WATER 547 



Surface Water. 



We have stated that rain water flows in part over the surface of 

 the ground into natural drainage channels, such as rivers and lakes. 

 The water becomes polluted according to the media through which it 

 passes. 



Lakes. These when natural or artificial form the purest supply 

 Avhen containing upland waters. Their purity is the result of : — 



(i) The action of light, destroying surface bacteria. 



(2) The subsidence of suspended matters entangling bacteria. 



(3) The draining of underground waters which have already 



undergone filtration discharge into it. 



(4) Its free oxidation. 



Alg^e are apt tO' form in all standing waters and should be removed 

 by screening and filtration. It is better to store underground w^ater 

 in covered reservoirs, 12 feet deep at least, witli perpendicular sides 

 to diminish the growth of alga^ when this is troublesome. Copper 

 sulphate can be added also in the strength of i in 1,000,000. 



The catchment area should be cuUi^'ated and be as free as possible 

 from dwellings of man or animals. 



Open conduits for water conveyance should be immediately 

 condemned. 



There should not be anv peat in the catchment area, for not only 

 does it make the water brackish but it is particularly solvent of lead. 

 If the presence of peat cannot be avoided, add lime to the water. 



Rivers. Tropical rivers are usually of greater volume and are 

 less polluted than rivers in luirope, hence the water is purer. 



The fouling of the water by dead bodies in India and Africa is 

 becoming a thing of the past. 



The purity of these waters is due to : — 



(i) The great dilution of any pollution that reaches them. 



(2) The great volumes of filtered water that reach them. 



(3) The gradual subsidence of organic matter to the bottom and 



sides where it undergoes disintegration. 



(4) The oxidation and bactericidal influence of sun and light. 



There is no river long enough in the United Kingdom to 

 become pure by oxidation alone. 

 The water from small shallow rivers is always dangerous and 

 should be avoided. In any case a pipe to draw river water should 

 have its intake as near to the middle of the bed of the river as possible 

 and above any villages that may be on or near its banks. 

 The water should always be boiled and filtered. 

 Tanks and Ponds. Water from surface drainage is often directed 

 into tanks and ponds as is the case at Calcutta, but they are ahvays 



