great extent by the process of freezing, and ice that has been frozen for a few months is considered perfectly safe to use by some authorities even though its source had been polluted.

The amount of rainfall per minute, hour, day, month, season, or year varies greatly in all parts of the globe. Only a general notion can be given here of the amounts and variations in the United States. A knowledge of the rainfall in connection with buildings is useful where the supply is taken from a small stream or river; where rainwater is collected from roofs and stored in cisterns for domestic or industrial uses; and where the roof areas are so large as to materially effect the size of rain leaders (down spouts).

year. West

The annual rainfall east of the Appalachian Range is 40 to 50 in., well distributed throughout the of this range and east of the Mississippi River it amounts to about 30 in., on the great plains 15 to 30 in., between the Rockies and Sierras, 10 to 20 in., and on the Pacific slope, 20 to 100 in. The higher limits are usually along the coast lines and about the Great Lakes.

The data given in Table 1 should be used only as a general guide. For more detailed information, see United States Weather Bureau reports and works on Hydrology.

3. Ground Water.-Ground water is a term applied to waters whose source is from the ground, whether from springs, seeps, or deep or shallow wells. Ground waters are always colder in summer and warmer in winter than surface waters, except those very deep-seated waters, such as boiling springs and geysers. The temperature of ground water below a depth of 50 ft. is the same as the mean annual temperature for the locality and does not vary from season to season. The temperature below this depth increases 1 deg. F. for each 60 ft. The outer crust of the earth to a depth of 50 ft. is influenced by the seasonal variations of temperature. From these facts it is possible to predict from what depth any water comes by comparing its temperature with the mean temperature for that locality. Ground waters are the most largely used sources of water for domestic purposes. A good potable water is one which is free from pollution and contains some mineral matter which makes it pleasing to the taste. Distilled water is very flat and insipid.

3a. Drilled Wells.-Drilled wells derive their water from rock or consolidated formations, such as sandstones, conglomerates, limestones, and trap rocks. The soft, caving formations above the rock are cased off with standard wrought-iron pipe or well casing, which should be driven firmly into the rock formation. Where possible it is preferable to extend the casing through any soft rock formation, such as shales lying above the water-bearing rock. In some cases wells are "grouted" from the surface of the water-bearing formation to the surface of the ground. In the case where the grouting is to be done on a new well, that portion of the well that must be grouted is drilled to a large diameter, say 12 to 20 in., and to a depth sufficient to prevent leakage, seepage, etc. The depth to be grouted is usually predeter nined by a knowledge of the strata and should preferably extend to or near the top of the waterbearing formation from which it is desired to obtain the water. An outside casing must be used where the formations above the rock cannot be kept from caving long enough to permit of the completion of the work.

The hole at the bottom of the grout should be made smaller, of a size just large enough to receive the "liner" which should be seated firmly in the rock to a depth of at least 2 ft. The liner can be made of any grade of metal sufficiently strong to withstand handling and prevent collapse. The tops of both the outside and inside casings should reach the surface of the ground, or to the point where the water is to be taken off. In the case of flowing wells, the liner may be used as the discharge or suction pipe if pumping is done, and would be directly connected to the pump or horizontal pipe. The space between the two casings should be at least 2 in., but more is desirable. The space between the inner casing (liner) and the drill hole is filled with neat cement grout which is forced into the space by a hand pump.

The following apparatus is necessary: a line of 14-in. pipe, sufficient to reach within 10 ft. of the bottom of the space to be grouted, a hand or tank pump, and a half barrel or water-tight box of about the same size, in which the grout is placed. The 14-in. pipe is hung between the liner and the outside of the well and is so connected that it can be readily raised as the filling progresses. The grout is made of cement of such a consistency that it will pour readily and no sand is added to the grout while the pump is in use.

The yield of a well depends upon its diameter, the water-bearing strata intersected, the depth of the well (as effecting friction), and the ability of the pump to lower the water.

Where the water-bearing strata is of a uniform nature or texture, such as sandstone, the yield varies with the physical structure of the strata and directly with its thickness. When the water-bearing strata is not uniform, such as limestone and granite, the yield depends upon the number and width of the fissures.

The theoretical relationship of these quantities is difficult of expression and interpretation, but for rough estimates the empirical formula

TL
C

holds approximately true for sandstone formations where the lowering of the water by pumping is not over 20 ftand the diameter of the well is 6 to 10 in. In this formula

For potsdam sandstone, 20 is about right for a value of C. Where L is more than 20

The amount of water that can be taken from a well per unit of time depends very largely, in the case of small wells, upon the character of the pumping machinery. For example, a 6-in. well may have a large capacity due to a great depth of water-bearing rock pierced by the drill, yet with a single acting deep-well pump, it is difficult to secure much over 100 gal. per min. The size of a well should therefore be made amply large if drilled before the quantity of water required, the method of pumping, and the depth and character of the water-bearing rock are known or closely approximated.

The capacity of wells in limestone or traprock formations is difficult to predetermine owing to the fact that the extent and number of fissures to be encountered are unknown. The quantity of water will usually vary approximately with the thickness of the formation pierced and with the square root of the lowering. A good procedure is to test the well by pumping before its completion, and noting (1) discharge of test pump in gallons per minute, (2) the lowering of the water in feet, and (3) the thickness of the water-bearing strata encountered. Then from this data, predict the conditions which would take place if the desired quantity were obtained.

The capacity of a well does not vary greatly with the diameter so far as the ground resistance to flow is concerned. A large diameter is of advantage chiefly in reducing the velocity of flow within the well, thus reducing the friction and the advantage of placing a large pump. These wells may be classed as either deep (non-flowing) wells, or artesian (flowing) wells, depending upon whether the water in the water-bearing formation is under enough pressure to bring the water to the surface.

In cases where the proposed well is within or near a well established (developed) community, a careful study of the local conditions as to existing wells, will aid greatly in arriving at the proper diameter and probable depth. Quicksand, clays, shales, slates, and close-textured granites, should not be depended upon as sources of water for any purpose, not even a country residence. The total yield of ground water that may be collected varies from 0.1 to 0.5 million gal. per day per square mile, and at one locality, from 1 to 3 million gal, per day.

36. Driven and Tubular Wells.-Driven and tubular wells secure their water from the loose formations above the solid rock, such as sand, gravel, or a mixture of these. Driven wells consist of a "point" attached to a screen, often called a "well point" or "well screen," which in turn is attached to several feet (as may be needed) of wrought-iron or steel pipe having a diameter to suit the "well point." These wells are seldom less than 114 or more than 4 in. in diameter. The points have openings 40, 60, or 80 meshes to the inch; the small sizes are usually 3 to 5 ft. long, and the larger sizes 8 to 10 ft. long.

Driven wells are usually relatively shallow, but in some cases have been driven to depths of from 400 to 500 ft. Driving to such depths is very liable to damage the screen. The amount of water to be obtained from such wells is very difficult to estimate for the principal reason that little or nothing is known about the character of the water-bearing material that is to be encountered. Even if the water-bearing strata is present, the possibility of getting the point entirely within it is quite an uncertainty. Where possible, it is a much better plan to use a tubular well. This is put down in much the same way as the casing of a drilled well with the exception that a specially-designed shoe with a flange is used When the water-bearing sand or gravel is reached, the point is dropped into the casing and is either driven below the end of the casing or the casing is pulled back to the top of the screen, or both processes of exposing the screen may be used. By this method much deeper and larger wells may be used, the point may be pulled up and examined, and a very definite knowledge of the water-bearing formation may be had by noting the character of the drillings and the behavior of the water levels when bucketing.

The amount of water to be obtained from wells of this kind varies greatly according to the porosity and coarseness of the sand or gravel. From tests of a large number of wells of this class it was found that with 60-mesh screens it was possible to secure 1⁄4 gal. per min. per sq. ft. of screen surface per foot of lowering of the water in the well; that is to say, a well point having 5 sq. ft. of screen and a lowering of the water of 10 ft. would supply approxi

mately 12 gal. per min. Wells in coarse gravel will often supply very much more than this amount. Forty wells located in coarse sand and gravel, yielded under test an average of 0.684 gal. per min. per sq. ft. of screen surface per foot of lowering, with a minimum of 0.45 gal. and maximum of 1.152 gal. per min.

Where the size of the sand grains is small or the porosity low, the capacity of this type of well can be greatly increased by packing the well screen in selected gravel. This can be successfully done by using a well casing 6 in. or more larger in diameter than the outside of the screen, and when the proper depth is reached, the well screen attached to pipe of slightly smaller diameter, is put in place. Then selected gravel to 3% in. in size is placed about and for some distance above the screen. The large casing is then drawn by jacks and the well is complete.

In the same formation and in identically the same place, wells of this kind have been known to give 6 times as much water as wells having a 60-mesh well screen but without gravel. The screens used with the gravel-packed wells are usually coarser than 40 meshes per inch. Besides the common gauze mesh screens, there are a number of patented screens such as the "Cook," "Johnson," and "Bowler."

3c. Dug or Open Wells.-Dug or open wells are usually relatively large in diameter, and shallow. The supply of water comes from the bottom and little or none from the sides unless it is excavated in rock formations. Wells of this kind are usually "curbed" with wood, brick, masonry, or concrete. The most successful manner of construction is to make a ring of concrete of the desired size, and after it has set sufficiently (3 to 4 days), lower it to the desired depth by excavating in the center. Such wells have been constructed 18 ft. in diameter by 60 ft. deep. Dug or open wells have the advantage of providing some storage of water, as well as a supply. Their capacity depends upon the physical characteristics and thickness of the waterbearing material, upon the lowering of the water, and upon the means provided for its entrance into the well. Metal screens may be placed in the walls near the bottom and the area of the water-bearing material, if of sand or gravel, may be covered with crushed rock or gravel to keep the fine sand from flowing in. The well should be properly covered to protect it from contamination.

The relative capacity of a well in the same formation, and with the same depth and extent of lowering, varies so far as ground friction is concerned, about as shown in the accompanying table, assuming that the water comes in through the sides and bottom of the well and that the yield of a well 1 ft. in diameter is unity.

From this table it is seen that the possible yield of a well of this type increases about as the diameter, but very much slower than the surface of water-bearing material encountered by the walls of the well. The yield of a well is often determined, not by the above consideration, but by the velocity of water at which fine sand may be carried into the well. This condition should be guarded against by properly-designed screens in the walls of the well and by assorted layers of gravel placed over the bottom of the well wherever the water-bearing material is fine sand.

4. Springs. Springs may be divided from an hydraulic standpoint into gravity and artesian (pressure) springs; and from a physical sense, into seepage, tubular, and fissure springs. Gravity springs are not confined between impervious beds, but flow because the ground-water surface is intersected by the surface of ground, as at the base of a hill or along a bank of a stream. Artesian (pressure) springs are confined between impervious beds, are relatively deep seated, and partake more or less of the characteristics of artesian wells. Such springs, if confined in a pipe or concrete basin, may be forced to rise several feet above the surface of the ground.

Seepage springs may be either gravity or artesian, but usually are of the gravity type and spread out over considerable area, as on a bench at the foot of a bluff along a river bank. This condition is usually accompanied by a soft spongy ground, abundant vegetation, and an oily scum due to decomposition of vegetable matter with the presence of iron or manganese. Many seepage springs are very deceiving as to the actual quantity of water flowing, owing to the relatively large area covered and apparent large quantity of water. Tubular springs are formed due to the solubility of some part of a rock formation or by the opening left by a decayed root. In limestone, the passages may extend for miles. These springs are often periodical-that is, fluctuating with the rainfall.

Fissure springs are always in rock formation and are usually artesian. They escape along bedding planes, joints, and cleavages in the rock formation and the waters are usually free from organic contamination, but are often highly mineralized. The requisite and qualifying conditions for the formation of springs are essentially the same as

those for an artesian well; viz., a sufficient rainfall, a collecting area, a porous inclined bed of sand or rock with an opening for escape of the water at the lower edge, or in case of an overlying impervious stratum, an upward passage for the water.

Springs may be contaminated principally in two ways: (1) by direct wash of pollution into the water as it merges from the ground, and (2) by infiltration of polluted water on the catchment area. The first may be prevented by protecting the spring, and the second may be remedied if the exact location of the collecting area is known and the source of pollution removed.

5. Infiltration Galleries.-Infiltration galleries are really horizontal wells excavated below the level of the ground water. They are constructed so as to leave an open space within the ground water horizon into which the water can percolate through the porous sides or openings left for that purpose. They are often made of brick, stone masonry, or some kind of pipe, such as vitrified clay pipe. The supply that can be obtained from a gallery of this kind depends upon its length, depth below the natural water level, and upon the characterstics of the waterbearing material. This may vary from a fraction of a gallon to several gallons per foot of gallery. (A 12-in. vitrified pipe 300 ft. long laid in a medium sized sand, supplied % gal. per ft. when the water level was lowered 6 ft.)

The Los Angeles Water Company has a vitrified pipe infiltration gallery 4500 ft. long which is reported to yield an average of 0.75 gal. per min. per ft. of gallery. The Crystall Springs Water Company, at about the same location, has 5368 ft. of similar gallery of vitrified pipe 15 to 24 in. in diameter, yielding 0.755 gal. per min. per ft. of gallery. At Grand Rapids, Wis., a 12-in, vitrified pipe gallery 960 ft. long, laid in extremely fine sand of uniform size, yielded only 20 of a gallon per min. per ft. of gallery.

6. Surface Waters.-The waters of lakes, ponds, rivers, and streams are very liable to be polluted and unfit for domestic use. Waters of this character, however, can often be used for commercial and industrial purposes without treatment, or at the most, by filtration through pressure filters at rapid rates. In rare instances, waters from an extremely large lake or from a river flowing from an unpopulated territory, are safe without purification. Where a safe water supply is insufficient in amount for all purposes for a building, water for flushing of toilets, scrubbing, etc., can be taken from a surface water supply and the bubblers supplied from another source or a part of the surface water supply treated for this purpose. Before such a source of supply is chosen, or better still, before the site of the proposed industry or institution is chosen, the quantity and permanency of the supply should be investigated.

The factors affecting the flow of a stream are: drainage area, slope of surface, character of subsoil, temperature (evaporation), and rainfall. Where the stream is large as compared to the demands of the industry, no detailed investigation might be necessary, but where the stream is small and varies in discharge with the seasons, a careful investigation should be made; also the character of the water, such as turbidity, color, and mineral and organic matter content, should be determined and compared with the needs of the industry.

Surface waters as a rule contain very much more organic matter and very much less mineral matter than do ground waters. These organic matters are largely of a nitrogenous nature and are difficult to remove when once passed into the nitrate form.

A conservative estimate of the quantity of water available from any stream would be 10 to 50% of the precipitation, depending upon the locality and nature of the stream. The rainfall should be taken for the average driest years. On streams where the minimum flow is likely to be less or just equal to the demand, a small dam should be constructed across the stream to impound water to tide over the driest spells.

PURIFICATION OF WATER

7. Impurities of Water.-Water may be considered to be impure from either a chemical or a sanitary standpoint. Mineral matters contained in waters, if in sufficient quantities, may interfere with steam making and industrial uses, and if it contains large quantities of alkalies, it will even be unfit for domestic use. The impurities which we are more concerned about are those of a sanitary nature and are of organic origin. A water may be ever so clear and of excellent taste and yet contain thousands of disease-producing germs.

8. Sources of Pollution.-A polluted water is one which contains the wastes from human habitation. It may not necessarily contain disease-producing germs or matters in which they are usually found, but may contain such other wastes as to make the water unwholesome. "Contaminated" is sometimes used synonymously for the word "polluted," but it is stronger

and means that the water has and does contain wastes which might cause disease or disorders. Besides the sewage carried away from buildings by sewerage systems, there are other sources of pollution, such as outhouses, slop drains (from kitchens), industrial wastes, decaying animal matter, and drainage from farm buildings and yards. The discharge from sewerage systems usually pollutes only surface waters, whereas the other sources pollute ground waters as well. If a water supply must be taken from a surface water, it should be taken, in case of a stream, far enough above the outlet of the sewer to be sure that none of the sewage will be drawn into the intake, and in case of a lake or other body of water, the intake should be in deep water and removed as far as practicable from the sewer outlet, even though the sewage be treated or purified.

In the case of ground waters, it is more difficult to trace sources of pollution to a water supply. In general, ground waters flow in the direction of the slope of the ground, i.e., into ravines, dry runs, and valleys. In all cases the source of water supply should be located so that the surface drainage will be away from, rather than towards the well, spring, or point on stream where the supply is taken.

9. Aeration. The usual process of purifying water by aeration is to discharge it into the air so as to break it up into a fine spray or a thin film. The process is used to oxidize organic matter, remove gases, such as hydrogen sulphide, and carbonic acid gas, and odors produced by aquatic vegetation. It is also used in some cases as a part of the process of removing iron and manganese.

10. Sedimentation.-Many surface waters, owing to the nature of the ground over which they flow, contain large quantities of suspended matter which may or may not be of a polluting nature. Many rivers flowing through a country where the surface material is largely clay, contain large quantities of finely divided clay in suspension. The process of sedimentation either natural or artificial, is used to remove as much of this material as possible. In some cases for industrial use, it is the only process needed, while in other cases it is a preliminary process to filtration, coagulation, or both.

Sedimentation is divided into two types, intermittent and continuous. The first gives no better results than the second, but in some cases for industrial use, it is more convenient where the use is intermittent, to fill a tank or basin, let it stand from 12 to 24 hr., and then draw off the clarified water. Continuous sedimentation is the most satisfactory process where large quantities are needed continuously. As a preliminary process to coagulation or filtration, the period should be from 6 to 24 hr., depending upon the fineness of the sediment and upon the relative cost of sedimentation basin and subsequent process. Fuller says that the economical limit of plain sedimentation is 24 hr., during which time 75% of the suspended matter is removed.

For large impounding reservoirs where the water is to be used for a domestic supply without further treatment, the period of sedimentation should be three or more days, depending upon the fineness of the sediment. The percentage of removal is greatest when the amount of suspended matter is greatest. Twelve hours subsidence removes about 33%, and 24 hr. removes from 59 to 83%. The process should take place in a basin or tank of proper size for quantity to be treated. The inlet and outlet should be made so that the velocity will be a minimum; baffles should be used to prevent currents from forming; and screens should be provided to keep out leaves and floating matter from entering the outlet.

11. Chemical Treatment.-Chemical treatment may be used for the following purposes: (1) removal of bacteria and polluting organic matter; (2) removal of suspended matterclay, etc. (producing turbidity), and vegetable compounds (producing color); and (3) removal of iron, manganese, and the salts producing hardness.

This treatment may be used with or without sedimentation depending upon the nature of the case. For (2) it is necessary in all cases to provide some sedimentation for effective work. The coagulants used for purposes (1) and (2) are sulphate of alumina, sulphates of iron, calcium carbonate, and sodium carbonate. Sulphate of alumina is most commonly used alone where the alkalinity is high enough to produce the necessary floc or precipitate. The amount of the chemicals used varies from a fraction of a grain to 6 or 8 grains per gal. depending upon the character of the water. Little work along these lines should be attempted by the architect or engineer without the aid of a chemist.

12. Filtration-Action and Function.-Sand and gravel have been found to be the most satisfactory materials to use in the process of filtration. When water is passed through a layer of these materials, a large portion of the suspended matter, bacteria, and color are removed. Even colorless organic matter in the colloidal form is partially removed. The function of the filter is to strain out the bacteria and suspended particles and this function is performed not only by the sand, but by the very organic matter which it is sought to remove, the organic matter forming a sort of gelatinous substance around the sand grains. That organic matter does