Boggabri Coal have got an open-cut pit full of water, stopping mining for a considerable period. Accordingly, they have applied for a permit to pump it out to the Namoi River.
Clearly, if they go ahead, this will need close monitoring, as sulphidic minerals are found in nearly all geological environments, including sedimentary rock (and associated with coal seams). Acid Mine Drainage (AMD) is the greatest negative impact of any disturbance of the subsoil and regolith. Sulphidic minerals (notably Pyrite FeS2) are oxidised on exposure to air, and then rainfall causes acidification of the oxidised pyrite to form sulphuric acid (H2SO4). In its simplest form, this chemical reaction is as given here:
The product Fe(OH)3, iron(III) hydroxide (orange), precipitates as a solid, insoluble mineral by which the alkalinity component is immobilized, while the acidity remains active in the sulfuric acid. The process of acidification is accompanied by the formation of high amounts of aluminium (Al3+, released from clay minerals under influence of the acidity), which are harmful to vegetation. Other products of the chemical reaction are:
- Hydrogen sulfide (H2S), a smelly gas
- Sulfur (S), a yellow solid
- Iron(II) sulfide (FeS), a black/gray/blue solid
- Haematite (Fe2O3), a red solid
- Goethite (FeO.OH), a brown mineral
- Schwertmannite a brown mineral
- Iron sulfate compounds (e.g. jarosite)
- H-Clay (hydrogen clay, with a large fraction of adsorbed H+ ions, a stable mineral, but poor in nutrients)
The iron can be present in bivalent and trivalent forms (Fe2+, the ferrous ion, and Fe3+, the ferric ion respectively). The ferrous form is soluble, whereas the ferric form is not. The more oxidized the soil becomes, the more the ferric forms dominate. Acid sulfate soils exhibit an array of colors ranging from black, brown, blue-gray, red, orange and yellow. The hydrogen clay can be improved by admitting sea water: the magnesium (Mg) and sodium (Na) in the sea water replaces the adsorbed hydrogen.