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Capillary attraction influences flow through all porous media whose particles are less than 2 inches in size. "If the effective size of the particles is less than 5 mm., as in all ordinary sands and many gravels, the actual velocity has been found to be approximately proportioned to the hydraulic slope. Velocities through clean uniform gravels larger than 2 inches and through rock fissures are approximately proportional to the square root of the slope. Laboratory experiments indicate the large effect of size of pore spaces, the velocity varying, with other conditions equal, about as the square of this size.

Although the laws of ground-water flow are well understood and, in the case of clean sands and gravels, have even been expressed with fair accuracy as mathematical formulas, they should be applied with caution to specific problems, due to the difficulty in securing correct information upon conditions largely concealed underground. Theoretical considerations should be coupled with test borings, records of existing wells, records of past and present levels of ground water, and data on local geology, if reasonably accurate predictions are to be made as to the hydraulics of a new development.

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Direct measurements of slope and velocity of flow The hydraulic slope can be determined by measuring the levels of water in a series of tight tubes sunk to the stratum, the slope being measured in the direction of greatest declivity.

Direct measurements of velocity of underflow involve the sinking of several borings along the line of flow. In one method a large dose of soluble salt is introduced at the upper hole and velocity is computed from the time interval required for the salt content in samples taken from the lower holes to become maximum. A quicker method, developed by Slichter, requires only two holes, into the upper of which an electrolyte is introduced. When the electrolyte reaches the lower hole there is a sudden increase in electrical conductivity, which is indicated upon an ammeter suitably connected with a battery and electrodes in the lower hole. Velocities of ground water flow are extremely slow, as compared with surface flows. In sands a velocity of 20 feet per day is relatively high and requires a steep gradient or coarse medium; in certain fine-texture sandstones estimated velocites are as low as 10 feet per year.

Yield

The yield of a ground water supply depends on the amount of rainfall which will enter the ground in the catchment area tributary to the point of interception; on the underground storage which can, by the method of interception, be made effective by smoothing out the seasonal or yearly variation in the amount of rainfall which enters the ground, and finally, on the capacity of the material in which the galleries or wells are located to transmit its contained water to the intercepting works.

Obviously these factors controlling yield generally cannot be as closely estimated as in the case of a surface supply.

Wherever possible, and particularly in the case of shallow wells in the drift sands and gravels, a pumping test at a rate approximating the desired yield should be run, with a sufficient number of scattered indicator wells driven for observation of the effect of the pumping in the lowering of the water-table.

Artesian water

When a water-bearing stratum having an elevated catchment area lies beneath an impervious cover the flow is confined somewhat as in a conduit, and if the resistance to flow though the stratum between such locality and the outlet be sufficient, the contained water will exert a pressure against the cover. The measure of this pressure is the height above the top of the confined stratum to the hydraulic gradient. Wells sunk to strata under such pressure are termed "artesian” wells, and water in them will rise in accordance with the pressure and may even overflow at or above the surface. Originally the term "artesian" was applied only to flowing wells, but the broader definition is now common. Most of the extensive underlying formations previously described are artesian in character.

Springs Springs may be considered in two general classes; gravity and artesian. Gravity springs are formed where the ground watertable intersects the ground surface as at the base of a hill or the margin of a stream. They may occur (as in the most important cases) at the outcrop of a porous stratum underlain by impermeable material, then receiving the entire flow of the stratum, or they may be mere surplus overflows of a stratum, the carrying capacity of which is less than its supply. Artesian springs occur under the same general conditions as artesian wells, being fed from confined strata from which the water breaks to the surface through faults or weak places in the cover.

Character of ground waters Deep-seated or artesian waters, except those from fissured rocks usually contain few bacteria. Waters in limestone and fissured rocks often carry pollution for many miles. Dug wells, or shallow wells, in surface deposits, even though in fine-grained material may be polluted by surface waters through rifts, craw-fish holes or other direct channels, and their adjoining catchment area should be protected for considerable distances around the well. Bacterial analyses from a new well are misleading unless the well is pumped for several months before sampling, as some such period is required to remove the effect of surface dirt left on the well casing and equipment. Increased draft on a surface well widens the tributary area and hence may increase pollution.

Ground-waters as a class have great solvent power and hence are hard and highly mineralized, although important exceptions occur. The most common dissolved minerals are salts of calcium and magnesium (forming hardness) sodium, iron and silica. The most common dissolved gases are carbon dioxide and sulphuretted hydrogen, both of which can be practically removed by suitable aeration. The temperature of ground water in strata some 50 feet below the ground surface is usually the mean temperature of the locality and in strata below this depth the temperature increases roughly 1°F. for each 60 feet of depth.

Works for oblaining ground-water Classification. The various forms of works used for obtaining ground-water may be classified as (1) works for utilizing the flow of springs, (2) infiltration galleries, (3) dug wells and (4) driven, drilled and bored wells.

Works to utilize springs. The foremost object in works of this kind is the protection of the spring against surface pollution and is usually accomplished by enclosing the spring in a covered masonry basin or gallery. Secondary objects may be means to increase yield, the providing of a convenient chamber for the connection of outlet pipe, and in some cases the providing of a settling basin.

Infiltration galleries. Infiltration galleries are horizontal galleries or pipes constructed below the water-table and provided with numerous small openings to admit the water. Their practical application is usually limited to depths suitable for excavation in open cut. They should be located well below the minimum level of the water-table so that ample head will be available, after making allowance for some probable clogging, to produce flow. Graded gravel and sand are usually placed around the openings, to exclude fine material. Masonry and vitrified pipe, being permanent and uninjured by water, are preferable to other materials. Access or inspection manholes and valves for cutting off various sections are desirable adjuncts. In some cases where relatively narrow underground streams occur, underground dams of sheet-piling or masonry walls are constructed immediately below the galleries, to make possible the interception of a greater proportion of the underflow. Galleries have frequently been constructed near surface streams for the purpose of obtaining stream-water filtered through the ground. Many of these have been disappointing, due either to low initial yield, gradual clogging of media or insufficient filtration. Porous substrata are necessary, but do not assure the permeability of the stream bed unless floods keep the bottom scoured clean of fine deposit.

The iron, present in most soils and gravels, is readily soluble in water from which the oxygen has been exhausted by the organic matter originally present in, or taken up, by the water on entering the ground through a muck-covered stream bed. When the water is again aerated by trickling down the surface slope of the cone of depression, this iron in insoluble form is deposited in the sands adjacent to the galleries or wells or in the well screens, and clogging and limitation in yield result.

The yield of galleries is about proportional to the length and to the amount of lowering of water-plane. Depending upon the lowering and upon the character of media the yield per minute per foot of length may vary from a small fraction of a gallon to several gallons. The advantages of galleries over a line of wells are that the yield for equal lowering is somewhat increased, the gallery takes

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the place of a collecting pipe, and the water after collection may be pumped with greater economy than by well-pumps. The disadvantages are that the first cost is high, the amount the water plane may be lowered is limited by the elevation of the gallery, the yield when once deteriorated cannot readily be improved and the system lacks flexibility in arrangement and operation.

Dug wells. Dug wells are relatively shallow, masonry-walled wells of diameters varying from 5 to 100 feet, built in excavations made from the surface. A common method is to sink the well by excavating inside of a circular steel shoe, on which the masonry wall is built as the well descends. The main supply is usually derived from the bottom which is left open, although openings in the sides are sometimes added. The bottom should be carried some distance below lowest ground-water level, to provide storage. The yield increases slowly and the cost rapidly with increase in diameter. The top of a dug well should be above the ground surface and surrounded with earth sloping away from the well.

A tight cover, preferably of concrete, should be provided to exclude dirt and prevent vegetable growths in the water. The advantages of dug wells over other types are storage capacity, convenience as pump-suction wells and somewhat increased yield. The disadvantages are high first cost, limited practicable depth and limited amount that water level may be lowered.

Driven, drilled and bored wells. An ordinary driven well consists of steel pipe, usually 1 to 4 inches in diameter, to the lower end of which a suitable point and a strainer are attached. It is driven by a hammer or by jetting and is seldom used for depths over 150 feet. Drilled wells are wells sunk in consolidated rock by means of special cutting tools. In the soft formations above the rock the wells are cased with steel pipe or casing. Bored wells are those sunk in unconsolidated formations by suitable boring tools. They are provided with casings set as, or after, the well is bored, and are equipped with screens or strainers set in the water-bearing medium. Drilled and bored wells are adaptable to much greater diameters and depths than driven wells. Selection of one of the three types is usually dictated by character and depth of material to be penetrated and by the desired size of well.

Hydraulics of wells. When a well tapping a non-artesian waterbearing stratum is pumped, the water table is lowered with a “cone

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