The presence of more than 1 and still more so of 2 grains of Chlorine per 100,000 of water is most suspicious, except in saline districts. Nitrites if present in an appreciable quantity indicate comparatively recent contamination by sewage. In deep well-water they may be produced by deoxidation of nitrates. Nitrates in upland surface waters should not be equivalent to more than ·03 of N. per 100,000; in shallow well-waters the amount varies greatly; in deep well-waters it may be excessive. As a rule it ought not to be equivalent to more than 5 parts of N. per 100,000 of water; but the significance of nitrates depends greatly on the source of the water and on the amount of the other constituents present.
Chemical analysis alone cannot ascertain the safety of a given drinking water. A minute amount of impurity inappreciable to analysis may be competent to produce disease; while another water may be drunk with impunity, which contains considerable organic matter. Chemical analysis “can tell us of impurity and hazard, but not of purity or safety” (Buchanan). An accurate opinion as to the character of a drinking water can only be expressed when one knows the amount of each chief constituent (as above), and whether these amounts deviate from the same water at other times or from other waters in the vicinity.
[CHAPTER XII.]
ORIGIN AND EFFECTS OF THE IMPURITIES OF WATER.
Origin of Impurities of Water.—Parkes classifies impurities of water as:
1. Those Received at the Source.—The character of water varies with the geological structures through which it has passed; with its origin from the subsoil or cultivated land, or deep wells, or graveyards, or near the sea, etc. It is a mistaken policy to commence with an impure water and proceed to purify it; though communities supplied from rivers may be compelled to submit to this. They must then insist on the most stringent measures of purification (see p. 96). Inorganic impurities are of much smaller consequence as regards health than organic; hence the great advantage of deep well-water over river water. It has been suggested, however, that when deep well-water becomes polluted, it is more dangerous than equally polluted river-water, because in the latter the normal bacteria of water are more abundant, and possibly interfere with the continued life in water of disease-producing bacteria. This statement is unproved; and if correct, is rather an indication for further precautions being taken to prevent access of pollution to deep wells, than in favour of the continued use of river-water.
2. Impurities of Transit from Source to Reservoir, acquired during the flow in rivers, canals, or other conduits. These impurities have been broadly divided by the Rivers Pollution Commissioners into “sewage” and “manufacturing;” the former including the solid and liquid excreta, the house and waste water, etc.; the latter including the refuse from manufacturing processes, as from dye and bleaching works, tanneries, etc.
3. Impurities of Storage, whether in wells, reservoirs, or cisterns. Organic impurities are commonly received at this stage. A well, for instance, drains the soil around it in the shape of an inverted cone, with a very broad base, unless the entrance of water from its sides is prevented.
4. Impurities of Distribution. Lead, and occasionally other metals, are dissolved by certain waters. If the pipes are left empty, as with an intermittent supply, sewage may be drawn into them; in a few cases coal-gas has found its way into the water pipes (page [76]).
Effects of Impure Water.—1. Effects of Mineral Impurities. Suspended Mineral matters in unfiltered water occasionally produce diarrhœa. The hill diarrhœa of some parts of India has been traced to water containing fine mica particles in suspension.