Notes on the Quantity of Water Available
In connection with the map printed in this issue, showing the area of the water shed at present available and which must be relied upon in future for the water supply of the new or Twenty-sixth ward of Brooklyn, we append the report of the engineers appointed to investigate this subject. It is as follows:
Geology of Long Island as Bearing on (Voter Supply.—This island is formed almost wholly by a deposit of boulders, gravel, sand and clay, brought from the northwest by glaciers. This material is torn away from the sides of valleys through which the glacier moves, is deposited upon it by landslides, or is torn up from the bottom. The forward end of the glacier in melting deposits this solid matter in an irregular intermingling of boulders, gravel, sand, etc. Erosion or cutting away of the higher portions of this deposit flatten out the side slopes, leave a central ridge and rearrange the finer gravel, sand and clay about the edges of the island here formed. The gravel and sand which forms the mass of this island is simply the more finely ground particles ot the larger masses of rock.
The geological proof of the origin of Long Island is found in the presence of trap boulders from the I’alisades, gneiss and marble from Westchester, shales from the Hudson groups, red sandstone from upper New Jersey, and other rocks transported here from distant but known localities. Rocks deeply scratched by friction with other rocks under some enormous impulse also occur on Long Island, and show the work of glacial action.
No water can possibly come from the mainland to Long Island, because of the non-existence of any continuous impervious clay strata, rising in the mainland and extending under the Sound to Long Island.
The only available water supply on Long Island is the water falling upon its surface as rain and stored up in the porous gravel and sand which forms the bulk of the island. The rain falls upon the surface and, excepting on the steep slopes of the central ridge, at once sinks into the ground. In the course of ages this porous soil has become saturated with rain water to the ocean level, and above that level as now described. Before this stored water was utilized by man the sea was its only point ot escape, and all rainfall upon the island unevaporated by the sun penetrated the soil or passed off at once by open streams. The water in the soil as it accumulated also sought the lowest point of escape, either the valley of some stream or the seashore. But this subterranean water has to pass through beds of sand, etc., and friction retards its flow. This retardation of flow causes a slope from the central ridge towards the sea in the surface of the saturated bed of water underlying the island. This slope has its summit practically under the central ridge, and dips toward tide and coincides with the tide at its outer limits. Careful investigation by Kirkwood and others show that this hydraulic slope is about five feet to one mile, in the parts nearest the ridge, and eight to twelve feet per mile nearer the streams emptying into tidewater.
All of the water supply of Long Island must come from this saturated bed of sand and gravel.
The quantity of water to be obtained from any locality on Long Island is not a question of guess, or ol seeking for possible water-bearing strata. It can be deduced closely from well-recognized laws, and the formation of the sub-soil determines the maximum limit of the supply. The average rainfall of Long Island is about forty-two inches per annum in depth. (The rainfall of i88qwas fifty-five inches. Hence an extra volume of water was stored in the ground, and the conditions were most favorable to the Long Island Water Supply Company, as compared with an average year. See report, Robt. Van Buren, l88q.) A certain percentage of the rainfall flows off in the streams or is evaporated by the sun ; another portion usually assumed at sixty per cent as the result of study is stored up in the saturated bed, and unless intercepted and utilized, finally feeds the springs and surface streams, or escapes into the sea at about tide level. Fresh water springs are found in the sea all along this coast. It is stated that this water surface is so accurately known that the exact depth at which water may be found at any point within the drainage area supplying the city of Brooklyn can be computed within a few feet of the actual depth.
The accompanying tracing shows the territory lying between Jamaica Bay and tidewater of the Sound. The lines drawn upon it from northeast to southwest near the cemeteries show approximately the summit of the slope in the saturated water-bearing material. To the north of this water flows naturally to the Sound and to the south of it to the bay. The profile on the tracing gives a section of the land from the Sound to the bay, and upon it is marked the approximate depth of the saturated ground from which water can be drawn. It should be realized that in order to have a continuous supply no more water can be pumped from the saturated water-bearing material than can be supplied to it continuously by the rain, which is the only source from which it can be replenished.
The map shows the approximate limits of the territory from which the water can be drawn to supp’y the pumps of the Long Island Water Supply Company.
The northern limit is determined by the line on the map just mentioned. The western limit is fixed by the location of Fresh Creek, into which salt water enters at high tide. The water lying to the west of the red line, near Van Sicklen avenue, will flow towards this creek and not towards the pumping station of the Long Island Water Supply Company. It will flow in that direction because it can reach ocean level with a steeper slope and therefore with less resistance than towards the pumps, provided these do not lower the water level below the level of tide. The eastern limit of the available watershed, also shown by the map line, is fixed by the Brooklyn Aqueduct, which receives into it the ground water flowing towards it from the north and northeast.
When the new wells sunk at the new water-works station at Ridgewood are used, they will draw the water from the same territory, the southern limit of which is approximately shown by the map line. It will, therefore, prevent a large area hereafter from draining towards the pumps of the Long Island Water Supply Company. The area then available for the latter is shown by a heavy line on the map, shaped nearly like a triangle.
As long as the area which thus serves the pumps is comparatively open, not paved nor having many buildings upon it, a greater portion of the rainwater will penetrate the ground and replenish the subterranean water supply than when the surface becomes more impervious through the laying of pavements, etc. In the latter case a large quantity of water would flow off directly to the ocean instead of sinking into the ground. This proportion will be still greater when sewers are built. It has been assumed that at present about 60 per cent of the rainwater penetrates the ground into the water bearing strata. As the suburb of Brooklyn becomes more built up this percentage will be correspondingly reduced.
Assuming, however, that 60 per cent of the 42 inches is available for a water supply this will amount to the following quantities: From the area at present draining to the pumping station of the Long Island Water Supply Company, measuring 2 55 square miles, it will be 3.067,578 gallons per day. After the new wells at the Ridgewood Pumping Station have come into use, the area available for the company will have been reduced to 1.0 square miles and therefore the supply to about 1,202,972 gallons per day. These figures approximate a maximum, as the most favorable conditions have been assumed, the actual available supply w ill probably be less.
D. McN. STAUFFER,
THK PROPOSED NEW WATER SUPPLY FOR PITTSFIELD. —Engineer M. M. Tidd of Boston has made his report to the board of public works of Pittsfield, Mass., on the proposed system of wells in the Sackett meadow, and also considered other sources of supply, lie says there is a doubt about the supply to be obtained by wells, though he has put in nearly fifty of these well systems without disappointment in the results. He thinks the water is abundant in the meadow and if this should be so a system of 75 wells would yield 1,000,000 gallons a day, lie would advise, however, a system of 25 wells at first, the others to be added if the supply proved sufficient to warrant others. The expense of such a system might be about $30,000 or more.
Mr. Tidd bedeves the pumps at the Ashley lake will be sufficient for this winter’s supply, but he says a permanent additional supply will be required as it is doubtful whether the lake fills up in a year if it is drawn as low as it will be by the pumps, and next summer there will be a shortage again unless heavy and prolonged spring rains or thaws make a greater supply than can be reasonably expected. Onota and I’ontoosuc lakes are suggested, but it is said the manufacturers along the river who use the water from the lakes for power would resent any diversion of the supply from the natural outlet.