Sodium hypochlorite is a very common method of disinfecting water especially in smaller drinking water suppliers. The quality, storage and use of these chemicals varies considerably. When used it will dissociate to produce some perchlorate ions. Wherever sodium hypochlorite and sodium hydroxide are used there is a potential for perchlorate release into the environment.
A study carried out by NSF International in the US, and published in the Journal of the American Water Works Association found that perchlorate was found in 90% of sodium hypochlorite used at water treatment plants with the concentration increasing as the sodium hypochlorite aged. Their conclusions were: “For utilities that routinely use their sodium hypochlorite within 45 days of manufacture, the contribution of perchlorate is likely to be negligible, unless there is some contamination of the original ingredients. Utilities or small systems that store sodium hypochlorite for longer periods may encounter significant levels of perchlorate in the finished drinking water. To minimize the perchlorate risk, sodium hypochlorite should be stored in the dark, at cool temperatures, diluted if possible, and used within a few weeks of manufacture. Storage tanks and piping should also be emptied of aged material and flushed to minimize the potential for contamination.” Link