Acid coal mine water treatment lagoon discharging into the river Pelenna, Wales, UK. When coal mines are closed they are often left to flood, causing iron and other metals in the mine to leach into the water. When water from the mines reaches the surface, the iron oxidises, forming an acidic solution known as acid mine drainage. Much of this acidic solution is neutralised by a significant amount of limestone present in UK coalfields. This neutralisation causes small particles of iron, known as ochre, to form. Passive settlement lagoons are used to separate out this ochre from the water. Reedbeds within the lagoons trap the particles of ochre, separating them from the flow of clean water into streams.