The health of Australia's Murray-Darling river system, already shockingly poor, has just taken a turn for the worse. In the past month, tracts of wetland at the mouth of the Murray have become as corrosive as battery acid, forming a yellow crust of sideronatrite, a mineral that only forms in extremely acid soil.

This latest indicator of the river's decline is detailed in reports to be released this week by the CSIRO Land and Water research institute in Adelaide, South Australia. For years drought and mismanagement have reduced water flows in the Murray-Darling system, altering salinity, temperature and nutrient levels. But in July last year, a team led by Rob Fitzpatrick, who wrote the new reports, found a new problem: falling water levels in Lakes Alexandrina and Albert at the Murray's mouth in South Australia were exposing the surrounding soils, rich in iron sulphide, to the air.

This has led to the production of 240,000 tonnes of sulphuric acid, says Fitzpatrick. "Acid dissolves aluminium, arsenic, zinc and lead, which could contaminate water supplies," he adds. The discovery of sideronatrite will fuel fears that the acid will seep into the lakes, killing aquatic life.

Fitzpatrick says a proposal to flush out the acid with seawater would only be a short-term fix, making the river even saltier than the sea. Two alternatives are being tested around Lake Albert: spreading lime and growing acid-resistant plants to neutralise the acid in the soil.