Frankland gives the following average proportions of nitrogen, as nitrates, occurring in 100,000 parts of various kinds of water:—
| Rain water | 0·007 |
| Upland surface water | 0·009 |
| Deep wells and springs | 0·400 |
| Surface water (cultivated districts) | 0·250 |
| Shallow wells (no average), 2 to 5 parts common. |
Other authorities regard the presence of more than 0·6 part of nitrogen as nitrates per 100,000 parts of water as indicating dangerous pollution.
At the International Pharmaceutical Congress held in Brussels,[128] the following standards of purity for potable water were recommended:—
1st. A water should be limpid, transparent, colourless, without smell, and free of matter in suspension.
2nd. It should be fresh, with a pleasant taste, and its temperature should not vary much, and certainly not be higher than 15°.
3rd. It should not contain noxious animal or vegetable matter, and especially none of these substances in a state of decomposition.
4th. It should not contain more than 6 to 10 mgrms. of organic matter per litre, expressed in terms of oxalic acid. It should not contain nitrogenous matter.
5th. The nitrogenous organic matter, oxidised with an alkaline solution of potassium permanganate, should not yield more than 0·01 part of albuminoid ammonia per 100,000.
6th. It should not assume a disagreeable smell after having been kept in an open or closed vessel.