Impacts of acid leachate on water quality and fisheries resources of a coastal creek in northern Australia

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Reclamation of ~700 ha of mostly tidal wetlands in north Queensland in the early 1970s disturbed potential acid sulfate soils resulting in acid leaching into coastal waterways. The wetlands had been isolated from tidal inundation through the construction of a levee with tidal gates on the major creeks. Acid leachate caused the pH values in impounded, freshwater reaches of Firewood Creek to drop to <4 for much of the year and increased total dissolved aluminium and iron concentrations. Flood rains caused large volumes of acid water from the creek to discharge into its tidal reaches, resulting in environmental problems including episodic fish kills. Factors contributing to the fish mortalities included a temporary reduction of pH, elevated iron and aluminium concentrations and occasional low dissolved-oxygen concentrations. Fish kills were observed on three occasions in three years but did not occur every time acid water was discharged. Fish species diversity in the tidal reaches also declined during the wet season and this correlated with high concentrations of dissolved aluminium. Although episodic flood events resulted in the acidification of the tidal reaches of Firewood Creek, no major deleterious effects on the main estuary were found. Rehabilitation options for this site include restoration of full or partial tidal flooding.