State of the Environment Report 2007
Acidification of wetlands, waterways, groundwater and agricultural drains are known to occur in the South West. Natural acidity may occur in some seasonal wetlands and salt lakes due to the natural seasonal wetting and drying cycle or the gradual build-up of sulfides, iron and other trace metals. However, most natural inland waters are neutral (neither acidic nor alkaline). Acidification may also be caused by lowering of watertables resulting in oxidation of natural sulfides in rocks, soils or iron rich groundwaters. This triggers chemical and microbiological reactions that generate significant amounts of sulfuric acid, which can be extremely damaging to the environment.
Acidified soils are often called 'acid sulfate soils'. In an undisturbed state below the watertable, these soils are benign and not acidic. Acid sulfate soils in WA commonly occur in low lying wetlands, swamps, estuaries, salt marshes and tidal flats, though they are not limited to coastal areas. Some acid sulfate soils have been found in South West agricultural areas, forming in response to rising watertables and land salinisation. Environmental problems begin to arise when soils are drained, excavated or exposed to air by lowering of the watertable. This may occur during dewatering processes, construction of drains, soil excavation, excessive groundwater use, or excessive planting of trees. Under these conditions the sulfide minerals react with oxygen to form sulfuric acid. Often the soil is unable to naturally neutralise the acidity. Water flowing through acidified soil can then produce acidic inland waters. Although rare, acidification may also result from contamination events and runoff from agriculturally-acidified soils (see 'Soil acidification').
Inland waters with increasing acidification can severely damage the environment, resulting in simplified ecosystems and a loss of biodiversity. Acid leaching of soil minerals such as aluminium, iron, nutrients, heavy metals and pesticides can also occur, presenting toxicity problems for plants and animals. Affected waters can become acidic chemical cocktails with potential to cause severe damage to infrastructure, water supplies and the environment.
Acidity is a reflection of hydrogen ion concentration in solution and is measured on the 'pH' scale. This varies from pH 0 (strongly acidic) to pH 14 (strongly alkaline), with pH 7 being neutral. As the pH scale is logarithmic, a fall of one pH unit (say from pH 5 to pH 4) represents a ten-fold increase in acidity. Even a small increase in acidity (i.e. a minor decline in pH) can have serious detrimental effects on biodiversity, especially where there is no history of acidity in an ecosystem (Psenner, 1994).
Most WA waterways are near neutral with a pH in the vicinity of 6.5-8.0. A few acidic waterways are found near Mandurah, Albany and on the Scott Coastal Plain (Figure IW5.1). Some natural acidity occurs due to seasonal wetting and drying of peaty soils in wetlands and waterways. In contrast, other waterways in the western Wheatbelt, the mid-west and Ord irrigation areas are slightly alkaline, their pH ranging from 8.0-8.5 (Figure IW5.1).
![Figure IW5.1: The pH of Western Australian waterways. [Data source: Department of Environment and Conservation [ver. 2005], Analysis: EPA, Presentation: EPA.]](/site/files/images/Figure-IW5.1_Large.jpg)
A snapshot survey of water quality in the Wheatbelt during the winter of 2003 identified very low pH (3.0-5.0) in lakes and waterways of the upper Lockhart and Upper Yilgarn rivers, and some areas of the North and East Mortlock rivers. The high acidity of inland waters of the eastern Wheatbelt reflects a broad problem that may be linked to rising saline watertables that are also acidic. Deep groundwater in the eastern Wheatbelt is considered naturally acidic since there is evidence of some naturally acidic salt lakes and deep groundwater of pH between 3.0-4.5 (Rogers & George, 2005). Agricultural drains were found to be most acidic in the eastern Wheatbelt (east of the line between Dalwallinu and Dumbleyung), with more than half the drains sampled being strongly acidic with an average pH of 3.0. Sampling of deep drains has also shown high levels of heavy metals (including arsenic, iron, aluminium, cobalt, copper, zinc, lead and uranium) that have leached under the acidic conditions.
Coastal groundwater acidity became a prominent issue in Perth in 2002 with the discovery that water from some household bores in the suburb of Stirling was killing garden plants. The groundwater was found to be acidic (with a pH as low as 1.9 in some places) and had become contaminated with aluminium and arsenic released from the soil (Appleyard et al., 2004). Nearby urban wetlands (Spoonbill Lakes) had also acidified. Heavy use of garden bores and dewatering of a nearby wetland for a housing development caused the acidification.
Investigations have also shown that extensive acidification of shallow groundwater is also occurring on the crests of the Gnangara and Jandakot mounds, with pH levels as low as 2.4 and high levels of dissolved aluminium and arsenic present (Appleyard, 2004 & 2005). High aluminium levels can kill plants and wetland aquatic fauna. High arsenic levels pose a risk to human health if the groundwater is used for drinking water, and represents a long-term toxicological problem for ecosystems. Several wetlands on the Gnangara Mound are now permanently acidified. For example, Lake Gnangara has been acidified to a pH of less than 4 since the late 1970s, and Mariginiup and Jandabup lakes have both had temporary acidification events (McHugh, 2004). In other wetlands and waterways the source of acidity is likely to be disturbed acid sulfate soils, which have been detected in wetlands and waterways adjacent to the Swan-Canning, Peel-Harvey, Leschenault and Vasse-Wonnerup estuaries, the Scott Coastal plain and low lying coastal areas on the south coast near Albany (Department of Environment, 2004b).
Acid sulfate soils are often not identified until land is in the process of being redeveloped, when affected vegetation starts to die, or when major infrastructure damage occurs. For this reason the problem is best managed through the identification of potential risk areas. The Department of Environment has been undertaking soil investigations to map the extent of acid sulfate soil risk areas (Figure IW5.2).
![Figure IW5.2: Risk of acid sulfate soils (ASS). [Data source: Department of Environment and Conservation [ver. 2007]; Presentation: EPA.]](/site/files/images/Figure-IW5.2_Large.jpg)
Acidification of ground and surface waters in coastal plain environments is largely a result of poorly planned and managed land use development. In urban areas, increased and more widespread acidification of inland waters is likely with increasing groundwater use, prolonged periods of low rainfall and the continued use of dewatering practices to enable development of wetlands. Lowering of water tables and subsequent drying of wetland soils leads to acidification when soils become wet again. Continued development of wetlands will place nearby areas under significant acidification pressures.
In coastal agricultural areas there are pressures increasing water acidification due to over design of existing drainage networks in combination with continuing declines in annual rainfall. Existing drains, that were designed and installed during a time of high rainfall, are now resulting in the excess lowering of water tables. If disturbed, areas with acid sulfate soils are likely to export acidity to wetlands and waterways.
In agricultural areas, disposal of acid drainage water is posing a significant problem. Fluctuating water tables in the Wheatbelt, caused by altered water regimes, climate variability and management interventions (such as deep drainage and pumping) are increasing the generation of acidic groundwater. Drainage water discharged into waterways and wetlands has the potential to impact on the health of the receiving ecosystem. The risk of downstream impacts may also be much larger when the cumulative effects of many acid drains are added together.
Engineering Evaluation Initiative: is a project that aims to deliver improved engineering options to manage salinity, manage the safe disposal of saline and acidic water, and address regional drainage planning. It has also commissioned the Collaborative Research Centre for Landscape Environments and Mineral Exploration to assess the geochemical risk (acidity and trace metals) posed by saline acid waters in the Wheatbelt. It focuses on waters likely to be discharged by deep drains constructed to mitigate salinity.
Acid sulfate soil mapping: The Department of Environment and Conservation is currently identifying and mapping the extent of acid sulfate soil risk in coastal areas under high pressure from development.
Planning responses: The Western Australian Planning Commission has released a bulletin containing planning measures to ensure that acid sulfate soils are identified, investigated and managed during land development processes (Western Australian Planning Commission, 2003).
Acid sulfate soils framework: A Proposed Framework for Managing Acid Sulfate Soils (Department of Environment, 2004b) has been developed outlining institutional arrangements, monitoring, planning and educational requirements to effectively manage acid sulfate soils. The department has also developed a series of guidance documents to assist in the identification, management and treatment of acid sulfate soils.
Natural Heritage Trust/National Action Plan for Salinity and Water Quality (NHT/NAP) programs: Regional Natural Resource Management groups in the South West, Swan, Avon, South Coast, and Northern Agricultural areas have recognised acidification or acid sulfate soils as a threat or potential threat to inland waters. Many regional groups will address this as part of estuary, waterway and wetland management. The regional groups are developing projects for management of affected waterways and protection of valued natural assets at risk.
Acidification is a growing problem and requires sound land use planning. Acid leachate (including heavy metals, minerals, nutrients and pesticides) can impact groundwater and surface waters causing ecological damage to aquatic and riparian ecosystems; damage to estuarine fisheries and aquaculture activities; contamination of groundwater supplies; reduction in agricultural productivity from acidic conditions and metal contamination (predominantly by aluminium); and damage to infrastructure through corrosion of building foundations, bridge supports, jetties, roads, dams, water pumps and underground pipes. Some impacts caused by subtle changes in acidity over a long period may not be apparent until irreversible changes have already occurred. Economic impacts of acidification of inland waters can occur through loss of economic water supplies, increased water treatment costs and damage to infrastructure. In the Wheatbelt the combined effects of acidification and salinisation in inland waters will rapidly accelerate corrosion of infrastructure, impact water supplies and the loss of biodiversity.
4.17 Review and implement the Proposed Framework for Managing Acid Sulfate Soils, with a view to using a risk-based approach and incorporating effective mitigation and management options.
4.18 Modify the Engineering Evaluation Initiative to incorporate ecological, economic and social risks and management options for managing acid inland waters in the Wheatbelt.
