Old contamination of water is indicated by -

WATER POLLUTION Pure uncontaminated water does not occur in nature. It contains impurities of various kinds - natural and man- made. The natural impurities are not essentially dangerous. These comprise dissolved gases (e.g. nitrogen, carbon dioxide, hydrogen sulphide, etc. which may be picked up during rainfall), and dissolved minerals (e.g., salts of calcium, magnesium, sodium, etc.) which are natural constituents of wa'ter following its contact with soil; and suspended impurities (e.g., clay, silt, sand and mud), and microscopic organisms. These impurities are derived from the atmosphere, catchment area and the soil. * A more serious aspect of water pollution is that caused by human activity - urbanization and industrialization. The sources of pollution resulting from these are : (a) sewage, which contains decomposable organic matter and pathogenic agents (b) industrial and trade wastes, which contain toxic agents ranging from metal salts to complex synthetic organic chemicals (c) agricultural pollutants, which comprise feilizers and pesticides, and (d) physical pollutants, viz heat (thermal pollution) and radioactive substances Ref: Park 23rd edition Pgno : 710