More than 50% of the world's population already live in cities, and the proportion is rising extremely rapidly towards developed country levels of more than 90%. Groundwater from wells is the major source of water supply for many of these cities, however, groundwater is polluted by the cities that overlie it and sewerage systems are often absent, or leak. Industrial landuse implies the use of large quantities of chemicals, and it is the poor disposal and accidental spillage of these chemicals which results in the contamination of virtually every industrial site in the world. Much of this pollution migrates deeper to damage groundwater. Groundwater has slow turnover time, often measured in decades and centuries, reducing the self-cleansing capacity. Urban aquifers are therefore vitally important but very fragile, easily damaged and slow to repair. Urban groundwater problems and solutions vary greatly around the world. Mature cities