Chromium is the one contaminant where a system can be under the federal limit and over a state limit at the same time, because it is two numbers, not one. The federal maximum contaminant level for total chromium is 0.1 milligrams per liter, set in 40 CFR § 141.62(b). But the form of chromium that made national headlines, hexavalent chromium, or chromium-6, has no federal MCL at all. It is regulated state by state, and California has set it at 0.010 mg/L. If you are not sure what your last chromium readings showed, or whether your state even requires you to test for chromium-6, check your water system's compliance status free at orevant.com before you read on.
Total chromium is the federal number
Total chromium is the sum of all chromium forms in the water, and the federal MCL is 0.1 mg/L (100 µg/L) under 40 CFR § 141.62(b). It is an MCL, not an action level, which means a confirmed sample at or above it is a health-based violation with a public notice attached. Because chromium is a chronic contaminant rather than an acute one, the notice runs on the Tier 2 clock: 30 days, not the 24-hour Tier 1 clock of nitrate or E. coli. See the Tier 1, 2, and 3 public notice deadlines and the difference between an MCL and an action level.
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Chromium-6 is the state number, and California wrote the book
Chromium occurs in water in two main forms: trivalent chromium (chromium-3), an essential nutrient at low doses, and hexavalent chromium (chromium-6), the form EPA classifies as likely carcinogenic when ingested. Chromium-6 became a household name after the groundwater contamination in Hinkley, California, made famous by the film Erin Brockovich. There is still no federal MCL for chromium-6 alone, but California adopted one: 0.010 mg/L (10 µg/L), effective July 1, 2014. A California system can therefore be under the 0.1 mg/L total chromium MCL and still exceed the chromium-6 limit, and the exceedance is a violation on its own. States without a chromium-6 MCL still hold you to the total chromium number.
How chromium is monitored
Chromium is an inorganic chemical (IOC) monitored under 40 CFR § 141.23. For groundwater systems the standardized monitoring framework calls for routine sampling every three years; surface water systems run on a longer cycle. The practical risk is the same one that applies to arsenic and radionuclides: if your wells have never been screened for the full IOC list, or the last sample is years old, your status is effectively unknown, and unknown is the most expensive status to be in when the state asks. See how the same monitoring logic applies to arsenic and radionuclides.
What an exceedance actually triggers
A confirmed chromium MCL exceedance lands on your SDWIS record as a health-based violation, and it starts a sequence: a Tier 2 public notice to every customer, a compliance schedule with your primacy agency, and a treatment or source change to get back under the limit. See how to read your SDWIS record so you know what an inspector or a buyer sees. The cheapest fix is to confirm your number before the exceedance is discovered by a routine sample or, worse, by the state's own monitoring.
Treatment options for chromium
Chromium-6 is removed in a few established ways. The most common is to reduce chromium-6 to chromium-3 with a reducing agent, then remove the chromium-3 by coagulation and filtration. Anion exchange and reverse osmosis also remove chromium-6. Which one fits depends on your source water chemistry and what else is in it, which is exactly the kind of decision a compliance roadmap should map before you spend on equipment.
Prefer to see the public record first? Check your water system free at orevant.com.
FAQ
Is there a federal MCL for chromium-6?
No. The federal MCL covers total chromium (0.1 mg/L, 40 CFR § 141.62). Hexavalent chromium alone has no federal MCL; California set one at 0.010 mg/L effective July 1, 2014, and other states may set their own.
Do I have to test for chromium-6 separately from total chromium?
It depends on your state. Total chromium is part of the standard IOC sampling set. Where a state regulates chromium-6, as California does, it requires chromium-6 specific sampling on its own schedule. Confirm your state's requirement with your primacy agency.
What notice applies if I exceed the chromium MCL?
Chromium is a chronic contaminant, so an exceedance triggers a Tier 2 public notice due within 30 days, filed with your state and delivered to customers, rather than the 24 hour Tier 1 clock of an acute contaminant like nitrate.
We are under the total chromium MCL. Are we compliant?
Under the federal rule, yes, but if your state regulates chromium-6 you must meet that separate, much lower number too. Meeting the federal total chromium MCL does not clear a state chromium-6 exceedance.