Harmful Algal Blooms and Drinking Water: What Shows Up in Official Records

What makes a bloom 'harmful'

Some cyanobacteria (blue-green algae) produce toxins — called cyanotoxins — including microcystins, cylindrospermopsin, and anatoxins. These compounds can affect liver and nervous system function at sufficient concentrations. Not all algal blooms produce toxins; laboratory analysis is required to confirm cyanotoxin presence.

Federal standards and the gap

EPA has not established federal MCLs for cyanotoxins in finished drinking water. In 2015, EPA issued health advisories for microcystin-LR and cylindrospermopsin. Some states — including Ohio, Minnesota, and Oregon — have established their own advisory or action levels. Without federal MCLs, cyanotoxin detections do not generate federal violation records.

How HAB events appear in records

HAB-related data may appear in: state-issued public notices or do-not-use advisories (separate from SDWIS); source water assessment reports submitted to state agencies; special monitoring records if a state requires cyanotoxin testing; and Consumer Confidence Reports that describe source water quality challenges.

What this does not mean

• The absence of a cyanotoxin record in federal databases does not mean HABs have not affected a utility's source water.
• Water Utility Report does not determine whether a water supply is safe or unsafe to drink.
• State advisories may apply even when no federal violation record exists.

What to check next

• Search your utility's source water type and recent monitoring records on Water Utility Report.
• Check your state environmental agency's HAB monitoring and advisory pages.
• Review your utility's Consumer Confidence Report for source water quality descriptions.
• If you receive a public notice or advisory from your utility, follow its guidance.