What are good indications that water is safe?
In an emergency it should be assumed that all water is at risk of contamination and this includes piped supplies. Thus until told by authorities or having inspected and tested the water it should ideally be treated.
The quality of urban drinking-water supplies is particularly at risk following structurally damaging disasters. Water treatment works may be damaged, causing untreated or partially treated water to be distributed, and sewers and water transmission pipes may be broken, causing contamination of drinking-water in the distribution system.
Floods may contaminate wells, boreholes and surface water sources with faecal matter washed from the ground surface or from overflowing latrines and sewers.
Where there is evidence of faecal contamination of the drinking-water supply, it may be necessary either to modify the treatment of existing sources or to temporarily use alternative sources of drinking-water. It may be necessary to increase disinfection at source or to rechlorinate during distribution. In emergencies, such as during outbreaks of potentially waterborne disease or when faecal contamination of a drinking-water supply is detected, the concentration of free chlorine should be increased to greater than 0.5 mg/litre throughout the system as a minimum immediate response.