Inorganic contaminants, or IOCs, are the metals and minerals EPA regulates under a single monitoring framework, and they trip up more small systems than any contaminant class except disinfection byproducts. The list is short but the numbers are unforgiving: several are measured in parts per billion, and two of them carry an acute, 24 hour public notice clock rather than the 30 day chronic one. If you are not sure what your last IOC round showed, check your water system's compliance status free at orevant.com before you read on.
The full IOC MCL list, in one place
The inorganic contaminant maximum contaminant levels all live in one subsection of the rulebook, 40 CFR § 141.62(b). They are:
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- Antimony — 0.006 mg/L
- Arsenic — 0.010 mg/L
- Asbestos — 7 million fibers per liter (fibers longer than 10 micrometers)
- Barium — 2 mg/L
- Beryllium — 0.004 mg/L
- Cadmium — 0.005 mg/L
- Chromium (total) — 0.1 mg/L
- Cyanide (as free cyanide) — 0.2 mg/L
- Fluoride — 4.0 mg/L
- Mercury (inorganic) — 0.002 mg/L
- Nitrate (as nitrogen) — 10 mg/L
- Nitrite (as nitrogen) — 1.0 mg/L
- Nitrate + nitrite combined (as nitrogen) — 10 mg/L
- Selenium — 0.05 mg/L
- Thallium — 0.002 mg/L
Most of these are naturally occurring in soil and rock and leach into groundwater, which is why groundwater systems carry the bulk of IOC risk. See the individual explainers for arsenic, chromium and chromium-6, and fluoride, and the difference between an MCL and an action level for the framework these numbers sit in.
Two of them are acute, the rest are chronic
Nitrate and nitrite are the exceptions in the IOC list, and the exception matters for public notice. Because nitrate converts to nitrite in the body and can reduce the blood's ability to carry oxygen in infants, a nitrate or nitrite exceedance is an acute violation with a Tier 1 public notice due within 24 hours. Every other IOC is a chronic contaminant, which runs on the Tier 2 clock of 30 days. See what a nitrate violation obligates you to do and the Tier 1, 2, and 3 public notice deadlines.
How IOC monitoring actually works
IOC sampling runs under the standardized monitoring framework in 40 CFR § 141.23. For groundwater systems the routine schedule is one round every three years, while surface water systems sample annually, and each cycle is a nine year compliance period. A system with a clean history can qualify for reduced, nine year monitoring, but the privilege is earned, not automatic, and it is lost the first time a sample comes back over an MCL. Nitrate is the exception again: it is monitored more frequently because it is acute and because it moves with shallow groundwater, and it is the IOC a small system is most likely to exceed. See how monitoring frequency works for coliform for the same logic applied to the rule most systems violate.
IOCs are not radionuclides, and they are not secondary standards
Three contaminant classes get confused because they all live in groundwater. Inorganic contaminants (40 CFR § 141.62, monitored under § 141.23) are the metals and minerals above. Radionuclides such as combined radium-226/228 (5 pCi/L), gross alpha (15 pCi/L), and uranium (30 µg/L) are a separate class with their own rule and their own sampling. And secondary standards like the 2.0 mg/L fluoride level or the 0.05 mg/L manganese level are aesthetic guidelines under 40 CFR § 143.3, not enforceable MCLs. Knowing which bucket a number falls in is the difference between a real violation and a customer complaint. See the radionuclides explainer and how a secondary standard differs.
The cheapest fix is a current sampling record
An IOC that has never been screened is not clean, it is unknown, and unknown is the most expensive status when the state asks. The $199 compliance scan maps your system's open violations and monitoring schedule against federal and state timelines so you know which IOCs you owe a sample on and which result is already over the line, before a routine sample or the state's own monitoring finds it first.
Prefer to see the public record first? Check your water system free at orevant.com.
FAQ
Which inorganic contaminants are acute?
Nitrate and nitrite. Because of the infant health risk, an exceedance triggers a Tier 1 public notice due within 24 hours. Every other IOC is chronic and runs on the 30 day Tier 2 clock.
Are radionuclides inorganic contaminants?
No. Radionuclides (radium, gross alpha, uranium) are a separate class with their own rule and monitoring schedule, even though they, like IOCs, occur in groundwater. See the radionuclides explainer.
How often does my system have to sample for IOCs?
Under 40 CFR § 141.23, groundwater systems sample IOCs once every three years and surface water systems annually, on a nine year compliance cycle. A clean history can earn reduced nine year monitoring. Nitrate is sampled more frequently.
We have never tested for IOCs. Are we in violation?
Missing a required IOC sample is a monitoring and reporting violation on its own, even if the water is clean, and it stays on your SDWIS record until corrected. Confirm your schedule and catch up on sampling rather than waiting for the state to flag it.