Concept · Ch 10
Acid-sulfate soils (the buried acid)
Waterlogged coastal muds are full of iron sulfides that sit perfectly harmless as long as they stay wet. Drain them and let the air in, and the sulfides oxidise to sulfuric acid — which the next big rain flushes into the creeks in a slug strong enough to kill fish.
Here is one of the meanest tricks the coast plays, and it is a powerful argument against draining a swamp on a whim. Many of the low, waterlogged soils of the coastal lowlands are rich in iron sulfides, built up over thousands of years in the airless mud. Left wet, they are entirely harmless and sit there doing nothing.
Drain them, though — dig a channel, drop the water table, let the air in — and the oxygen reacts with those sulfides to produce, of all things, sulfuric acid. The drained soil turns sour, and the next big rain flushes that acid, along with dissolved iron and aluminium, into the waterways in a slug strong enough to strip the oxygen from the water and the protective slime from a fish’s skin. These are acid-sulfate soils, the buried landmine beneath a great deal of drained coastal floodplain in eastern Australia — the Maroochy included, where the “red-spot” ulcers on bream and flathead tend to follow the acid pulses after rain. A swamp is storing water and carbon and, the real surprise, keeping a chemical hazard safely asleep besides. Wake it carelessly and the bill arrives, years later, as dead fish and corroded drains.
In depth
Over thousands of years, the airless mud of coastal floodplains and estuaries builds up a store of iron sulfides — chiefly pyrite (FeS₂) — laid down where bacteria reduced the sulfate in trapped sea water. As long as the soil stays waterlogged and airless, the pyrite is inert and does nothing at all.
Drainage is what arms it. Dig a channel, lower the water table or let the flat dry out, and oxygen reaches the sulfides and oxidises them to sulfuric acid, along with dissolved iron and aluminium. The acid sits in the drained soil until the next heavy rain, which flushes it into the waterways in a concentrated pulse: the water can drop to pH 3–4, the dissolved metals strip oxygen from it, and the acid strips the protective mucus from a fish's skin. On the Maroochy floodplain the outbreaks of "red-spot" ulcers on bream and flathead around Bli Bli tend to follow exactly these acid pulses after rain.
State it honestly. The acid is the trigger and aggravator, not the sole cause: red-spot (epizootic ulcerative syndrome) is driven by a waterborne mould, and the acid pulse is what stresses the fish enough to let the mould take hold. Climate sharpens the trap — a long, hot drought bakes and exposes more sulfide, and the drought-breaking downpour then flushes a bigger slug out all at once. The remedy is the mechanism run backwards: keep the sulfides drowned. That is a large part of why re-wetting drained floodplain (as the Blue Heart is doing on the Maroochy) is not only habitat restoration but hazard control — a swamp, it turns out, is also keeping a chemical landmine safely asleep.
Primary sources & further reading — the doorway
- McPhee, D. (2017). Environmental History and Ecology of Moreton Bay. CSIRO Publishing. — Estuarine water-quality and fisheries impacts in the bay region — where acid pulses do their damage.