A number of scientific studies, together with the regulatory impact assessment supporting the consultation on proposals to revise nitrate vulnerable zones regulations, estimated that agriculture contributes approximately 61 per cent of the nitrogen entering surface waters in England. Discharges from sewage treatment-works account for up to 30 per cent. The remainder comes from a variety of sources, including road transport. According to the National Atmospheric Emissions Inventory, road transport accounted for about 34 per cent of total nitrogen oxide emissions to air in 2005. Data are not available on the amount of nitrogen entering surface waters through aerial deposition.