water quality during the past 5 years because of the scale of the problem and the cost of agricultural reform (GBRMPA 2014).
Pesticide contamination in the Great Barrier Reef catchment is widespread, and concentrations of the herbicide diuron exceed guideline trigger values at multiple sites (Smith et al. 2012). Diuron is used in the Queensland sugar cane industry and interferes with the photosynthetic activity of a wide range of organisms (Duke et al. 2005). Current understanding of nontarget effects of toxins is limited, and there is a need for greater knowledge of the risks of toxins in coastal areas to inform guidelines and management. Ideally, management should target the source of inputs, such as the Queensland bylaw that limits the use of diuron by the sugar cane industry.
Pollution in southern states receives less attention than issues related to flow regulation, but it is still a major problem in some areas. A high proportion of South Australian estuaries are in poor condition, and pollution as a legacy of industrial activity, land clearing and mining is common in Tasmania. In Victoria, pollution is not part of the Index of Estuarine Condition program (DEPI 2013), despite being a significant pressure in some estuaries. There are large and obvious impacts of pollution in the southern half of Western Australia, particularly in the south-west (Brearley 2005, Ward et al. 2010).
Much of far northern Australia is relatively unaffected by pollution because development pressure is low. Most of the Northern Territory coastline is unmodified and largely free from contaminants, although Darwin Harbour is an exception (DLRM 2013, 2014). Water quality in Darwin Harbour and its tributaries is generally good, but increased sedimentation and nutrients arise from dredging, sewage discharge, and wet-season stormwater flows (DLRM 2014), and herbicides and metals have been detected at low concentrations (French et al. 2015). In the dry season, the potential for poor water quality increases with water residence time (the length of time a parcel of water resides in a particular area) (Fortune 2010). The Northern Australia Development initiative will present various challenges in the form of pollutants from agricultural and extractive industries.
Improvements have been made in managing point-source pollutants, but diffuse pollutant sources (e.g. agriculture and storm water) make it difficult to reduce pressure in the presence of a growing population and catchment development. Remediation of contaminated sediments is expensive, so is usually only attempted for high-priority issues, such as the removal of dioxins from sediments in Homebush Bay, New South Wales. Management improvements exist or are planned in some states and regions, but lack of research funding and monitoring limits our understanding of their effectiveness. Legacy effects may continue for decades after management begins, which is detrimental to the public and political will required to sustain efforts. Australia has no national program of measurement or monitoring of contaminants in aquatic or terrestrial environments; however, the National Pollution Inventory does consolidate the reporting of emissions of 93 toxic substances from medium and large industries.
The input and environmental impact of pollutants are expected to rise with increasing catchment modification to support growing populations. Pollution is also likely to increase with land reclamation, dredging, waste disposal, agriculture and storm water (including floods and high flows), all of which increase with coastal population and development. Water quality could be improved by better water and sediment quality guidelines, monitoring, risk assessment tools, analytical techniques and measures to address the shortage of management action.