It is expected that some regions may have significant reductions in precipitation or major variations in the timing of wet and dry seasons with many aspects of the economy, environment, and society dependent on water resources, such changes in the hydrological cycle will have the potential to severely impact upon environmental quality, economic development, and social well-being. Water treatment is the alteration of a water source in order to achieve a quality that meets specified goals. The treatment of public drinking water to remove pathogenic, or disease causing, microorganisms began about that time. Treatment methods included sand filtration as well as the use of chlorine for disinfection. The virtual elimination of diseases such as cholera and typhoid in developed countries proved the success of this water treatment technology. In developing countries, waterborne disease is still the principal water quality concern. Although approximately 98 percent of liquid fresh water exists as groundwater, much of it occurs very deep. This makes pumping very expensive, preventing the full development and use of all groundwater resources.