Clean, fresh water is essential to life on Earth. Water pollution is a global threat to this vital resource. Acid rain is a prime example of how activities in one geographic area can impact conditions globally. Acid rain primarily results from the transformation of industrial pollutants such as sulfur dioxide (SO2) and nitrogen oxides into other compounds such as sulfuric acid (H2SO4), and nitric acid (HNO3). This transformation occurs as these pollutants are transported in the atmosphere over distances of hundreds to thousands of kilometers. For example, sulfur dioxide emissions from industrial processes and the burning of fossil fuels have resulted in extensive acid rain and accompanying water pollution problems in southeastern Canada and the northeastern US. These emissions have global implications: more than half of the acid deposition in eastern Canada originates from emissions in the United States. Even slight changes in the pH of lakes and rivers can cause the loss of fish and invertebrates which are important links in the food chain. Acid rain is also responsible for extensive loss of forest cover in that region.