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silt and clay reaching the watercourse. Moreover, the humus top layer of soil developed acts as a filter and natural storage medium for water falling on it and may thus tend to regulate stream flow. The effect of forests on flood prevention is a much controverted question. Forestation properly maintained on any watershed does, however, suppress the increase of human population in such areas and thus tends to hold in check the potential or actual dangers of human pollution. If rigidly enforced, such forestation may eliminate all resident population and thus greatly reduce the probability of dangerous pollution.

Natural filtration is a safeguard of great value in the purification of water. The continuous action of this agency is well illustrated in the low bacterial content and turbidity of ground waters; hence their popularity for drinking-water supplies. The extent of filtration depends largely on the nature of the filtering medium or geological composition of the soil. Fine sandy soils, being more porous, filter and retain larger quantities of water than closely compacted ones, and hence are more efficient in this respect. Clay and closely compacted soils, while excellent filters, allow such small volumes of water to pass that they are deficient from the standpoint of quantity; the greater part of the rain falling on them is shed as surface water.

The process of natural filtration is rather complex and makes use of various agencies for effecting the result. There is first of all a straining action by which larger particles, and some smaller ones as well, are prevented from passing through the filtering layers with the water. In addition there is a great reduction of velocity of the liquid which causes deposition of suspended matter and, by increasing the time. of flow, permits the death of organisms to proceed within the filtering medium. Moreover, the grains of the filtering material finely divide the water passing through, and thus facilitate the oxidation of impurities by means of the oyxgen carried in solution in the water. Filtration is therefore a combination of mechanical, biological, and chemical processes.

Aeration is a natural safeguard that is most active in self-purification

processes. The benefits are those caused by bringing the oxygen contained in the air into intimate contact with the impurities in the water, and thus hastening the oxidation of unstable compounds by providing ample supplies of oxygen for the biochemical reactions taking place. Since oxygen is being continuously absorbed by water surfaces for this purpose, the extension of exposed water surface facilitates aeration. This extension may be effected by waterfalls, riffles, or turbulence caused by irregularities in stream channels, or even by artificial devices. Aeration may at times have a certain mechanical effect by releasing gases that are held in solution and that impart odors and tastes, such as hydrogen sulphide or sulphur dioxide. Ex

cessive carbon dioxide content of underground waters may also be released by aeration.

Impounding or storage of water affords a means for a certain amount of natural purification by facilitating sedimentation of impurities. Finely divided suspended matter, as well as some bacteria that will not settle in moving streams, will be dropped to a large extent where the velocity is sufficiently reduced. Storage also may permit the action of light to bleach and reduce color. Storage also provides time and generally suitable environment for the activities of bacteria-consuming organisms and biochemical reactions that have a beneficial effect upon quality.

Another important factor in self-purification, acting as a natural safeguard, is the extensive variety of microscopic living forms that move about in the water and are commonly known as plankton. Plankton are of two general types-one being food producers which convert inorganic matter into organic compounds, and the other, food consumers or animals which feed on the bacterial flora. The complete significance of plankton life and activity is not understood. However, some of the plant forms liberate oxygen from certain compounds and make it available for oxidation of organic matter in the water. The animal forms, by feeding on the bacterial content, enormously increase the normal death rate of all bacteria in the water, pathogenic forms as well as others, and thus have a distinct bearing on self-purification. This class of plankton is most numerous where its food supply is most plentiful, and hence fluctuates in number with the bacterial content with which it comes in contact.

Cost.

A primary factor in the selection of a source of water supply, and, in fact, often the governing one, is that of cost. Regardless of many other considerations, cost must always be taken into account and will at once rule out, in many cases, otherwise satisfactory sources. Costs should be carefully analyzed for any project and each item should be given its proper weight, especially in those instances where certain factors tend to offset the first cost of apparently more expensive systems considered from the standpoint of installation alone. Thus it might be cheaper to expend considerably more money to secure a naturally soft water than to install a cheaper hard-water supply and then pay for excessive soap waste and increased expense incurred by industrial users.

The factor of cost will also determine whether it is advisable to neglect largely the factors of existing pollution in a source and provide for the artificial purification of the water before use, rather than attempt to eliminate such sources of pollution or seek a source where

such pollution does not exist. Because of the scarcity of naturally pure sources of supply and the prohibitive cost of making such sources satisfactory, various means have been developed for artificially purifying water. A careful analysis of costs will determine just which device or combination of devices should be used for securing a satisfactory supply in any individual case. Cost is therefore a factor that will usually have great weight in the selection of a source.

Summary.

The selection of a source of water supply depends on a number of factors, all of which should be given due consideration and weight before a choice is made.

Determination of the nature and extent of use of the proposed supply will at once indicate the inadvisability of selecting certain inadequate or unsuitable sources. Such determinations will also make evident the necessity for accurate knowledge concerning the quantity and quality of water to be expected from various sources. The possibility of pollution of the source is also a factor to be carefully considered, as well as the natural safeguards that exist or can be inaugurated that tend to counteract such pollution.

Finally, the relative cost of supplies developed from various sources, carefully balanced against their advantages, will determine largely the source to be chosen. This factor may outweigh many other considerations and indicate the advisability of artificial purification rather than the selection of an uncontaminated source.

References.

NOTE. The following list of references is in nowise a complete bibliography on the subject of sources of water supply. The matters discussed in the paper will, however, be found more fully treated in the references quoted.

References to general treatises on water supply.

1. The Value of Pure Water. Geo. C. Whipple. John Wiley & Sons, New York. 2. The Microscopy of Drinking Water. Geo. C. Whipple. John Wiley & Sons, New York.

3. Water Supplies. Samuel Rideal. John Wiley & Sons, New York.

4. Water Supply. 14th Edition. W. R. Mason. John Wiley & Sons, New York.

5. Public Water Supplies. Turneaure and Russell. John Wiley & Sons, New York.

6. Water Purification. J. W. Ellms. McGraw Hill Book Co., New York.

7. Water Works Handbook. Flinn, Weston, and Bogert. McGraw Hill Book Co., New York.

8. Standard Methods of Water Analysis. American Public Health Association, Boston, Mass.

9. Water Supply Papers, U. S. Geological Survey, Washington, D. C.

10. Possible Sources of Water Supply for Sacramento, Calif. Report of Hyde, Wilhelm, and Miller, Engrs., April 15, 1916.

References to current technical literature.

NOTE.--In the list of articles quoted, the publication in which such article appears is designated by letter, which refers to the corresponding journal in the following list:

(a) Engineering News, New York.

(b) Engineering Record, New York.

(c) Engineering and Contracting, Chicago, Ill.

(d) Municipal Engineering, Indianapolis, Ind.

(e) Canadian Engineer, Toronto, Ont.

(f) Engineering, London, Eng.

(g) The Engineer, London, Eng.

(h) The Surveyor and Municipal and County Engineer, London, Eng. (i) Transactions American Society Civil Engineers, New York.

(j) Transactions American Institute Mining Engineers, New York.

(k) Journal, Boston Society Civil Engineers, Boston, Mass.

(1) Journal, New England Water Works Association, Boston, Mass.
(m) Journal, Association of Engineering Societies, Philadelphia, Pa.
(n) Proceedings, American Water Works Association, Troy, N. Y.
(0) American Journal of Public Health, Boston, Mass.

(p) Journal, Franklin Institute, Philadelphia, Pa.

(q) Journal, Engineers Club of Philadelphia, Philadelphia, Pa.

(r) Proceedings Indiana Sanitary and Water Supply Association, Indianapolis, Ind.

(8) Transactions American Microscopical Society, Washington, D. C.

(t) Cornell Civil Engineer, Ithaca, N. Y.

(u) Journal of Electricity, Power, and Gas, San Francisco, Calif.

(v) Coal Age, New York.

(w) Metal Worker, New York.

(x) Power, New York.

(y) Professional Memoirs, Corps of Engineers, U. S. A., Washington, D. C.

(z) Architects and Builders' Magazine, New York.

(aa) American Naturalist, Lancaster, Pa.

(bb) Technology Quarterly, Cambridge, Mass.

(cc) Proceedings Institute Civil Engineers, London, Eng.

SOURCES OF SUPPLY.

11. A Study of a Ground Water Supply: (b) August 3, 1907.

12. The Artesian Water Supply of Australia: (b) November 16, 1907.

13. The Water Supply System of Los Angeles, Calif.: (b) February 29, 1907. 14. A Notable Ground Water Development at Pueblo, Colo.: (b) April 4, 1908. 15. Geologic Basis for Artesian Prediction, N. H. Darton: (b) May 30, 1908.

16. Troubles With Deep Wells: (b) November 26, 1910.

17. Basic Principles of Ground Water Collection: (n) 1913.

18. Sooke Lake Water Supply for Victoria, B. C.: (e) July 23, 1914.

19. The Ground Waters, James Kemp: () Vol. 45.

20. The Growing Value of Ground Water Supples. L. J. Le Conte: (n) December,

1911.

21. Ground Water Supplies, Wm. S. Johnson: (k) May, 1915.

22. Underflow Water Supply at Moline, Kans., W. L. Benham: (a) May 13, 1915. 23. Studies of Artesian Waters in Chicago and Surrounding Territory, Carl B. Anderson and F. W. De Wolf: (n) June, 1915.

21. The Possibility of an Improved Water from Deep Wells in Northern Illinois, C. B. Williams: (n) June, 1915.

25. Experience with Artesian Well Water at Elgin, Ill., R. R. Perkin: (n) June, 1915.

26. The New Well Water Supply of Galveston, Tex.: (c) November 17, 1915.

27. Water from Gravel Wells, C. W. Wiles: (n) December, 1915.

28. A Notable Artesian Water Supply Development at Ogden, Utah, Frank J. Hendershot: (c) December 15, 1915.

29. The Brooklyn Water Supply, Wm. F. Laase: (1) December, 1915.

30. Water Supply of 500,000 Gallons Daily from River Springs: (a) May 4, 1916.

31. Deep Bore Wells for Public Water Supply Purposes, W. H. Maxwell: (g) May 5, 1916.

32. Spring Water for Fort Huachuca: (a) June 1, 1916.

33. The Solution of Water Supply Problem: Merits of Methods Considered for Supplying and Storing Water from a Distant Spring: (u) June 16, 1916.

34. An Economical Water Supply from Flowing Wells: (a) June 29, 1916.

35. Underground Waters: (u) August 19, 1916.

36. New York City Water Supply: The Catskill Mountains Scheme, W. J. E. Binnie: (h) January 5, 1917.

37. Developing a Supply by Tapping Water-Bearing Strata with Tile Pipe Lines, L. V. Martin: (e) February 14, 1917.

38. New York City's Catskill Mountain Water Supply, Alfred D. Flynn: (q) March, 1918: (y) May, 1918.

39. The Catskill Water Supply System, J. Waldo Smith: (n) June, 1918.

40. History of the Artesian Water Supply at Savannah, Ga., E. R. Conant: (n) June, 1918.

41. Artesian Wells for Water Supply with Special Reference to the Artesian Wells of Wisconsin. W. G. Kirschoffer: (d) October, 1918.

QUANTITY OF WATER.

42. Flood Control and Conservation of Water Applied to Passaic River, Morris R. Sherrerd: (b) December 1, 1906.

43. Quarantative Estimation of Ground Waters for Public Supplies, M. L. Fuller: (7) June, 1913.

44. Notes on the Relation Between Rainfall and Yield, D. Halton Thompson: (ƒ) September 4, 1914.

45. Snow Survey Provides Basis for Close Forecast of Watersheds Yield, J. E. Church, jr.: (b) April 17, 1915.

46. Computing Run-off from Rainfall and other Physical Data, Adolf F. Meyer: (i) Vol. 79, 1915.

47. Flow of Water into Wells, N. Werenskiold: (a) August 10, 1916.

48. Storm Water Storage in Texas. J. C. Nagle: (1) December, 1916.

49. Rational Study of Rainfall Data Makes Possible Better Estimates of Water Yield, Robert E. Horton: (u) August 2, 1917.

50. The Yield of Catchment Areas, E. P. Hill: (cc) Paper No. 3641.

QUALITY OF WATER.

51. Odor and Color of Surface Waters, F. M. Drown: (/) Vol. 2, 1888.

52. Odors and Tastes of Surface Waters with Special Reference to Anabaena, D. D. Jackson and J. W. Ellms: (bb) Vol. 10, December, 1897.

53. On the Amount of Dissolved Oxygen and Carbonic Acid Dissolved in Natur. 1 Waters and the Effect of These Gases upon the Occurrence of Micro-organisms, G. C. Whipple and H. N. Parker: (8) May. 1902.

54. Life or Death in "Pure" Country Drinking Water, G. E. Walsh: (z) June, 1906. 55. Prevention of the Growth of Algae in Water Supplies: (b) September 8, 1906.

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