Protecting Public Health and the Environment.

Cyanotoxins: Information for Public Water Systems

Most of Idaho’s drinking water comes from ground water sources. However, approximately 5% of public water systems in Idaho draw from surface water that may be at risk for cyanobacteria blooms, commonly referred to as harmful algal blooms (HABs). The cyanobacteria are naturally occurring and are often referred to as blue-algae but they are actually bacteria that photosynthesize like algae and plants. . Under certain conditions, however, cyanobacteria can grow rapidly and produce toxins called cyanotoxins that pose a risk to human health.

Cyanotoxins are not currently regulated for public water systems. Potential risks to public drinking water exist. Cyanotoxins have been reported in Idaho’s surface waters and in July 2018, one public drinking water system was impacted by a HAB; however, cyanotoxin levels in the treated (finished) water were below health advisory levels. Unregulated private drinking water sources that receive drinking water from surface water sources are also at risk from cyanotoxins.

Public health advisories are coordinated by the local public health districts and DEQ regional offices. Advisories are issued when cyanotoxin concentrations in a water body are above acceptable levels established by the US Environmental Protection Agency (EPA). For more information on drinking water advisories, visit DEQ’s Drinking Water Health Advisory webpage.

Due to the public health risk associated with cyanotoxins, EPA established nonregulatory drinking water public health advisories in 2015 for two types of cyanotoxins (Cylindrospermopsin and Microcystin). EPA also issued “Recommendations for Public Water Systems to Manage Cyanotoxins in Drinking Water” for the protection of public health.

The recommended management steps include the following:

  • Evaluate a systems source water (e.g., water quality parameters, climate, land use)
  • Prepare for when blooms are likely to occur by evaluating the current treatment process for vulnerabilities and visually observing source water for bloom occurrence
  • Monitor for cyanotoxins in raw and treated water
  • Make treatment adjustments, if needed, and communicate this with the public

For more information on cyanobacteria, HABs, and current health advisories for recreational waters, visit DEQ's surface water webpage.


Lake Lowell bloom in 2016

Fernan Lake bloom in 2016

Staff Contacts

Drinking Water Analyst
Maureen Pepper
DEQ State Office
Water Quality Division
1410 N. Hilton
Boise, ID 83706
(208) 373-0174

DEQ Resource

Cyanobacteria: Facts and Response Actions for Idaho's Public Water System Operators

Public Water System Resources

A Water Utility Manger’s Guide to Cyanotoxins - Water Research Foundation, April 2015

Fast Facts: Blue-green Algae Q&A for Water System Operators - Oregon Department of Health

First water filter certified to reduce microcystins in drinking waterNSF International

Idaho Department of Health and Welfare, HABs Educational Materials

Laboratory Resources

Information for Public Water Systems - Ohio EPA HABs Program - extensive website and materials for public water systems

Managing Cyanotoxins in Drinking Water: A Technical Guidance Manual for Drinking Water Professionals - Water Environmental Federation, September 2016

Recommendations for Public Water Systems to Manage Cyanotoxins in Drinking Water - EPA, June 2015

Understanding Cyanobacteria and Cyanotoxins - Water Research Foundation, August 2016

Related Pages

Cyanobacteria Harmful Algal Blooms

Recreation Water Quality Health Advisories