Cyanotoxins are the one drinking-water contaminant class with no federal maximum contaminant level, which is exactly why a surface-water operator can sit on a harmful algal bloom and believe there is nothing to do. There is no MCL for microcystins or cylindrospermopsin on the federal books today. What exists instead is a set of 10-day EPA health advisories, a monitoring cycle that already ran, and state thresholds that change from one primacy agency to the next. If your system draws from a lake or reservoir, check your water system's compliance status free at orevant.com before you read on.
There is no MCL, but there is a number
Under the Safe Drinking Water Act, EPA publishes health advisories for contaminants that are not subject to a national primary drinking water regulation (42 U.S.C. § 300g-1(b)(1)(F)). In 2015 EPA issued 10-day drinking water health advisories for two cyanotoxins. For microcystins: 0.3 µg/L for bottle-fed infants and pre-school children, and 1.6 µg/L for school-age children and adults. For cylindrospermopsin: 0.7 µg/L for infants and pre-school children, and 3.0 µg/L for school-age children and adults. A health advisory is not an enforceable standard. It is the concentration below which adverse health effects are not anticipated over the stated duration, with a margin of safety built in. See the difference between an MCL and an action level to place these numbers against the enforceable rulebook.
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UCMR4 already monitored for it
The Fourth Unregulated Contaminant Monitoring Rule (UCMR4) ran cyanotoxin assessment monitoring from 2018 through 2020, testing for ten cyanotoxins and one cyanotoxin group across systems that draw from surface water or groundwater under the direct influence of surface water. Large systems serving more than 10,000 people sampled directly, and a representative set of smaller systems was included to estimate national occurrence. The results put microcystins in the public record as the cyanotoxin most often detected. See what the next UCMR cycle means for small systems.
What a bloom actually obligates you to do
A harmful algal bloom in your source is not itself a violation, but it changes what you are responsible for the moment it appears. If you use surface water, you are already required to monitor turbidity and hold a disinfectant residual, and a bloom attacks both at once: algae spikes turbidity and consumes chlorine, which is how a bloom becomes a disinfectant residual failure and a coliform positive downstream. See the turbidity limits and the chlorine residual floor. Where a state has set its own cyanotoxin threshold, a confirmed exceedance can require a do-not-drink or do-not-use advisory and public notice on the state's clock. See the Tier 1, 2, and 3 public notice deadlines.
How a small system stays ahead of bloom season
Blooms peak in late summer and early fall, when warm water, sunlight, and nutrient load align. The systems that handle them are the ones that know their source is vulnerable before the first scum line appears. A checklist tells you the advisory numbers. It does not tell you what is already open on your system's public record, what your turbidity and residual history shows, or whether your state has a cyanotoxin threshold you are already near. That is the gap the scan closes.
Prefer to see the public record first? Check your water system free at orevant.com.
FAQ
Is there a federal MCL for microcystin?
No. There is no federal maximum contaminant level for cyanotoxins today. The 2015 EPA health advisories (0.3 and 1.6 µg/L microcystins; 0.7 and 3.0 µg/L cylindrospermopsin) are guidance, not enforceable standards, though states may set their own thresholds.
Does my surface water system have to monitor for cyanotoxins?
Only if your primacy agency requires it or you were in the UCMR4 assessment window. UCMR4 monitored for cyanotoxins from 2018 to 2020; routine federal monitoring is not currently required for most systems.
A bloom showed up in our reservoir. Are we in violation?
A bloom itself is not a violation, but it can drive the turbidity and disinfectant residual failures that are, and it can trigger state do-not-drink advisories where thresholds exist. Act on it before it becomes a treatment technique failure.
What do the microgram-per-liter numbers mean for my system?
They are 10-day health advisories. For microcystins, 0.3 µg/L protects bottle-fed infants and 1.6 µg/L protects older children and adults. For cylindrospermopsin, 0.7 µg/L and 3.0 µg/L. They are the numbers states and utilities use to decide when to act.