The following table gives the amounts of suspended matters for various classes of waters corresponding to the turbidities stated, which have been deduced from the experience of the author. It is very likely that ratios different from the above would be obtained with waters in which the sediment was of different character.
| Turbidity, Platinum-wire Standard. | Suspended Matters: Parts in 100,000. | |||
|---|---|---|---|---|
| Settled Waters. | River Waters, Finest Sediment. | River Waters, Average Sediment. | River Waters, Coarsest Sediment. | |
| 0.01 | 0.16 | |||
| 0.05 | 0.80 | 0.85 | 1.30 | 2.40 |
| 0.10 | 1.60 | 1.75 | 2.60 | 4.90 |
| 0.20 | 3.20 | 3.60 | 5.50 | 10.00 |
| 0.30 | 4.80 | 5.70 | 8.50 | 15.00 |
| 0.40 | 6.40 | 7.80 | 11.60 | 21.00 |
| 0.50 | 8.00 | 10.00 | 15.00 | 26.00 |
| 1.00 | 16.00 | 23.00 | 36.00 | 59.00 |
| 1.50 | 24.00 | 40.00 | 62.00 | 97.00 |
| 2.00 | 32.00 | 61.00 | 94.00 | 140.00 |
| 3.00 | 48.00 | 110.00 | 175.00 | 250.00 |
SOURCE OF TURBIDITY.
Much turbidity originates in plowed fields of clayey soil, or in fields upon which crops are growing. If it has not rained for some days, and the surface-soil is comparatively dry, the first rain that falls upon such land is absorbed by the pores of the soil until they are filled. If the rain is not heavy, but little runs off over the surface. If, however, the rain continues rapidly after the surface-soil is saturated, the excess runs off over the surface to the nearest watercourse. The impact of the rain-drops upon the soil loosens the particles, and the water flowing off carries some of them in suspension, and the water is said to be muddy.
The particles carried off in this way are extremely small. Mr. George W. Fuller, in his report upon water purification at Louisville, estimates that many of them are not more than a hundred thousandth of an inch in diameter, and not more than a tenth as large as common water bacteria.
The turbidity of the water flowing from a field of loose soil may be 2.00 or more; that is to say, the wire is hidden by a depth of half an inch of water or less. When the water reaches the nearest watercourse it meets with water from other kinds of land, such as woodlands and grassed fields, and these waters are less turbid. The water in the first little watercourse is thus a mixture and has a turbidity of perhaps 1.00.
The conditions which control the turbidity of any brook are numerous and complicated. The turbidity of a stream receiving various brooks depends upon the turbidities of all the waters coming into it. Generally speaking, the turbidity of a river depends directly upon the turbidities of its feeders, and is not affected materially by erosion of its bed or by sedimentation in it. There are, of course, some streams which in times of great floods cut their banks, and all streams pick up and move about from place to place more or less of the sand and other coarse materials upon their bottoms. The materials thus moved, however, have but little influence upon the turbidity.
After the rain is over some of the water held by the soil will find its way to the watercourses by underground channels, and will prevent the stream from drying up between rains, but the average volume of the stream-flows between rains will be much less than the volumes during the rains when the water is most turbid.
Fig. 19.—Fluctuations in Turbidity of the Water of the Allegheny River at Pittsburg during 1898.