There is increasing concern regarding the presence of metals, particularly heavy metals in drinking water. In addition to the well-known toxic effects of lead, which are discussed at length in the Best Practice Guide on the Control of Lead in Drinking Water (in this series of Guides), the latest WHO guide values for maximum mean concentrations of arsenic, nickel and others in drinking water have been lowered compared to previous versions. European Union, USA and National standards for drinking water have followed the trend based largely on the same toxicological information. There is currently growing interest in the presence of hexavalent chromium in drinking water following its finding in some drinking waters in the USA above the national upper limit for total chromium and research suggesting that this limit needs to be considerably lowered. Some metals, particularly iron and manganese are associated with the production of discoloured (dirty) water in distribution systems and are therefore very important to the production of water with acceptable aesthetic qualities. Others (iron and aluminium) are key to the treatment of raw waters to drinking water standards. In most cases it is not possible to continuously monitor waters for metals and suitable sampling programmes must be designed and carried out to give results representative of the true water quality.
