This section is from the book "Beverages And Their Adulteration Origin, Composition, Manufacture, Natural, Artificial, Fermented, Distilled, Alkaloidal And Fruit Juices", by Harvey W. Wiley. Also available from Amazon: Beverages And Their Adulteration.
When water is confined so as to be practically motionless except as it may be moved by the wind or by the application of other forces to the surface, there is apt to be an accumulation of debris, which manifests itself in various ways. When the depreciation of the water is so marked as to become visible it is called stagnant. The condition which arises is revealed to the senses in various ways; the water may become foul smelling and thus the stagnant condition be revealed through the sense of smell. The growths of various flora, such as algae in various forms, is also a mark of stagnancy. In such cases the water becomes coated with a scum, usually green, which indicates the abnormal growth of organisms of the kind mentioned. The debris of organic life, either of the bacterial flora or the flora represented by the type of the algae by decay also adds additional matters to render the water less desirable. This condition of stagnancy often occurs in reservoirs of water which are intended for potable purposes, especially during the hot months. The mere growth of simple forms of algae cannot in any sense be considered as a contamination in the sense of being injurious to health. These forms of life as a rule are perfectly harmless. They become objectionable solely by their abundance and by their decay. The appearance of the water is not attractive and no one would be willing to consume a water thus infected, even if convinced of its harmless character. During the winter months the low temperatures prevent the growth of the different kinds of organisms mentioned, and thus conditions of stagnancy are not so often revealed. Especially upon farms where the water for the animals is contained in ponds the stagnant conditions are apt to obtain during the summer months. In city supplies where the water is held in reservoirs for the purpose of settling or otherwise, the condition may also obtain.