4.1 Salinisation of inland waters

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Priority Rating: 1

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Description

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Salinisation is the process that increases dissolved salts in inland waters. It occurs naturally in arid parts of WA where salt lakes exist. Native flora and fauna have gradually adapted to these naturally saline ecosystems. Problems arise when previously unaffected land and water become salinised, and associated flora and fauna are unable to cope.

Salinisation of land and water is largely due to widespread land clearing and replacement of native vegetation with shallow rooted annual crops and pastures that use less water. This alters the natural water balance of the land. Rainfall not used by vegetation seeps into the soil, passes the root zone and adds to groundwater stores. Over time, groundwater stores increase at a much faster rate than would otherwise occur naturally, causing watertables to rise and bringing salt stored in the soil to the surface. This is expressed as saline seeps and salt scalds, which allow for rapid runoff of salty water to nearby streams, rivers and wetlands. Salt is also transported by groundwater flow although this process occurs at a much slower rate. For this reason it may take up to several decades for waterways and wetlands to become salinised following widespread clearing of vegetation. Salinisation of inland waters typically affects the South West agricultural parts of the State, where dryland salinisation occurs.

Objectives

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The State Salinity Strategy (Government of Western Australia, 2000) lists specific goals for managing the impact of land salinisation in the South West agricultural region. Objectives relevant to inland waters include:

• To protect and restore key water resources to ensure salinity levels are kept to a level that permits safe, potable water supplies in perpetuity;
• To protect and restore high value wetlands, waterways and natural vegetation, and maintain natural (biological and physical) diversity in areas affected by salinisation.

Condition

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Indicator IW4: Status and trends in salinity of inland waters.

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River salinity shows a clear pattern through the South West (Figure IW1.1). Waterways originating in high rainfall (>900 millimetres) coastal areas and forests are generally fresh, with very little or no salt. Further from the coast rainfall decreases, a greater proportion of land has been cleared, and waterways have become more saline. Some of the larger waterways that originate in cleared inland areas (such as the Avon, Frankland, Blackwood, Kent and Murray rivers) are severely impacted in upper reaches and remain brackish to saline approaching the coast. Waterways and wetlands in the eastern Wheatbelt, east of the Meckering Line, have salinities that are about three to six times higher than those in the western Wheatbelt. Much of the salinity in eastern Wheatbelt inland waters is natural and salinity levels are increasing more slowly than those to the west (Commander et al., 2002).

Figure IW1.1: Salinity in rivers of the South West.

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Data source: Department of Environment - Stream salinity condition [ver. 2004], water quality sites [ver. 2005], Department of Agriculture - remnant vegetation [ver. 1996]. Analysis: EPA - water quality sites; Presentation: EPA.

Table IW1.1: Salinity trends for South West rivers for which long-term monitoring data are available.

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Data source: Mayer, Ruprecht & Bari (2005). Note: a) Trends are statistically significant at 95% confidence limit except for those denoted with (*) which are not statistically significant.

Collectively South West waterways discharge about 4700 gigalitres (GL) of water and 7.5 million tonnes of salt each year. Of the total volume discharged about 44% of flow is fresh, 10% is of marginal quality, 21% is brackish and 25% is saline (Mayer, Ruprecht & Bari, 2005). Many waterways have become progressively more salty over time. The salinity of the Avon River in the Wheatbelt, for example, is estimated to be 20-40 times pre-clearing levels (Schofield, Ruprecht & Loh, 1988), and the Blackwood River, which was once fresh, now exports one million tonnes of salt each year (Figure IW1.2; Mayer, Ruprecht & Bari, 2005). Predictions of future trends have shown that, without any remedial action, salinities in five major South West rivers may increase by 15-65% above current levels (National Dryland Salinity Program, 1997).

Figure IW1.2: Salinity in the Blackwood River, 1940-2002.

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Data source: Mayer, Ruprecht & Bari (2005). Note: mg/L TDS, a measure of salinity, represents milligrams per litre of total dissolved salts; 'flow-weighted' means the data is adjusted for the effects of flow.

Detecting long-term salinity trends in waterways requires dedicated monitoring programs over many years (Table IW1.1). The number of salinity sites being monitored across WA has reduced by about one-third over the past 5 years due to funding constraints. Determining salinity levels in many waterways now relies more on infrequent sampling, which is not suitable for detecting long-term trends. The exceptions are those rivers within water supply catchments and water resource recovery catchments that have monitoring sites continuously measuring salinity levels over time. These are limited and do not reflect the true spatial extent of the problem.

Irrigation salinity is already a problem in some irrigated areas with poorly drained soils and shallow, naturally saline groundwater. About 20% of the southern Swan Coastal Plain, from Gingin to Dunsborough, is considered to be at risk of irrigation salinity (Government of Western Australia, 2000). Salt scalds are already evident in some areas of the Harvey River catchment (Mayer, Ruprecht & Bari, 2005). Irrigation salinity is also a risk in the Ord River Irrigation Area with groundwater salinity levels being elevated in some places. Dewatering bores are now required in some irrigated areas to reduce the risk of a rising saline groundwater table.

Salinisation of Wheatbelt wetlands is less well-documented, but there is enough anecdotal and scientific evidence to indicate it is severe and widespread (Lane & Munro, 1983; Sanders, 1991; Lane et al., 2004). For example, salinity levels in Lake Bryde appear to have increased 10-fold between 1981-94. Many other wetlands that were once fresh enough to support native vegetation prior to salinisation are now salt lakes supporting minimal vegetation (Halse, Pearson & Patrick, 1993). Substantial loss of fringing vegetation and waterbird populations is now evident at Coyrecup, Coomelberrup, Walymouring, Eganu, Dumbleyung and Parkeyerring wetlands (S Halse, Department of Conservation and Land Management, pers. comm.).

Long-term monitoring of wetlands in the conservation estate shows that 12% increased in salinity between 1977-2000 (Lane et al., 2004). Without successful management activity, other valuable wetlands (including Fraser's, Noobijup, Yaalup, Wheatfield and Kulicup wetlands) are expected to become salt affected within 10 years, leading to substantial declines in biodiversity (S Halse, Department of Conservation and Land Management, pers. comm.).

Superficial groundwater salinity is highly variable throughout the State and most is naturally brackish to saline (Figure IW1.3). Pockets of superficial fresh groundwater are found along the Swan Coastal Plain, the South West corner of the State and parts of the Pilbara and Kimberley. Deeper groundwater is less well understood. Groundwater salinity tends to be stable, except where catchment hydrology has been altered or high rates of water extraction occur (see 'Altered water regimes' and 'Water supply').

Figure IW1.3: Groundwater salinity of superficial aquifers across Western Australia.

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Data source: Department of Water (2000); Presentation: EPA.

Pressures

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The extent of salt-affected land has a direct relationship to the extent of inland waters affected by salinity (see 'Land salinisation'). Historical clearing of perennial native vegetation and increased use of deep drainage are the most long-term and severe pressures for salinisation of inland waters in the South West. For example, 93% of native vegetation has been cleared in the Avon-Wheatbelt bioregion, resulting in a significant land salinisation problem and salinised wetlands and waterways (Department of Environment, 2004a). The increasing extent of land being impacted by salinity will eventually result in more waterways and wetlands becoming impacted.

Analysis of 3000 bore sites in the South West agricultural region showed no significant trend to decreasing salinity, with most showing rising or stable groundwater salinities. Modelling indicates about 16% of land (4.65 million hectares) in this region had shallow watertables in 2000 (Department of Agriculture, 2001). Assuming groundwater trends remain steady, it is projected that 20% (6.37 million hectares) of the region will have shallow water tables by 2020, and 33% (13.66 million hectares) by 2050. These figures were calculated on the basis that watertables were less than 2 m from the soil surface or between 2-5 m and rising.

In contrast, there is recent evidence to suggest that the drying climate in the South West has lowered local groundwater levels in some places. A decrease in winter rainfall and general lack of 'wet winters' is likely to have slowed the rate of salinisation for some wetlands and waterways, especially for central and eastern Wheatbelt areas. Some areas cleared a long time ago appear to be reaching groundwater equilibrium, while the trend of rising groundwater continues in recently cleared areas, such as high rainfall parts of the Wheatbelt and South Coast (Indian Ocean Climate Initiative, 2005b).

Deep drains and associated engineering works are increasingly seen by many farmers as a viable option to manage salinity and enhance productivity on their land. It has been reported that deep drains can lower the watertable up to 500 m from the drain, but this varies considerably depending on soil type and landscape characteristics. Some farm deep drains currently discharge saline water into downstream waterways and wetlands. Recent surveys (Department of Agriculture, 2006) indicate that about 27% of farmers in the South West are using deep drainage. It has been estimated that there are 14 000 kilometres (km) of agricultural drains in the South West agricultural region in 2007, with about 1000 km currently being constructed every year (extrapolated from Dogramaci & Degens, 2003).

The attraction of using deep drainage as a quick fix for agricultural production may have unintended long-term consequences for the environment. Drainage water discharged into natural waterways and wetlands has the potential to severely impact on the health of the receiving ecosystem through addition of water, salt, nutrients, sediment, heavy metals and acidity. The risk of downstream impacts may also be much larger through the cumulative effects of many deep drains. There are widely differing opinions about the efficiency of deep drainage; however, the environmental impacts are real and deep drainage remains subject to environmental harm provisions under the Environmental Protection Act 1986 and may result in prosecution.

Current responses

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State Salinity Strategy: was released in 2000 by the State Salinity Council, whose aim was to reduce and manage salinity in the South West agricultural region. It promoted a partnership-based approach, development of prioritisation tools, improved protection and conservation mechanisms, further research and development, and ways to help the farming community move to more sustainable production systems. In response to the strategy, the State Government formed the Natural Resource Management Council, initiated a prioritisation methodology (known as the Salinity Investment Framework), and promoted a stronger strategic role for natural resource management regional groups in managing salinity.

Natural Heritage Trust/National Action Plan for Salinity and Water Quality (NHT/NAP) programs: These two Commonwealth programs aim to ensure on-ground environmental improvements occur via a targeted strategic approach at the regional level. Natural resource management groups in the South West, South Coast, Swan, Avon and Northern Agricultural areas have recognised salinisation of inland waters as a threat to natural resources and have projects to manage and protect valued natural assets considered at risk. Irrigation salinity has been recognised by the Rangelands regional group as a threat in the Ord River catchment.

Engineering Evaluation Initiative: is a State Government project to examine a range of engineering options to mitigate land salinisation. These include deep drains, groundwater pumping, diversion and surface water management. It is also investigating options for safe saline water disposal and regional drainage planning. The project is evaluating deep drains at Morawa, Beacon, Pithara, and Dumbleyung.

Wheatbelt Drainage Council: has been recently established to provide policy and assessment advice to the State Government on the most appropriate and accountable approaches to deep drainage planning, implementation and management. The Council is in the process of developing a framework to evaluate deep drainage proposals.

Agriculture extension program: The Department of Agriculture and Food promotes application of best practice by farmers to mitigate and prevent land salinisation. Recent surveys (Department of Agriculture, 2006) indicate that 35% of Wheatbelt farmers undertake regular monitoring of the watertable. The percentage of Wheatbelt farmers managing water on valley floors using surface drains (49%) and deep drains (27%) appears to have increased in 2006. Interestingly, the percentage of farmers using deep drains in higher rainfall parts of the South West more than doubled in 2006 to 27%.

Research: Several organisations are currently undertaking research on the salinisation of inland waterways and wetlands. Murdoch University has recently completed projects that improve understanding of the effects of increased salinisation on inland aquatic systems. Another project is investigating Wheatbelt deep drains and whether they are an effective strategy for alleviating salinised areas of the Wheatbelt. The CSIRO Water for a Healthy Country program is undertaking research on evaluating Wheatbelt drainage and developing models to help assess drainage plans.

Recovery catchment programs (water, biodiversity): These programs focus on the stabilisation and recovery of important natural resource assets already impacted by salinity. In relation to water resources, action is currently being taken in the Collie, Denmark, Warren, Kent and Helena rivers to stabilise and reduce salt levels flowing into dams. Recent analysis indicates that land clearing controls and reforestation in the Collie and Denmark river catchments have halted the increasing trend in salinity (Figures IW1.4 & IW1.5). The Denmark River represents a breakthrough in salinity management because it is the first time in Australia that river salinity levels have been reversed due to planting of trees, community and government actions, and application of engineering solutions (Bari et al., 2004).

In relation to biodiversity, the wetlands of Buntine-Marchagee, Drummond, Lake Bryde, Muir-Unicup and Lake Warden complexes and Lake Toolibin are receiving attention to protect and recover their unique conservation values. An estimated 80% reduction in salt load entering Lake Toolibin has occurred using combined surface water diversion and groundwater pumping, thereby preventing damage to valued flora, fauna and habitat (Wallace, 2002).

Figure IW1.4: Salinity levels and average trend in the Denmark River since 1955, and projected trends under various management options.

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Data source: Bari et al. (2004). Note: Salinity is depicted in milligrams per litre (mg/L).

Figure IW1.5: Salinity levels and trend in the Collie River since 1952, and projected trends based on various management options.

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Data source: Maugher et al. (2001). Note: Salinity is depicted in milligrams per litre (mg/L).

Implications

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Excessive salinisation of inland waters results in a catastrophic collapse of aquatic ecosystems. Not only does waterway and wetland vegetation die because of rising salinity, but habitat is lost and plant and animal populations decline (Cale, Halse & Walker, 2004). Many plant and animal species in salt-affected parts of the Wheatbelt are at risk of extinction. For example, many areas of remnant native vegetation in the Wheatbelt (perhaps 80% on private land and 50% on public land) are at risk of death from rising saline water tables. Estimates suggest that 450 species of vascular plants are at risk of extinction. Salinisation of Wheatbelt wetlands has resulted in a 50% decrease in the number of waterbird species utilising them (Government of Western Australia, 2000). Salinisation exacerbates soil erosion processes leading to filling of river pools and wetlands and, along with altered water regimes, can also lead to an increased risk of flooding. Although flood risk studies are limited in salinised catchments (Bowman & Ruprecht, 2000), there are estimates that a two- to four-fold increase in shallow saline water tables may lead to a doubling in flood flows (George et al., 1999).

Removal of saline water using deep drainage is proving an extremely contentious issue in the Wheatbelt. There is a perceived lack of government direction, consistency and unity on the matter, and this is creating a sense of community frustration and confusion. Further direction and regulation is urgently required for deep drainage. Economic costs are also incurred because infrastructure such as bridge supports, buildings, drains, dams, weirs and pumping equipment is corroded by saline water. Many agricultural towns have been historically built in low-lying parts of catchments or floodplains, and are therefore susceptible to rising saline groundwater tables. The durability of sealed roads may be reduced by up to 75% in areas with rising saline groundwater. More than one-third (36%) of the South West's dams (domestic, agricultural or industrial) have become brackish or saline and a further 16% are of marginal quality (Government of Western Australia, 2000). To compensate for the loss of water supplies, development of additional water storages or extraction must occur, at a significant financial cost to the community.

Suggested responses

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4.5 Develop and implement a strategic policy and governance framework for agricultural deep drainage which fully incorporates environmental principles.

4.6 Develop and implement an evaluation framework to consider deep drainage in the context of integrated solutions where such options are environmentally sound.

4.7 Enhance routine monitoring of waterways and wetlands currently impacted by, and at future risk of, salinisation.

See also 'Land salinisation':

• Continue to implement the recommendations of the State Salinity Strategy, recognising it as a component of the proposed State Soil Protection Policy. Although significant progress has been made, more focus should be given to integrating engineering and plant based solutions to cater for a variety of farm and catchment characteristics, and promoting alternative farming systems in low to moderate rainfall areas.
• Continue the Land Monitor program to provide five-yearly updates of the extent of land salinisation in the South West.  Land Monitor remains one of the few tools to accurately measure regional trends in land salinisation and to evaluate the effectiveness of actions.