This is of particular concern in private groundwater wells located in agricultural regions with overlying permeable soils. The US Environmental Protection Agency requires that public water systems not exceed a nitrate concentration of 10 mg/L, because nitrate above this level, causes infant methaemoglobinaemia. Transformation products and metabolites of man-made chemicals that are produced from biological, chemical and physical breakdown reactions. Technical Paper of the Intergovernmental Panel on Climate Change, IPCC Secretariat, Geneva, 210 pp.