By doubling the length of the watershed we have thus doubled the length of time during which the water is turbid, and have increased the average turbidity of the water taken for water-works purposes from 0.19 to 0.26, although the average turbidity of all the water running off remains exactly the same.
If now we assume a watershed so long that three days are required for the water from the most remote points to reach the intake, with computations as above, water taken for water-works purposes will have an average turbidity of 0.32; and with still longer watersheds this amount will increase, until with a watershed so long that ten days, or the interval between rains, are required for the water from the upper portions to reach the intake, the average turbidity of the water taken for water-works purposes will reach the average turbidity of the run-off, namely, 0.57.
In the above computations the numbers taken are round ones, and of course do not represent closely actual conditions. They do serve, however, to illustrate clearly the principle that the larger the watershed, other things being equal, the more muddy will be the water obtained from it for water-works purposes, and the longer will be the periods of muddy water, and the shorter the periods of clear water between them.
It cannot be too strongly emphasized that the period of duration of muddy water is, in general, dependent upon the length of time necessary for the muddy water to run out of the stream system after it is once in it, and be replaced by clear water; and that the settling out of the mud in the river has very little to do with it.
Muddy waters result principally from the action of rains upon the surface of ground capable of being washed, and the turbidities of the stream at any point below will occur at the times when the muddy waters reach it in the natural course of flow, and will disappear again when the muddy waters present in the stream system at the end of the rain have run out, and have been replaced with clear water from underground sources, or from clearer surface sources.
THE AMOUNTS OF SUSPENDED MATTERS IN WATER.
There is a large class of waters, including most lake and reservoir waters, and surface-waters from certain geological formations, which are almost free from suspended matters and turbidities. That is to say, the average turbidities are less than 0.10, and the average suspended matters are less than 2 parts in 100,000, and are often only small fractions of these figures. This class includes the raw waters of the supplies of many English cities drawn from impounding reservoirs, and also the waters of the rivers Thames and Lea at London, and the raw waters used by both of the Berlin water-works, and in the United States the waters of the great lakes except at special points near the mouths of rivers, nearly all New England waters, and many other waters along the Atlantic coast and elsewhere where the geological formations are favorable.
Data regarding the suspended matters in these waters are extremely meagre. The official examinations of the London waters contain no records of suspended matters, although the clearness of filtered waters is daily reported. Dibden, in his analytical investigations of the London water-supply, mentioned in his book upon “The Purification of Sewage and Water,” reports the average suspended matters in the water of the Thames near the water-works intakes as 0.77 part in 100,000. No figures are available for the raw waters used by the Berlin water-works, but both are taken from lakes, and are generally quite clear. Even in times of floods of the rivers feeding the lakes, the turbidities are not very high, because the gathering grounds for the waters are almost entirely of a sandy nature, yielding waters with low turbidities, and further, the streams flow through successions of lakes before finally reaching the lakes from which the waters are taken. It is safe to assume that the suspended matters and turbidities do not exceed those of the London waters. Even at times when somewhat turbid water is obtained, due to agitation by heavy winds, the suspended matter is mainly of a sandy nature, readily removed by settling, and it does not seriously interfere with filtration.
The examinations of the Massachusetts State Board of Health, with a very few exceptions, contain no statements of suspended matters. This is due to the fact that the suspended matters, in most of the waters, are so small in amount as to make them hardly capable of determination by the ordinary gravimetric processes, and the determinations if made would have but little value. The Merrimac River at Lawrence, at the time of the greatest flood in fifty years, carried silt to the amount of about 111 parts in 100,000. This was for a very short time, and the suspended matter consisted almost entirely of sand, which deposited in banks, the deposited sand having an effective size of 0.04 or 0.05 millimeter. No clayey matter is ever carried in quantity by the river.
The reports of the Connecticut State Board of Health also contain no records of suspended matters for the same reason. It may be safely said that the average suspended matters of New England waters are almost always less than 1 part in 100,000.