the metals used in aircraft construction will corrode causing safety issues. Snow dump sites A snow dump site is a location where snow is dumped as a part of the snow removal process. Designated sites are sometimes required to prevent water and ground pollution because the snow collected on roads typically contain a variety of grit, de-icing chemicals, vehicle fluids, engine emissions, and litter. Some pollutants become diluted and wash away with the melt-water and some concentrate at the dump site. Dumping into fresh water is "...almost universally prohibited due to the serious impact that deicer salts can have on freshwater aquatic life." In the United States dumping snow into water bodies is not specifically prohibited by the Clean Water Act or the Ocean Dumping Act. The states and local governments determine their own dumping policies. Snow dump site selection is based on the availability of suitable land and pollution prevention factors that may include distances from lakes and streams, installation of silt fences, soil and aquifer type, and other factors. Some sites may use filters and settling ponds to help prevent the pollution from spreading. Occasionally an excessive amount of snow must be dumped and is sometimes allowed to be legally dumped into water bodies on an "emergency only" basis. Surface treatments The surface is treated primarily by snow removal. Roads are also treated by spreading various materials on the surface. These materials generally fall into two categories: chemical and inert. Chemical (including salt) distribution induces freezing-point depression, causing ice and snow to melt at a lower temperature. Chemical treatment can be applied as a preventive measure and/or after snowfall. Inert materials (i.e. sand, brash, slag) make the surface irregular to improve traction. Both types can be applied together, but the inert materials tend to lower traction once the snow and ice has melted. Chemical treatment materials include: Sodium chloride (common table salt, NaCl) Calcium chloride (CaCl2) Potassium chloride (KCl) Magnesium chloride (MgCl2) Ammonium nitrate (NH4NO3) Ammonium sulfate [(NH4)2SO4] Potassium acetate (CH3COOK) Urea [(NH2)2CO] Propylene glycol (C3H8O2) Calcium magnesium acetate (C4H6O2Ca and C4H6O2Mg) Sodium ferrocyanide (hydrous, Na4Fe(CN)6•10H2O) Methyl alpha-D-glucopyranoside (C7H14O6) In the European Union, 98% of chemical treatment materials used in 2000 were sodium chloride in various forms. It is effective down to −5 °C, at the most −7 °C. For colder temperatures, calcium chloride (CaCl2) is added to NaCl in some countries, but deployment is limited as it