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close this book Water purification, distribution and sewage disposal for Peace Corps volunteers
close this folder Section 1: Water supply sources
View the document Overview:
View the document Background information
View the document Evaluation of sources
View the document Factors influencing the quality of water
View the document The quantity of water
View the document Types of sources
View the document Development of water sources
View the document The basic requirements of a water supply
View the document Selection of the source of supply
View the document Lesson plans

Types of sources

SURFACE WATERS

Surface water sources are lakes and ponds, rivers, streams, and controlled catchments (cisterns).

Lakes and Ponds

A lake or pond is any standing body of inland water.

Quantity: Advantageous in that it is usually able to store water in wet periods for use in dry periods.

Quality: Generally poor. Normally turbidity and bacteria are the mayor pollutants. Use only when ground water sources and controlled catchments are not available or are insufficient or inadequate.

Development requirements: The ideal situation is that the watershed permits water of the highest quality to enter the pond. to approach this goal, the watershed should be clean, free from septic tanks, barns, privies, etc., protected against erosion and drainage from livestock areas, and livestock should be excluded (fencing if necessary).

Treatment requirements: No lake or pond water can be considered safe until it has been disinfected. Generally it is also necessary to remove turbidity.

Treatment processes:

1) Sedimentation plant - (w/o alum for coagulation) allows large particles of turbidity to settle out.

2) Filtration to remove turbidity and reduce bacterial content.

3) Disinfection

Warning: Lake and pond waters usually require extensive treatment and a floating intake structure. In many cases the filtration unit becomes clogged and must be cleaned. It is advisable to look elsewhere for a source of water.

Rivers and Streams

General: A stream or river is a body of running water on the surface of the earth, from higher to lower ground.

Quantity: Yield controlled by rate of minimum flow per day and year. Streams generally exhibit marked seasonal variation in flow.

Quality:. Generally poor. Chemical nature partially dependent on bedrock. Physical and bacteriological quality highly variable. Easily contaminated. Impossible to exert sanitary control over watershed.

Development requirements: Requires a submerged intake structure and in the case of small streams requires the construction of small diversion dams.

Treatment requirements: Same as lakes and ponds. Likely to be more turbid and have a greater quantity of chemical pollutants.

Treatment Processes: Same as the lakes and ponds,

Warning: Rivers and streams should be considered last as a potential source of water unless, of course, adequate treatment facilities already exist.

Springs

General: The outflow of water that has previously run or percolated through the pores of rocks. Two types:

1. Gravity-ground water flows over impervious stratum onto ground surface.

2. Artesian-water rise to surface after confinement between two impervious beds.

Quantity: (1) Yield of gravity springs fluctuates with rainfall Characteristically have low discharge. (2) Artesian springs tend to have a nearly constant yield.

Quality: Are subject to contamination near points of emergence. Poor to good. Usually contain dissolved minerals (especially calcium carbonate). Caution* springs emerging from limestone channels allow for very little natural filtration. May become highly turbid and polluted after heavy rains. Careful investigation recommended.

(* Gravity springs have the advantage in that they provide a gravity type distribution system. A filtration and storage unit can be constructed before point of emergence. Such a development is not feasible unless the spring has a substantial yield.)

Development requirements: Elimination of all sources of contamination near point of emergence. If gravity type, further development not recommended. Requirements - -(1) filtration-sometimes not necessary (2) disinfection

Processes: (1) slow-sand filter (2) chlorination

Catch Basins (Cisterns)

General: A sloping surface area for collection of rainfall runoff leading to a covered tank (cistern). Roofs are the most common collection areas.

Quantity: Dependent on amount and variation of rainfall, evaporation, infiltration, and replacement of the soil deficit. For covered surfaces, losses are less than fifteen percent. quality: Wholly dependent on the character of the surface of the area of collection.

Development requirements: Construction of a watertight, manholed covered, tank with outlet.

Treatment requirements: Tank must be periodically disinfected. With fenced catch areas of clay surface, filtration advisable before intake. With roofs, cement or other hard surfaces it is advisable to have some screen to catch leaves, etc. Hard surfaces should be cleaned periodically.

Treatment processes: Disinfection is the only necessary treatment.

GROUND WATER

Ground water serves the great majority of people who live in rural areas and have a water-supply system of one type or another. The reason is that, among the various sources of supply, ground-water is by far the most practical and safe in nature. Even in a highly industrialized country such as the USA, municipal ground-water installations far outnumber surface-water supplies. It is very probable that, for a long time to come, ground water will be the most important source of supply for most rural communities of the world.

The advantages of ground water are:

1. It is likely to be free of pathogenic bacteria;

2. Generally, it may be used without further treatment)

3. In many instances it can be found in the close vicinity of rural communities;

4. It is often most practical and economical to obtain and distribute;

5. The water-bearing stratum from which it is drawn usually provides a natural storage at the point of intake.

The disadvantages are:

1. Ground water is often high in mineral content;

2. It usually requires pumping.

In ground-water supply investigations design, the engineer is concerned with the following steps:

1. to find it in the required quantity and quality as near as possible to the center of consumption, in order to reduce transport costs;

2. to extract it by means of a system which produces the quantity required, safeguards the quality, and, at the same time, involves the least capital outlay;

3. to transport the water to the consumer in a way which requires the least amount of operational and maintenance skill and cost.


Fig. 7 Geological Formations

1 = Areas where there are good possibilities of obtaining water from infiltration galleries, well-point system

2 = Ground water is outcropping at this point, so that e flowing spring, is formed At the foot of river banks and hills other springs may possibly be found

3 = Top of ground-water table

4 = Area of infiltration to supply formation B

A = Non-confined (non-artesian) water bearing formation covered with top soil

B = Confined (artesian) water-bearing formation

C = Impervious rock, or herd pan formation

Ground water is that portion of the atmospheric precipitation, mostly rainfall, which has percolated into the earth to form underground deposits called aquifers (water-bearing formations) (See Fig. 8) These an be tapped by various means, to be discussed later; and, in the great majority of cases, they can be used without further treatment for individual and community water-supplies in rural areas. Fig. shows the occurrence and distribution of subsurface water.


Fig. 8A Occurrence and distribution of sub-surface water


Fig. 8B Occurrence and distribution of sub-surface water

BC = theoretical static level d confined water body

BC' = pressure gradient; indicates actual static level in wells piercing the conduit

Reproduced from Tolman, C. F. ( 1937) Ground water, p. 55, by kind permission of McGraw-Hill Book Co. Inc. New York

The great majority of wells for rural water-supplies take water from the "free-water zone". (Figs. 9, 10). These will usually be jetted, dug, driven, or bored wells. Infiltration galleries also take water from this zone. Drilled wells often penetrate the confined water aquifer. It is from this stratum that flowing wells are developed.

The aquifer must be supplied with an ample quantity of water if it is to serve as a source. It is simply a reservoir and can be depleted in the same manner as a surface reservoir if its supply is inferior to the demand placed on it. In rural areas this is very seldom a concern as the aquifer will usually be replenished sufficiently to supply the relatively small demands of rural communities. An element of greater significance for the engineer searching for ground water pertains to the characteristics of the soil formation of the aquifer, i.e., to the ability of the aquifer to give up water and, therefore, to serve as a reliable source of supply.

The quantity of water that can be extracted from an aquifer will depend on (l) its porosity and permeability, and (2) the draw-down in the well, The porosity and the permeability, of a formation are limited by nature; and while conditions may be altered somewhat in the immediate vicinity of a well intake, the general nature of the aquifer is fixed and cannot be modified. The draw-down in a well, however, can be varied within the limits of the thickness of the aquifer, the penetration of the well into the aquifer, and the capacity of the pump used (Fig. 9,10).

Ground formations, however, have a certain tendency to hold the water and to give up only a Part of it. This characteristic of a soil formation is called permeability; it is the quality of a formation which controls the passage of water through it. From a knowledge of hydraulics, it is obvious that water will pass through large openings more easily than it does through small ones.

Clays and topsoils have high porosity (large volume of voids) but low permeability (very small opening between particles), so that water passes through them with great difficulty. Gravels and sands, on the other hand, are permeable and therefore allow ground water to pass with relative ease. This type of formation is also porous, as can be seen above, so that it can store lame quantities of water. These, then, are the water-bearing formations most amenable to the development of wells and most Important to the engineer in searching for a rural community water-supply. Sandstone is both porous and pervious and therefore an excellent aquifer which can be tapped to produce large quantities of water, especially if it is confined as shown in Fig. 9 (formation B) and Fig. 10 . Where it is known, for example, that sandstone underlies an area, and where no other readily available source is found, a test hole into this stratum would be a good risk. Chalk formations in the British Isles and in Haiti are known to produce reasonable quantities of water.


Fig. 9 Shallow well in tree water zone

A = Ground surface

B = Top layers of soil

C = Water-bearing stratum

D = Impervious stratum

E = Thickness of water bearing

F = Water table

G = Draw-down

H = Depth d penetration of well, into aquifer

I = Draw-down cone

J = Curve of maximum draw-down stratum

P = Pump

R = Radius of circle of influence


Fig. 10 Well tapping confined water

A = Ground surface

B = Top layers of soil

C = Water-bearing stratum

D = Impervious stratum

E = Thickness of water bearing

F = Water table

G = Draw-down

H = Depth d penetration of well, into aquifer

I = Draw-down cone

J = Curve of maximum draw-down stratum

P = Pump

R = Radius of circle of influence

Except for unusual geological features or underground dams, it can be said that, in any drainage basin, ground-water always flows towards the principal streams (Fig. 7 ). While there are exceptions to this rule, the best place to look for shallow ground-water is at the bottom of draws and valleys. It is in this area that pockets of sand and gravel may have been deposited. If these are close to the present stream or in an old course, they will probably be well supplied. Underground sampling by boring or jetting in these areas will usually be profitable. In this way, samples of the underground formation can be taken and examined to determine the characteristics of the aquifer and its ability to supply the quantity of water needed. Fortunately, a great many small towns in rural, underdeveloped areas have been built along natural watercourses, so that the possibility of finding available ground-water as a source of supply may be somewhat improved in such areas.