Summary
Oyster farmers on the Macleay River in NSW have learned to live with acid pulses after rain. What no one has noticed is what else might be moving through the water with them.
Steve Macarthur has worked the oyster leases on the lower Macleay River in NSW long enough to recognise when the river is behaving differently. When the water turns the colour of weak tea, he doesn’t need a test kit to tell him conditions have changed.
What the oyster farmer is seeing has a long history, and it begins upstream. A 2025 UNSW Water Research Laboratory analysis of the Clybucca Wetlands — 18 kilometres upstream of the ocean entrance, and prepared for the NSW Department of Primary Industries and Regional Development — found that one event between April and July 2024 discharged the equivalent of about 170 Olympic swimming pools of acid, or about 13 million cubic metres, at pH 3, as acidic as vinegar, and up to ph 4.5, into the Macleay River estuary.
Bar chart: acid from Clybucca into the Macleay estuary, in Olympic pools of pH 3 acid. Four events: 7, 37, 170 and 14 pools. The April to July 2024 event was the largest.
Drainage works at Clybucca expanded dramatically during the 1960s and ’70s, lowering water levels across areas containing acid sulfate soils. When sulfide-bearing soils that have remained waterlogged for thousands of years are exposed to oxygen, the reaction produces sulfuric acid, which rain later flushes into the estuary.
Acid is not the only thing the wetland can send downstream. After prolonged flooding, decomposing plant matter can strip the oxygen from the water: the same report estimates about 11 million cubic metres of water containing no dissolved oxygen left Clybucca over 10 days in March 2022, and warns that, in a worst-case scenario, blackwater from the wetland could deoxygenate the entire Macleay Arm.
“You learn to work around it,” Macarthur says. “After a lot of rain, you keep an eye on the water and on the oysters, because conditions can change pretty quickly.
“For us, the health of the river and the health of the oysters are tied together.”
Unshucked oysters. Photo: Mark Stebnicki/Pexels.
But acidity, it turns out, is only one part of what the Macleay carries.
A catchment with two kinds of memory
Rivers carry more than water, whatever the land around them has been through, long after the people responsible have disappeared.
More than 150 kilometres upstream sits the Hillgrove mineral field near Armidale, mined for gold and antimony since the 19th century. It’s where millions of tonnes of mine waste containing arsenic and antimony, which is used in lead alloys, were deposited around waterways including Bakers Creek, and which has been traced for more than 300 kilometres downstream.
That history is chemically distinct from the acid sulfate soil problem at Clybucca: the metals came down through sediment from historic mining upstream, while the acidity is generated locally.
Hillgrove’s mine waste doesn’t make acid itself; the carbonate in the host rock buffers it, so water leaving the mine stays close to neutral.
Two very different stories, but sooner or later, they end up in the same river.
The link between them is geography, not cause: the arsenic and antimony that washed down from Hillgrove now sit in the same low-lying floodplain where drained soils make acid, and the 2024 study that measured them did so in a seasonal acid sulfate soil wetland on the lower Macleay.
Professor Scott Johnston, an environmental hydrogeochemist at Southern Cross University, points to iron.
The wetland is super dynamic and its basic chemistry flips between dry-acid and wet-neutral conditions. Those flips are what can make very interesting and dynamic things happen.
Professor Scott Johnston
Arsenic and antimony are “super dynamic in part because they hang out with iron oxides and iron sulfide minerals … and [acid sulfate soil] wetlands are loaded with both,” he says.
Both metals are “redox sensitive elements,” Johnston adds, as are the abundant iron and sulfur in the system from [acid sulfate soils]. Their behaviour depends on whether conditions are oxidising or reducing.
“The redox and pH fluctuations with wet and dry seasons are key,” he says.
In a 2024 study in the Journal of Hazardous Materials, Johnston, Gretchen Wichman, Edward Burton and Damien Maher calculated somewhere between 113 and 192 tonnes of arsenic, and 14 to 24 tonnes of antimony, sitting in just the top 10 centimetres of soil across the lower floodplain, with arsenic more extractable than antimony, a sign it may be more available to move through the environment.
For Macarthur, that’s a different problem altogether, one that doesn’t announce itself the way acid water does.
“You hear about the acid water because you can actually see the change in the river,” he says. “Arsenic or antimony is different. You’re not going to look over the side of the boat and know it’s there.
“That’s probably the part that concerns me most.”
The Macleay winds through more than 350 kilometres of NSW’s mid-north coast before reaching the sea. Photo: Tiana Tanevska.
What the river won’t say plainly
The question is what happens when the two histories share a floodplain.
According to Johnston, the answer is genuinely complicated, not the simple yes-or-no it might seem.
“Potentially it can,” he says of whether the process mobilises the arsenic and antimony.
“Mobilisation is more likely during redox transitions, i.e. from wet to dry or dry to wet, as this changes … the key minerals (and pH) that can ‘host’ [arsenic and antimony].”
Under dry, acidic conditions, iron minerals can trap the metals in place; under wet, oxygen-poor conditions, they can destabilise and release what they were holding. But they can also be taken up again, he says, as iron sulfides re-form in the organic-rich sediments near the surface.
“The wetland is super dynamic and its basic chemistry flips between dry-acid and wet-neutral conditions,” Johnston says. “Those flips are what can make very interesting and dynamic things happen.
“Their mobility is connected, but the actual behaviour is very complex and dynamic.”
Johnston keeps the two problems in proportion.
Acid pulses could “quite possibly, at times” carry elevated arsenic or antimony downstream, he says, but he doubts the impact would come close to the damage already done by low pH and low oxygen alone.
“These [pH and dissolved oxygen] are like the main meal in terms of water quality,” he says. “Any [arsenic or antimony] is likely a minor condiment on the side.”
I don’t think anyone expects a river to be perfect… but if we know there are problems sitting in the catchment, we should be looking for them.
Steve Macarthur
What troubles him more is a simpler gap: nobody has actually been checking for it when the acid arrives.
The Water Research Laboratory’s 117-page Clybucca report lists oyster mortality and reduced oyster growth among the known effects of acid sulfate runoff.
It never once mentions arsenic or antimony.
Asked if current monitoring tests for arsenic or antimony during acid events, his answer is blunt: “Nope. Not that I am aware of.”
He adds: “I think there is a substantive practical gap in our understanding of whether seasonal mobilisation of [arsenic or antimony] occurs in this system or not, because it has not been systematically monitored.
“We know [arsenic and antimony] are elevated (vs background) in the soils. We know there is potential for seasonal mobilisation. But who knows what happens in practice?
“It might be that most of the [arsenic and antimony], once deposited in the wetland, largely stays in the wetland … Or, perhaps, under the right conditions, some is re-mobilised and exported back out to the river. Likely a bit of both I would guess.”
Antimony is measured further upstream. A 2024 study in Hydrological Processes by Wichman, Johnston and Damien Maher sampled the river at Bellbrook, about 168 kilometres downstream of Hillgrove and well upstream of the oyster leases, 496 times between 2016 and 2023.
Antimony exceeded the Australian drinking-water guideline of 3 micrograms per litre in 17 per cent of samples, though that is a drinking-water benchmark, not an oyster-farming one.
Bar chart of dissolved antimony passing Bellbrook on the Macleay River each year, in kilograms.
The same study estimated that flushing the mine-derived antimony out of the upper catchment could take roughly 600 to 1,000 years, and that a return to pre-mining conditions would take centuries to millennia.
How much moves depends on the weather: about 24 kilograms of dissolved antimony passed Bellbrook in the 2019 drought, against about 5,174 kilograms in the very wet 2022. And an estimated 10 to 45 per cent of the particulate antimony exported from Hillgrove since about 1880 has settled on the lower coastal floodplain, the low-lying country Clybucca drains.
Johnston likens it to an egg, saying: “One cannot easily ‘uncrack’ an egg once you have cracked it.”
The lesson, he says, is “don’t crack it in the first place”.
One body of research describes a wetland releasing enormous volumes of acidic water; the other shows the same floodplain holds a substantial legacy of arsenic and antimony.
Neither proves every acid event carries dangerous concentrations of metals into oyster-growing waters, but together, they raise a question nobody has answered: is anybody routinely checking?
Along the Macleay River people who grow oysters depend on them for their livelihood. Photo: Tiana Tanevska.
Living with two rivers at once
For people working the Macleay, that isn’t abstract.
Oysters filter the water around them their whole lives, and the people who grow them depend on it for their livelihood and everyone’s confidence in what they produce.
“At the end of the day, this is where our oysters come from and it’s where we work every day,” Macarthur says. “You want to know what’s in the water.
“If there’s something there that needs to be monitored, I’d rather we know about it than find out years down the track.”
The Macleay runs almost 300 kilometres from the Great Dividing Range to the sea, with a catchment covering 11,450 square kilometres, according to the NSW Department of Climate Change, Energy, the Environment and Water.
Some of what it carries disappears with the next tide; some stays in the sediment for generations. Research suggests the arsenic and antimony already dispersed through the system could persist for centuries.
Water fowl along the Macleay. Photo: Tiana Tanevska.
At Clybucca, authorities are still working through drainage decisions made decades ago, and the lower Macleay remains an unusual meeting point between two environmental legacies: one carried down from mining country far upstream, the other generated beneath the floodplain itself.
The NSW Department of Primary Industries and Regional Development has previously said it received no reports of stock losses from Macleay oyster producers linked to the April–July 2024 acid pulse, and urged farmers to report any losses through the state’s natural disaster assistance scheme.
It’s a response about the acid itself, not a claim, one way or the other, about arsenic or antimony, which nobody has yet asked them to test for.
The Macleay would not be the first NSW estuary to face this. In 2001, after major flooding on the Richmond River on the state’s far north coast, NSW Fisheries closed the river to all forms of fishing for three months following a fish kill that a Greens NSW report on the river called “unprecedented in Australia”.
The report blamed low dissolved oxygen, linking such blackwater surges to floodplain drainage exposing acid sulfate soils, and says oyster farming on the river was said to be in a state of recession when the report was published.
It is a different river and a different event, and the report is an advocacy document rather than a government finding, but it shows what drained floodplains can do to an estuary’s fisheries after a flood.
Who the river supports
The Macleay’s oyster industry is small by state standards. In 2023–24 its farms produced 61,771 dozen Sydney rock oysters worth about $867,000, roughly 1 per cent of the NSW total, according to DPIRD’s aquaculture production report.
But oysters spend their whole lives in the water around them, and the UNSW report lists reduced growth and oyster mortality among the known effects of acid runoff.
So far, the evidence of harm to that industry is limited. A 2010 Kempsey Council study found acid from the Clybucca floodgates had affected oysters only in the uppermost harvest area, over a limited period, and DPIRD says it received no reports of stock losses from Macleay oyster producers after the 2024 acid pulse.
But the UNSW report warns that, in a worst case, blackwater from Clybucca could strip the oxygen from the entire Macleay Arm, and it lists forced closure of estuary fishing among the known consequences.
Others work the same water. The 2010 council study put the estuary’s commercial fishery at about $376,000 a year, shared among 12 to 15 permit holders, and the Department of Primary Industries confirmed about 1,000 fish died in Clybucca Creek in February 2020 after heavy rain.
Graziers on the floodplain pay too: with reports acid water can scald pastures and corrode concrete and steel, including bridge pylons and pipes.
Who answers for it is less clear. As the Macleay Argus reported in February, the Clybucca Wetlands are not the responsibility of any single organisation: Kempsey Shire Council manages the floodgates, and Transport for NSW is preparing to hand management to the National Parks and Wildlife Service, though that transfer is not yet final.
The river supports a lot of people down here. Looking after it matters.
Steve Macarthur
A draft 2011 NSW government briefing acknowledged “a plume of material containing heavy metals in the river system from the Hillgrove area to the Pacific Ocean”.
A review of public documents for this story could not find anyone testing for arsenic or antimony when the acid comes through.
For Macarthur, the answer doesn’t need to be complicated.
“I don’t think anyone expects a river to be perfect, especially after a big flood or a lot of rain,” he says. “But if we know there are problems sitting in the catchment, we should be looking for them.
“The river supports a lot of people down here. Looking after it matters.”
Scientists have already estimated where the arsenic and antimony sit across the lower floodplain, and monitoring stations at Clybucca can already detect the moment an acid event begins.
What’s missing is putting the two together. Until then, the Macleay keeps doing what it has always done after rain, and Macarthur keeps watching the water, the way he always has, for whatever it decides to show him.
Main image of the Macleay floodplain, where drainage schemes built more than 60 years ago continue to shape the river today. Image credits: OffTrack Photography.
