4.5 Acid sulfate soils

Progress has been made in remediating the areas worst affected by actual acid sulfate soils, though much remains to be done to halt leakage of acid and prevent further degradation.

Virtually all estuaries along the NSW coast have the potential to leak acid and create profound water quality and ecosystem problems if acid sulfate soils are exposed or drained. Acid sulfate soil 'hot spots' have been identified in about half of the estuaries from the Tweed to the Shoalhaven.

Remedial action has been carried out on seven of the worst hot spots but more needs to done. The extent of actual acid sulfate soils is stable or declining, but vigilance is required to prevent the problem growing.

NSW indicators

Introduction

Acid sulfate soils formed naturally along the NSW coastline over the last 10,000 years and are still being formed in mangrove and saltmarsh areas. 'Potential' (not yet exposed to air) acid sulfate soils are usually safe unless dug up or drained. However, the large-scale drainage of coastal floodplains – for flood mitigation, infrastructure and urban expansion, as well as agriculture – has exposed large areas of potential acid sulfate soils to air. This has resulted in the development of 'actual' (highly acidic) acid sulfate soils, as sulfides in the soil are oxidised to sulfuric acid. Acid leachate, plus the aluminium, iron and heavy metals released from these areas, can find their way into waterways and cause significant environmental and economic problems.

The known effects of acid leachate include massive kills of fish and crustaceans, increased susceptibility of fish to diseases such as red spot disease; damage to, or death of, oysters; and the release of heavy metals from contaminated sediments (EPA 2003). Acid sulfate soils can also kill or restrict plant growth (DPI 2006b). They have been responsible for the corrosion and deterioration of iron, steel and concrete structures, such as pipes and bridges. In 2002, acid leachate was estimated to cost the NSW fishing industry up to $23 million per year (DLWC 2002a). It also affects tourism, aquaculture, cropping, grazing, dairying, extractive industries and urban development.

A less well-known and more restricted form of acid sulfate soil occurs in inland environments and is linked to irrigation or land clearing and rising watertables (Fitzpatrick 1993). Salinity and waterlogging in these 'scalded' discharge areas can lead to the mineralisation of sulfur and iron, transforming once productive agricultural soils into locally degraded acid sulfate soils (Fitzpatrick 2002) (not to be confused with induced soil acidity – see Land 4.4).

An emerging issue related to acid sulfate soils in both coastal and inland areas is the presence of monosulfidic black oozes. These highly reactive sediments are often found in the bottom of drains in acid sulfate soil landscapes and can cause rapid and complete deoxygenation of waters when mobilised (Sullivan et al. 2002a). They have been considered a significant factor in the deoxygenation of North Coast rivers, such as the Richmond, contributing to fish kills in these areas in 2001 (Sullivan & Bush 2001). Monosulfidic black oozes have also been discovered in inland drainage systems associated with saline areas, which may have serious implications for inland waters and aquatic ecosystems (Sullivan et al. 2002b).

Current status and trends

The area of the NSW coast potentially affected by acid sulfate soils has not been updated since SoE 2003, and nor has the area affected by actual acid sulfate soils, though there has been a demonstrated improvement in environmental outcomes during the period as a result of remediation programs.

Approximately 400,000 hectares of coastal NSW contained, or was at risk of, acid sulfate soils in 2002 (DLWC 2002b). Of this, over 260,000 hectares was at high risk with about 150,000 hectares of this area under agricultural production. The largest areas were on the coastal floodplains of northern NSW, particularly the Tweed, Richmond, Clarence, Macleay, Hastings, Manning and Hunter rivers. The potential extent of acid sulfate soils and worst-affected problem areas or 'hot spots' are identified in Map 4.6. These priority areas cover approximately 55,000 hectares where land disturbance has created actual acid sulfate soils and resulted in acid drainage and associated problems (DNR 2005).

Map 4.6: Acid sulfate soil priority management areas and areas at risk

Source: Modified from DLWC data 2003; adapted from Naylor et al. 1998; Davies & Mumby 1999; Tulau 1999a; Tulau 1999b; Tulau 1999c; Tulau 1999d; Tulau 1999e; Tulau 1999f; Tulau 1999g

Response to the issue

The NSW Acid Sulfate Soils Management Advisory Committee, which was successful in establishing, overseeing and coordinating activities relating to the management of acid sulfate soils, was disbanded in 2003, with the transfer of responsibility for setting acid sulphate soils priorities and targets to CMAs.

Stage 1 of the Acid Sulfate Soils Hot Spots Remediation Program was completed in 2005. Over $3 million has been spent to reduce the frequency, intensity and duration of acid discharges from seven hot spots in the Tweed, Clarence, Macleay, Hastings, Manning and Shoalhaven catchments. Table 4.2 describes the remediation projects in each catchment under Stage 1.

Table 4.2: Stage 1 of the Acid Sulfate Soil Hot Spots Remediation Program

Source: DNR 2005

Monitoring since 2002 has gauged the success of the remediation programs. Continuous, real-time telemetered water quality data is available for each of the six North Coast hot spots (DNR 2005). This monitoring provides localised assessment, but a statewide assessment has not been carried out. At Broughton Creek in the Shoalhaven estuary, the University of Wollongong has undertaken water quality monitoring in targeted areas and has made all relevant data collected available for use in the hot spots program. Early post-remediation monitoring data, particularly from Partridge Creek near Port Macquarie and from the Broughton Creek hot spot, has indicated that on-ground works, where completed, have successfully contained acid release. Where works have not been completed, the problems remain (DNR 2005). Early consultation with landholders in affected areas is crucial to the success of remediation works.

Local government and industry alike are directly affected by the environmental and economic consequences of acid sulfate soils and have continued to make a significant contribution to containing and remediating affected areas. Both Hastings and Shoalhaven councils for example have made substantial funds available to the hot spot remediation program in their areas (see Table 4.2) (DNR 2005). As a critical component of acid sulfate soils policy, local councils along the coast have generally ensured that their local environmental plans (LEPs) adequately address the likely impact of new development on acid sulfate soils. These measures are believed to have both slowed the rate of construction of new drainage works and improved the environmental performance of those works that have proceeded. Best practice guidelines initiated by industry are continuing to play an important role in preventing further disturbance of acid sulfate soils, and industry groups – sugar, dairy and tea tree – continue to promote responsible development among their members (ASSMAC 1999; NSW Agriculture 2000).

Landholder involvement is another integral component of projects undertaken to rehabilitate coastal acid sulfate soils (DNR 2005). Where there has been a long-term cooperative approach, community projects to address coastal soils have resulted in greater landholder ownership of the problem and greater participation in finding solutions. For example, landholders and other stakeholders on the Macleay floodplain alone have undertaken 17 remediation projects with most of the highest risk soils under active management. The projects cover approximately 9000 hectares (28%) of the floodplain (DNR 2005).

The statewide natural resource management targets relevant to acid sulfate soils are 'By 2015 there is an improvement in soil condition' and 'By 2015 there is an increase in the area of land that is managed within its capability'.

Future directions

A three-tiered approach will continue to be necessary to limit environmental degradation from acid sulfate soils and to prevent potential acid sulfate soils from becoming actual acid sulfate soils:

• minimise the disturbance of potential acid sulfate soils associated with drainage systems – dredging and urban development are significant ongoing risk factors
• encourage further cooperation by local councils administering LEPs, partnerships between CMAs and local government, and further adoption of best practice land management, supported by key industries
• provide extension officers to advise landholders, industry and contractors, as this approach has been demonstrated to be an essential component of any strategy.

Further remediation will be necessary. Less than one-third of the known hot spots were addressed under Stage 1 of the remediation program. It is too early to gauge whether new institutional roles and responsibilities and investment responses by the CMAs will be adequate to address the risks. It should be recognised that decentralised investment will place more pressure on the design and implementation of long-term monitoring and assessment programs and on the sharing of monitoring data, and coordination between CMAs will be important.

Monosulfidic black oozes and inland acid sulfate are emerging issues that need more research and monitoring.