The average composition of the four classes of unpolluted waters is given by the same authorities as follows. Their estimations are in parts per 100,000, but may be converted in grains per gallon by multiplying by 7 and dividing by 10:
| Total solid im- purity nitrites | Organic Carbon | Organic Nitrogen | Ammonia | Nitrogen as nitrates and | Total combined nitrogen | |
| Rain water | 2·95 | ·070 | ·015 | ·029 | ·003 | ·042 |
| Upland surface water | 9·67 | ·322 | ·032 | ·002 | ·009 | ·042 |
| Deep-well water | 43·78 | ·061 | ·018 | ·012 | ·495 | ·522 |
| Spring water | 28·20 | ·056 | ·013 | ·001 | ·383 | ·396 |
| Previous sewage or animal contamination | Chlorine | Hardness | |||
| Temporary | Permanent | Total | |||
| Rain water | 42 | ·22 | ·4 | ·5 | ·3 |
| Upland surface water | 10 | 1·13 | 1·5 | 4·3 | 5·4 |
| Deep-well water | 474 | 5·11 | 15·8 | 9·2 | 25· |
| Spring water | 3559 | 2·49 | 11·0 | 7·5 | 18·5 |
Rain water contains, among natural waters, the smallest amount of solid matter in solution. From the columns headed “Organic Carbon” and “Organic Nitrogen” it will be seen that even rain collected with special precautions, away from any large town, is by no means free from organic matter. Rain water collected from roofs and stored in underground tanks is often very impure.
Surface waters form the main supply of rivers. If collected from high uncultivated districts they are usually unpolluted with animal matter. The organic matter is usually peaty, is sometimes very small, but is liable to considerable variations with the season, and is occasionally present in excessive quantities, discolouring the water and rendering it unpalatable. From their softness these waters are admirably adapted for manufacturing purposes. The amount of solid matter in solution ranges from 2 to 7 grains per gallon.
Surface water from cultivated land, contains on an average less organic matter than upland surface water, but the pollution, being derived from manure and other objectionable matter, is more harmful.
River water consists of the above, aided by springs, and most frequently the drainage of towns on its banks. The amount of solid matter varies from 10 to 30 grains per gallon. In Thames water there are on the average about 20 grains.
Wells, if shallow, are usually a most undesirable supply. Unless far from any house they are contaminated by drainage, and sometimes, from proximity to cesspools, contain more animal matter than ordinary town sewage. They are, as a class, hard waters, the polluted ones excessively so.
Wells of 100 feet deep and upwards are, as a class, very superior waters, the filtration and oxidation of so great a depth of soil having removed the greater part of the organic matter. The hardness varies with the strata, but, as a class, the deep wells are softer than the shallow.
Spring water greatly resembles deep well water, possessing all its good qualities in a higher degree. Spring and deep well water are very uniform in quality, and little affected by climatic changes.