Animals at the top of the food chain may be the first indicator that the water is polluted. Sediment or soil particles carried to a stream bed, lake, or ocean, can also be a pollutant when present in large enough amounts. Soil erosion is the typical reason for this and can be produced by many things. The removal of trees near waterways is a major contributor to erosion. The Earth's topsoils can also find their way into streams by means of heavy rain runoff and floods in or near croplands, strip mines and roads.
Some organisms occur in nature and are only considered pollutants when found in drinking water.
One such parasite is Cryptosporidium parvum, which caused more than 400,000 illnesses and 100 deaths in Milwaukee in 1993 when it contaminated the drinking water supply. EcoWorld , All Rights Reserved EcoWorld and EcoWorld Tours are registered Trademarks of EcoWorld Inc.