If no contamination be present in the water of a river, it forms a good source of water-supply; being running water, it is always fairly well aërated, and is not usually so hard as spring-water.
Even if sewage has entered a river, it is asserted that it becomes a safe source of water-supply, after passage through filter beds, the sewage having been got rid of in four ways.
1st.—By subsidence, the organic matter settling to the bottom.
2nd.—By the influence of water-plants, which assimilate ammonia, nitrates, etc., and give out nascent oxygen.
3rd.—Oxidation. Doubtless a large amount of the nitrogenous matter does become oxidised in its course down a river, and in this condition is harmless. The river Seine becomes greatly polluted as it passes through Paris, but so far as chemical analysis can determine its condition, it is purer 30 miles below the city than it was before it received the sewage of the city.
4th.—It is highly probable that the germs (or micro-organisms) of enteric fever and other diseases known to be propagated by polluted water, are practically or wholly destroyed in the struggle for existence with the natural micro-organisms of river-water. When to this is added the fact that river-water supplied to large communities is carefully filtered through sand, after having been stored in reservoirs, in which the chief impurities have time to settle, it is not surprising that the experience of those communities like London, which are supplied with river-water, usually shows no evidence of evil ascribable to drinking this water. For over 30 years the inhabitants of London have been drinking filtered water from the river Lea and from the Thames above Teddington, and this gigantic experiment on a population which has increased from 2½ to 5 millions has not been accompanied by any conclusive evidence of evil effect.
In regard to the comparative merits of the various waters described, it will be useful to give here the classification made by the Rivers Pollution Commissioners in their sixth report:—
| Wholesome |  | 1. Spring Water |  | Very palatable. |
| 2. Deep-well Water | ||||
| 3. Upland Surface Water | | Moderately palatable. | ||
| Suspicious |  | 4. Stored Rain Water | ||
| 5. Surface Water from Cultivated Land |  | Palatable. | ||
| Dangerous | | 6. River Water to which Sewage gains access | ||
| 7. Shallow-well Water |
Passage through certain geological strata has a great influence in rendering water palatable, colourless, and wholesome by percolation.
The following strata are said by the Commissioners to be the most efficient:—(1) Chalk, (2) oolite, (3) greensand, (4) Hastings sand, (5) new red and conglomerate sandstone. Fissures or cracks in these strata may cause the water to pass through them unpurified by filtration.