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of depression" immediately adjacent to the well and a secondary depression which decreases with increased radius and disappears at the limit of the "circle of influence." If the well is artesian in character the pressure line assumes the same general form, although the water may not be lowered below the top of the water-bearing stratum. In either case the lowering continues and the circle of influence widens until the inflow from the stratum without the circle equals the amount of water pumped. (If the quantity of water naturally flowing in the stratum does not at least equal the pumpage, the lowering and widening effects will continue indefinitely, being limited only by the extent of the stratum.) If the water-bearing stratum is porous the ultimate amount of depression is approximately proportional to the pumpage rate, and this fact is important in estimating effects of changes in conditions. In fissured limestones the draw-down varies more nearly as the square of the pumpage rate.
When a number of closely-spaced wells in the same stratum are pumped simultaneously their circles of influence overlap, and their total yield for a given draw-down is much less than the sum of their yields when pumped separately. The amount of this mutual interference depends upon specific data. The effect is illustrated in the following example, which is based upon theoretic analysis by Slichter: Interference of 6-inch wells, radius of circle of influence 600 feet pumped down
Specific capacity of wells. The "specific capacity” of a well is the yield in gallons per minute divided by the amount, in feet, that the water-surface is drawn down. A specific capacity of 100 is high, , 20 is moderate and 5 is low. Values of 100 or higher may be obtained
. only in thick strata of coarse sands or in gravels.
Factors affecting yield of wells. The potential yield of a well is determined principally by the capacity of the water-bearing medium to transmit water to the sides of the well, assuming of course that yield is not limited by depletion of natural supply in the stratum. This transmitting capacity depends upon the character and thickness of the stratum and upon the amount that the water level may be drawn down.
The direct effect of varying the diameter is relatively small. The indirect effect of increasing diameter may be large, as in cases when the yield is limited by the size of the pump that may be placed in the well, or by insufficient strainer area. In deep, artesian wells the restricting of friction losses due to upward flow in the well may partly dictate diameter. In sand strata, high velocities clog the strainer openings and rearrange and compact the particles near the strainer, progressively decreasing the yield. This action is much less marked when velocities are moderate.
Gravel-packed wells. The yield and specific capacity of wells in unconsolidated sands may often be increased, and clogging troubles greatly reduced, by removing the sand from around the strainer and substituting selected, graded gravel. A coarser strainer is then used and the effective diameter of the well becomes that of the pocket of gravel. In fine sands the improvement is often much more than would appear likely from mere consideration of relative diameters, due, no doubt, to the fact that the velocity of water leaving the sands is reduced below the velocity at which finer particles are picked up. In some cases the gravel deposit extends out several feet from the strainer.
The general method has been used on wells of all depths up to several hundred feet. The procedure varies, but generally involves the pumping out of sand through an inner casing and the simultaneous feeding downward of gravel between the inner and an outer casing. The method is most applicable to shallow wells in fine sands. In coarse sands it effects little improvement and in deep wells it is costly.
Strainers. In unconsolidated materials some form of strainer is required to exclude the particles of sand and gravel from the well and to allow water to enter freely. In coarse gravel relatively large perforations in the lower part of the casing may suffice. More often the materials are finer and special strainers are necessary. These may be of brass wire mesh, but are usually cylinders of brass tubing containing a multitude of narrow, horizontal or vertical slots of any desired width. The openings are usually flared toward the inside of the strainer, to diminish clogging tendency. The outer width of openings varies from 0.004 inch upward according to the fineness
of the water-bearing medium. The net total area of strainer openings should be such that velocities through the opening will not exceed 2 or 3 inches per second.
In shallow wells of large diameter, when the water-bearing medium is gravel, or when gravel-packing is used, strainers are sometimes made of porous concrete. These consist of uniform gravel containing enough cement to bind the particles without filling the voids. Their advantage is freedom from corrosion.
Safeguards against pollution. Surface washing and the groundwater for a few feet below the surface are actual or potential sources of pollution and should be carefully excluded from wells. They may enter by direct overflow at the well top, through leaky casings or by working down to the water-bearing stratum in the space sometimes left outside the casing. Casings should be of considerable thickness to resist corrosion, should have tight joints, and should either be carried somewhat above the ground level or terminate in tight concrete pits whose sides extend above ground level. If wells are in a stream valley, flood levels should be investigated and casings carried above them. Casings of drilled or bored wells should extend down to the top of the water-bearing formation and the annular space outside the casing should be sealed with grout, puddle or fine sand, the means varying with local conditions. Gravity pipe lines used for the collection of water from wells should be tight and preferably made of cast iron. Leaky vitrified collecting pipes caused the notable typhoid epidemic at Salem, Ohio.
Ground vs. surface water. Compared with unfiltered surface water supplies, the relative advantage of ground water supplies as a class are that the natural water has low bacterial content, little color, uniformly low temperature and is clear and generally free from industrial wastes. The relative disadvantages are that (1) the water is likely to be hard or high in other objectionable dissolved minerals or gases, (2) the quantity available in most localities is limited, (3) works for obtaining ground water are subject to comparatively rapid depreciations, (4) methods of pumping from wells are relatively inefficient and (5) more or less speculation is involved in predicting such factors as the ultimate supply capacity of a stratum, the yield per well, the pumping lift and the amount of maintenance work required. The comparison is general and in a specific case it is suggested that the points mentioned be considered and proved or disproved separately.