Perchlorate is both a naturally occurring and synthetic chemical, used in fertilizer, rocket fuel, fireworks, and explosives, and at one time used to treat hyperthyroidism. It contaminates the soil around many military bases and defense plants, and the EPA has detected it in water supplies of at least half the states in the US.
In 2011, the Obama adminsitration reversed a Bush ruling that perchlorate is of no consequence to health and instructed the EPA to go about regulating levels in water.
In high concentrations, perchlorate can interfere with iodide uptake by the thyroid gland, reducing thyroid hormone production and interfering with normal growth and metabolism.
The best treatment is reverse osmosis. Anion exchange, carbon adsorption and possibly distillation can also be effective.
(whole house & well units)