Rainwater is considered a very pure form of water but it dissolves a number of impurities as it courses down the atmosphere and along catchments.  Most contamination of rainwater occurs after contact with the catchment surface (roof or ground) and during subsequent delivery and storage.

If precautions are taken to maintain the cleanliness of the catchment, delivery pipelines and the storage tank, rainwater is well suited for use in a number of non-potable domestic, commercial and industrial applications. Toilet flushing, washing and landscape irrigation are some of the activities for which rainwater can be used after preliminary treatment.

Quality of rainwater assumes greater importance if it is collected and used for potable purposes such as drinking, cooking and washing of vessels. While no water supply is 100 per cent safe at all times, one has to evaluate the “acceptable” level of risk while considering the potability of rainwater. This would depend upon the concentration of toxins present, the level of exposure and the vulnerability of the population. There continues to be considerable debate on whether rainwater meets the "acceptable" level of risk to qualify its use for drinking. There is consensus however, in that the catchment area should be properly maintained and the rainwater should be adequately treated to meet drinking water standards as per IS 10500, 1992 (reaffirmed 1993) before using it for drinking purposes. The quality of water should be checked in a certified laboratory.

If harvested water is being used for artificial recharge of groundwater, extreme care should be taken to ensure that the water for recharge is free from chemical and microbial pollutants that can contaminate the aquifer.  

Source: Rainwater Harvesting  - Trainers’ Manual published by Karnataka Urban Infrastructure Development and Finance Corporation

Chapter 9