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mangrove challenge

Acid sulfate soils

Photo of canal estates
Canal estates contribute to the acid sulfate soil problem.
Photo: Heather Janetzki, QM

Photo of acid sulfate soil
Do not disturb: With a pH less than 4, acid sulfate soils need careful managing.
Photo: NRW

Canal estate housing developments have been built in south-east Queensland estuaries since the 1950s. They are constructed by first excavating the soil, and then using this to raise the surrounding land surface to build houses. In the past, canal estates were built on reclaimed mangrove and saltmarsh areas. These areas are now protected by the Fisheries Act 1994, so in places like the Gold Coast reclaimed land such as golf courses are used.

In low-lying coastal areas of Queensland, to 5 m above sea level, acid sulfate soils often form once the soil is disturbed. This soil, usually mud or sand, was once deposited by the sea, and contains iron pyrites (or iron sulfide) made by bacteria in the low oxygen conditions. Kept covered, iron pyrites has no effect on the environment, but if oxygen enters the soil, sulfuric acid forms, and it becomes an acid sulfate soil. Excavation and drying out of coastal soils due to housing development and the building of walls and paths that stop sea water, allow oxygen to enter the soil, and cause this chemical reaction.

Acid sulfate soils often cause major environmental problems. In some cases, water from acid sulfate soils has a pH as low as 2. This leaches metals such as aluminium from the soil, and after rain, an acid-metal mixture can wash into streams and canals and kill fish and aquatic plants, cause disease in fish, and increase algal blooms.

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