and other metals. Reduction in agricultural productivity through metal contamination of soils (predominantly by aluminium). Damage to infrastructure through the corrosion of concrete and steel pipes, bridges and other sub-surface assets. Agricultural Impacts Potentially acid sulfate soils (also called cat-clays) are often not cultivated or, if they are, planted with rice, so that the soil can be kept wet preventing oxidation. Subsurface drainage of these soils is normally not advisable. When cultivated, acid sulfate soils cannot be kept wet continuously because of climatic dry spells and shortages of irrigation water, surface drainage may help to remove the acidic and toxic chemicals (formed in the dry spells) during rainy periods. In the long run surface drainage can help to reclaim acid sulfate soils. The indigenous population of Guinea Bissau has thus managed to develop the soils, but it has taken them many years of careful management and toil. In an article on cautious land drainage, the author describes the successful application of subsurface drainage in acid sulfate soils in coastal polders of Kerala state, India. Also in the Sunderbans, West Bengal, India, acid sulfate soils have been taken in agricultural use. A study in South Kalimantan, Indonesia, in a perhumid climate, has shown that the acid sulfate soils with a widely spaced subsurface drainage system have yielded promising results for the cultivation of upland rice, peanut and soybean. The local population, of old, had already settled in this area and were able to produce a variety of crops (including tree fruits), using hand-dug drains running from the river into the land until reaching the back swamps. The crop yields were modest, but provided enough income to make a decent living. Reclaimed acid sulfate soils have a well-developed soil structure thanks to the abundance of trivalent cations (mainly Al+3) which have a very strong flocculating effect; they are well permeable, but infertile due to the leaching that has occurred. In the second half of the 20th century, in many parts of the world, waterlogged and potentially acid sulfate soils have been drained aggressively to make them productive for agriculture. The results were disastrous. The soils are unproductive, the lands look barren and the water is very clear (again, due to the flocculating effect of Al+3), devoid of silt and life. The soils can be colorful, though. Construction When brickwork is persistently wet, as in foundations, retaining walls, parapets and chimneys, sulfates in