Protecting Public Health and the Environment.

Blue-Green Algae and Harmful Algal Blooms: 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 harmful algal blooms (HABs). These surface waters may also have recreational uses such as fishing and swimming. Cyanobacteria, also known as blue-green algae, naturally occur within freshwater ecosystems. Under certain conditions, however, cyanobacteria can grow rapidly and produce toxins that pose a risk to human health. These potentially toxic blooms are known as HABs.

Cyanobacteria are not currently regulated for public water systems. Although there have been no reported impacts to public drinking water systems in Idaho to date, the potential risk exists. Unregulated private drinking water sources that receive drinking water from surface water sources are also at potential risk from HABs.

Public health advisories are coordinated by the local public health districts and DEQ regional offices and are issued when an affected water body has blue-green algae concentrations above levels of concern. For more information on blue-green algae, HABs, and current health advisories for recreational waters, visit DEQ's surface water webpage.

In 2015, EPA issued nonregulatory public health advisories for two types of cyanobacteria (Cylindrospermopsin and Microsystin) and 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


Lake Lowell harmful algal bloom in 2016

Fernan Lake harmful algal bloom in 2016

Staff Contacts

Drinking Water Rule 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

Blue-Green Algae and Harmful Algal Blooms

Recreation Water Quality Health Advisories