In turn, increased acidity can dissolve metals, some of which are toxic. For example, in acidified water courses it may be dissolved aluminium which kills fish rather than the acid itself. The sites of manufacturing plants may become contaminated by raw materials, combustion products and wastes over the years they are running. Any attempt to redevelop the site may encounter asbestos, toxic metals or hazardous solvents. More pervasive, and often less identifiable, are the problems associated with landfill sites.
Many old dumps contain chemical and metal wastes as well as household refuse.
Water percolating through such dumps may dissolve toxic materials, producing leachates which contaminate water courses. More pervasive still is the decomposition of organic materials to produce methane, a gas which is explosive when mixed with air and which is a very effective greenhouse gas. Modern landfills can be managed to reduce water percolation and to collect methane for use as a fuel, but many old dumps are unrecorded and potentially hazardous. Pollution control is partly a matter of improving technology, for example, incineration of domestic waste can solve the problem of methane in landfills, as well as generating useful amounts of electricity, but adoption of improved controls may be costly and may require political pressure. Several different principles have been used or proposed.