Acid-sulfate soils (the buried acid)

Here is one of the meanest tricks the coast plays on anyone rash enough to drain a swamp, and it takes years to spring. Across the low, waterlogged floodplains and estuarine flats of the coastal lowlands, the airless mud has spent thousands of years quietly building a store of iron sulfides, chiefly pyrite, fool’s gold, laid down where bacteria in the oxygen-starved sediment stripped the sulfate out of trapped sea water. The Maroochy has some eight thousand hectares of it mapped between here and Caloundra. Left wet, this buried pyrite is the most harmless stuff imaginable. It sits there drowned and inert, doing nothing at all.

Drainage arms it. Dig a channel across the flat, lower the water table, let the ground bake dry in a drought, and air reaches the sulfides for the first time in millennia. Oxygen and pyrite do not coexist quietly. They react, and out comes sulfuric acid, with a load of dissolved iron and aluminium — of all the unlikely things to come welling up out of a paddock. This is not the tea-coloured acid of the wallum, the mild organic tannin leached from peat; this is genuine mineral acid, made on the spot, and a different order of sour.

The acid goes nowhere at first. It sits in the drained soil and waits, until the next heavy rain flushes it into the creeks and drains in a single concentrated slug that can drop the water to pH 3 or 4, strip the dissolved oxygen out of it, and peel the protective mucus off a fish’s skin. On the Maroochy floodplain the outbreaks of “red-spot”, the ulcers that leave bream and flathead around Bli Bli looking as though they have been shot, tend to follow these acid pulses.

Say it plainly, though, because the chemistry is easy to over-blame. The acid is the trigger, and it keeps making things worse, but it is not the assassin. Red-spot proper, epizootic ulcerative syndrome, is the work of a waterborne mould, Aphanomyces invadans; the acid pulse stresses and abrades the fish enough to let the mould take hold in the first place. The two arrive together, and the fish pays.

Climate sharpens the trap at both ends. The coast is promised more long, hot droughts. A drought like that bakes the flats harder and deeper, exposing and oxidising more sulfide than a normal year would, and then the downpour that finally breaks it flushes a bigger slug of the accumulated acid out in one hit. Deeper dries, fiercer wets. The same swing that empties the floodplain primes it to poison its own water the day it refills.

The remedy, mercifully, is the mechanism run backwards. Put the water back, keep the sulfides drowned, and they go back to doing nothing. This is a good part of why re-wetting drained floodplain, as the Blue Heart is doing across the lower Maroochy, does two jobs at once: it restores habitat and it defuses a hazard. A swamp, it turns out, does more than store floodwater and carbon and hide a nursery in its shallows. It also keeps a chemical landmine safely asleep. Wake it carelessly with a drain and a dry season, and the bill arrives years later, as dead fish and corroded steel.