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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

Factors influencing the quality of water

As water goes through its hydrologic cycle, it gathers numerous impurities. Dust, smoke, and gases fill the air and tend to contaminate rain, snow, hall, and sleet. As runoff, water picks up silt, chemicals, and disease organisms. As it enters the earth through seepage and infiltration, some of the suspended impurities may be filtered out, but at the same time, other minerals and chemicals are dissolved and carried along. It is now ground water in an underground deposit and, although it may now become less contaminated or polluted, it is not necessarily pure, and may contain disease organisms as well as harmful chemicals.

In addition to the impurities in water resulting from infiltration, many are contributed by an industrialized society. Garbage, sewage, industrial waste, insect sprays, and chemical, biological, and radiological agents are examples of these.

Impurities in water are either suspended or dissolved. The suspended impurities are usually more dangerous to health. They include mineral matter, disease organisms, silt, bacteria, and algae, and must be destroyed or removed from water that is to be consumed.

1) The main factors influencing the quality of a given water supply source are:

A) Nature of the surface geology; character of soils and rocks.

B) Character of vegetation; forests; cultivated and irrigated lands, including salinity, effect on irrigation water, etc.

C) Methods of sewage disposal whether by diversion from watershed or by treatment.

D) Character and efficiency of sewage-treatment works on watershed.

E) Proximity of sources of faecal pollution to intake of water supply.


The turbidity test is used to show the amount of suspended matter present in raw water, and also to determine the amount removed from treated water. The test may be made with the white porcelain cup with its black enameled dot, or by employing the turbidimeter and standard turbidity solution. Rapid approximate readings only can be made by using the cup.

Turbidity Determination by Use of Measuring Cup:

The measuring cup can be used to determine whether raw water has more or less than 100 turbidity units. If the black spot cannot be seen when the cup is filled to the top with the water sample, the turbidity is 100 units or over. The turbidity is less than 100 units if the outline of the black spot is visible.

Fig. 6 Turbidity Test

Waters having low turbidities, such as effluent from a purification unit, may be checked by adding about 1/2 inch of water to the cup and looking at the black spot, it appears as black as it was originally, the turbidity is less than 5 units. Turbidities over 5 unfits produce a graying or milky hue in the black spot.