ABSTRACT
A wide variety of pollutants are present in natural waters. These include sand, silt, clay, organic matter from decaying vegetation, and products of chemical conversions, among others. Natural puri‹cation processes are continually active in streams and rivers to reduce the levels of the pollutants to acceptable or negligible concentrations. These processes include dilution, sedimentation, ‹ltration, heat transfer, and chemical and biological conversions. These natural puri‹cation mechanisms are slow and can restore the health of water bodies over a period of time, depending on the concentration of the pollutants.
