Nature, culture and history

History

Trinity Inlet was once the valley of the Mulgrave River when the sea level was lower than today. Over time, land uplift diverted the mouth of the river south to its present location at Russell Heads.

As sea levels rose and then retreated over the past 10,000 years, iron sulfides developed along the moving coastline. As long as these were covered with water, they caused no problems. In the 1960s the area now known as East Trinity was a natural wetland covered by mangroves and samphire flats with melaleucas on the freshwater edges. Its creeks—Hills, Firewood and Magazine—were considered good fishing areas.

In the 1970s Colonial Sugar Refining Company (CSR) developed the East Trinity site for growing sugar cane. A rock levee—the bund wall—was built around the entire site to exclude salt water. Tidal gates were included on Hills and Firewood creeks to allow water to leave the site (but not to enter) and drainage systems were constructed to lower the water table and remove the salt.

Unfortunately, as the waterlogged soil dried out and was exposed to oxygen in the air, the naturally occurring iron sulfides in the acid sulfate soils (ASS) oxidised to produce sulfuric acid (similar to battery acid) and heavy metals, and the sugar cane failed to thrive. After rain, the acid and heavy metals were washed into the creeks causing fish kills. Eventually cane farming was abandoned.

The property was subsequently sold. Plans were developed, some of which included the development of a major resort complex and urban centre for up to 20,000 people. Lacking approvals, the property passed into the hands of various mortgagees.

The Queensland Government purchased the 940 ha East Trinity site in May 2000 to protect the Cairns Scenic Rim from development. The reserve is now managed by QPWS.

What are ASS?

ASS are part of the world's natural sulfur cycle. The ASS most commonly disturbed by coastal development were formed within the past 10,000 years, during and after the last major sea level rise which followed the melting of the massive polar ice caps, after the last Ice Age. They are created when salty water mixes with sediments containing organic matter, usually from mangroves or other coastal vegetation. Soils or sediments that contain iron sulfide minerals are known as acid sulfate soils.

These ASS are not a problem while they remain where they were formed—beneath the water table. However, when sulfides are exposed to the oxygen in air, sulfuric acid starts to form. This acidifies soil water, groundwater and, eventually, drainage waters and streams.

As it moves through the soil, the acid strips iron, aluminium, arsenic and any heavy metals from the soil, creating an acidic brew. When this remains in the soil it can be toxic to plants and soil organisms, and when flushed into waterways it can be lethal to aquatic life, particularly fish. In this way ASS can have devastating impacts on aquatic ecosystems. ASS can also pollute groundwater and corrode concrete and steel structures such as water pipes.

East Trinity ASS

Ninety per cent of East Trinity Reserve lies below 2 m above mean sea level and virtually all of this area contains at least some ASS. Being so low, much of the site is subject to flooding during cyclones and heavy rain, which restrict future land use options. Another feature of ASS is that when the soils dry they shrink and don't swell again when re-wetted. This means that much of the East Trinity Reserve is now lower than it was previously.

Remediation of ASS

Conventional remediation of ASS with lime would have cost hundreds of millions of dollars and required removal of almost all the vegetation at East Trinity. The department devised an innovative acid sulfate remediation plan involving controlled, lime-assisted tidal exchange and wetland creation. The plans were publicly supported by both the Great Barrier Reef Marine Park Authority and the Commonwealth Department of Environment and Energy (DoEE).

Controlled, lime-assisted tidal exchange uses tidal flushing to push water that has been treated with hydrated lime into areas that are acidic. The tide pushes water into the site through open floodgates on the incoming tide. The number of gates open is restricted so that the water inside the site only reaches a set maximum level. As the tide drops outside, the floodgates swing open and release water from inside the site. The hydrated lime is added before the tide pushes across acidified soil, and also as it returns to the creeks when the tide falls. This minimises acid and heavy metals leaving the reserve and entering Trinity Inlet.

More elevated areas of ASS will be treated using conventional methods such as the direct application of agricultural lime. This will ensure neighbouring properties are not affected by tidal waters.

The effectiveness of this acid discharge management is tested by measuring the pH of the water leaving the site. The DoEE has set a minimum discharge target pH of six and data from Hills Creek, where this process has been operating for about three years, shows that for 2005 the discharge water is meeting or bettering that target. The exception to this is when large amounts of rainwater, that naturally have a pH less than six, flow down the creek.

Ongoing tasks

QPWS has taken on the tasks associated with ongoing maintenance of East Trinity Reserve. These include the following activities:

• The 35-year-old bund wall, which plays an important role in the controlled reintroduction of tidal water to the site, is maintained and stabilised for protection from strong tidal flows. The road, floodgates, trash racks, culverts and bridges are also repaired and maintained as the concrete has been weakened by acid and salt. Access, tidal movement and drainage are also improved by the maintenance of internal roads, especially after the wet season.
• Pests are an ongoing problem at East Trinity, notably wild dogs and pigs. The latter have been well controlled with persistent trapping and removal, and damage has been greatly reduced.
• As any gardener knows, you just need to turn your back and the weeds take over! Sickle pod and pond apple are among the worst offenders at East Trinity, along with guava, African tulip trees and others. Sickle pod is sprayed and slashed but much of the pond apple cannot be sprayed due to its fondness for taking over wetland areas; weed spray should not be used near water. Techniques to remove pond apple include basal spraying, cutting and applying herbicide to the stumps and hand pulling but since a healthy population of estuarine crocodiles resides in the same locality, this is not always easy! Guava and African tulip are being progressively removed.
• Weeds are also managed with fire. Blocks are burned in rotation, in the cooler months, to produce a mosaic of burned and unburnt areas. While preventing wildfires, these controlled burns also destroy guava and reduce the numbers of pond apple seedlings. It also encourages natural regeneration, benefiting native grasses growing on the sand ridges. The tall, permanent melaleuca and cycad areas need fire every 10 years or so.
• QPWS also conducts crocodile monitoring at East Trinity Reserve, with periodic spotlighting counts. Several crocodiles have been seen in Firewood and Hills creeks. Hatchlings were also seen; East Trinity Reserve is a prime breeding site for these ancient reptiles.

The future

Remediation works at East Trinity Reserve will continue until the site is self-sufficient and stable. Tourism opportunities will be investigated in line with guarantees to maintain the green backdrop to Cairns.

Natural environment

Mangroves growing back

A 2006 vegetation survey of East Trinity Reserve has found many positive changes since the 2002 survey.

When salt water was excluded from the site 35 years ago, melaleuca trees—mostly broad-leaved tea-trees (Melaleuca leucadendra)—replaced the original mangrove communities. The incursion of salt water, associated with the remediation works, is reversing that process. Although the invading melaleucas are a tough species, they have died in areas where the reintroduction of tidal water has caused an increase in salinity. Life, however, is returning and the 2006 survey found that several mangrove species, including Avicennia marina and Excoecaria agallocha, are reinvading areas of former melaleuca forests. Mangrove ferns (Acrostichum aureum) are forming a dense ground cover on the margins of affected communities and are rapidly colonising bare areas.

The dynamics of mangrove communities are also changing. The increased tidal inundation has resulted in successional changes to the mangrove species as they react to changed water depths. In the northern section, black mangroves (Lumnitzera racemosa) have been dying off but other mangrove species are moving in to take over.

Not all melaleucas at the reserve have died, however. Natural areas of melaleuca and feather palm forest, situated at the margins of the reserve, were unaffected when the land was developed for cane growing and remain healthy. The regrowth melaleucas that occupy the margins of the tidally affected area are also flourishing.

When drained for sugar cane production, some areas were invaded by non-native grasses. These grasses have now been killed by the salt water and mangrove ferns along with salt-tolerant sedges and mangrove shrubs (Sonneratia alba, Excoecaria agallocha, Acanthus ilicifolius) have recolonised.

Seasonal wetland communities are now in much better condition than they were in 2002, providing excellent habitat for waterbirds. There are thick covers of wetland sedges such as bulkuru or water chestnut (Eleocharis dulcis)—a very important food source for magpie geese—and Schoenoplectus litoralis. The improved condition can be attributed largely to feral animal control; the evidence of soil churning by feral pigs in these communities is limited. QPWS continues to monitor numbers of feral animals and run control programs.

The department acknowledges David and Peter Stanton for providing this information.

Fish flourishing

When the bund wall was built around the site by CSR in the 1970s, tidal gates were included where creeks passed through it. These allowed water to leave the site, while preventing tidal salt water from moving back upstream.

Water quality inside the wall became so contaminated with acid and heavy metals that very few fish could survive and, each year, following pre-wet season storms, this contaminated water would flow out of the site through the tidal gates, also causing fish kills downstream of the wall.

The water quality in the creeks at East Trinity has dramatically improved since the rehabilitation work began. Proof of this is the discovery in the latest survey, of 38 species of estuarine and marine fish, including important angling species such as barramundi (Lates calcarifer) and mangrove jack (Lutjanus argentimaculatu ), in the previously barren creeks. They also hold healthy stocks of juvenile mud crabs (Scylla sp.).

As mangroves recolonise the creek banks and lower areas of East Trinity, fish nursery habitats are extended, promising increased stocks of healthy fish and happy fishers!

The department acknowledges John Russell from DAF for providing this information.

Wildlife moving in

Common tree snakes (Dendrelaphis punctulata) are not always green. They vary in colour from black to yellow with shades of green in between. Some specimens are even bright blue and these have been spotted on East Trinity Reserve.

Many large water pythons (Liasis mackloti) inhabit the reserve. These snakes are found across the top of Australia, sometimes in very large numbers. They do not have the usual patterned markings of other pythons but are a plain colour with a beautiful sheen in sunlight. They are non-venomous and kill their prey by constriction. Their favourite food is rats.

Ninety-six different bird species have been recorded on the reserve by local birdwatchers. Several breeding pairs of black-necked storks (Ephippiorhynchus asiaticus), also known as jabirus, have been seen at East Trinity. These conspicuous fish-eating birds have probably been attracted to the area by the resurgence of fish populations.