Acid sulfate soils (ASS) are natural sediments that contain iron sulfides. They are common along low lying areas adjacent to waterways, typically where the surface elevation is less than 5m above mean sea level. If left undisturbed, ASS do not present any risk, but when exposed to air the soil reacts with oxygen to create sulfuric acid. This can damage built structures, reduce agricultural productivity of land, and harm or kill animals and plants.