Perchlorate is the drinking-water contaminant with a state limit but no federal one, which means a system can be compliant in one state and out of compliance just across a state line. There is no maximum contaminant level for it on the federal books today, yet California and Massachusetts already regulate it, and EPA is moving toward a national standard after a court blocked its attempt to walk away from one. If your system draws groundwater near a former aerospace, defense, or fireworks site, or pulls from the Colorado River, check your water system's compliance status free at orevant.com before you read on.
What perchlorate does in the body
Perchlorate interferes with the thyroid gland's uptake of iodide, which lowers the production of thyroid hormone. That hormone drives prenatal and postnatal growth and development, normal metabolism, and mental function in adults, so the health concern centers on pregnant women, developing fetuses, infants, and anyone with an existing thyroid condition. It is this thyroid effect, not a cancer risk, that sits under every perchlorate standard in the country. See the difference between an MCL and an action level for how a number like this becomes enforceable.
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Where it comes from
Perchlorate and its salts are used in solid propellant for rockets, missiles, and fireworks, and in the production of matches, flares, pyrotechnics, ordnance, and explosives. The form that shows up in groundwater is ammonium perchlorate, a rocket propellant ingredient. California first detected perchlorate in drinking water wells in 1997 in eastern Sacramento County near the Aerojet General Corporation facility, and later across Los Angeles County at several aerospace and defense sites. Sampling also found perchlorate at 5 to 9 µg/L in Colorado River water, an important drinking source for southern California, traced to ammonium perchlorate manufacturing in Nevada. If your source is groundwater near a legacy industrial or defense site, perchlorate belongs on the short list of things you screen for.
The state standards
California was first. Its perchlorate maximum contaminant level is 6 µg/L (6 ppb), effective October 2007, with a public health goal later tightened to 1 µg/L in 2015 and a detection limit for reporting that dropped to 1 µg/L in January 2024. California also regulates perchlorate as an acute contaminant, which means a confirmed exceedance triggers fast confirmation resampling, immediate laboratory notification, and expedited public notice rather than the standard 30 day chronic clock. Massachusetts is stricter, with an MCL of 2 µg/L (0.002 mg/L). There is no federal MCL today, but the picture is moving: EPA's 2020 decision not to regulate perchlorate was vacated and remanded by the D.C. Circuit, and EPA has since proposed a national primary drinking water standard. The trend is toward a federal number, and toward more states following California and Massachusetts in the meantime. See how emerging contaminant funding covers perchlorate projects.
What a detection obligates you to do
A perchlorate detection above your state's limit is a health-based violation with a public notice attached, and in California it moves on the expedited clock because the contaminant is treated as acute. It also lands on your SDWIS record, where an inspector, a buyer, or a lender will read it. See how to read your SDWIS violation record and the Tier 1, 2, and 3 public notice deadlines. The cheapest position is to know your number before a routine sample, or the state's own monitoring, finds it first.
Treatment options
Perchlorate is most commonly removed with perchlorate-selective ion exchange, where a specialized resin strips perchlorate from the water and is regenerated with brine. Biological treatment, which uses microbes to reduce perchlorate to harmless chloride, and reverse osmosis also work. Which one fits depends on your source chemistry and what else is in the water, which is the kind of decision a compliance roadmap should map before you spend on equipment. See how the same treatment decision plays out for radionuclides.
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FAQ
Is there a federal MCL for perchlorate?
No. There is no federal maximum contaminant level for perchlorate today. California regulates it at 6 µg/L and Massachusetts at 2 µg/L, and EPA is moving toward a national standard after a court vacated its 2020 decision not to regulate.
Where does perchlorate in drinking water come from?
Mostly ammonium perchlorate, a rocket propellant ingredient, plus fireworks, pyrotechnics, explosives, and matches. Legacy aerospace, defense, and fireworks sites are the main groundwater sources, along with Colorado River water contaminated by Nevada manufacturing.
Does my system have to test for perchlorate?
If your state regulates perchlorate, as California and Massachusetts do, yes, on the state's schedule. If your state does not, there is no federal monitoring requirement, but systems near legacy industrial or defense sites should screen proactively. Confirm your state's requirement with your primacy agency.
How is perchlorate removed from drinking water?
The most common method is perchlorate-selective ion exchange, followed by biological treatment and reverse osmosis. The right fit depends on your source water chemistry and what else is present.