Top 5 Takeaways

  1. Persistent Cyanobacteria HABs: Clear Lake experienced multiple cyanobacteria harmful algal blooms (HABs) from June to November 2021, with microcystins levels often exceeding California’s health advisory limit.
  2. Impact on Private Drinking Water: Homes around Clear Lake, using private water systems or wells, faced uncertainty regarding water safety due to these blooms.
  3. Cal-WATCH Project Findings: Analysis of tap water from homes with private lake water intakes and near-shore wells revealed microcystins in 22 of 31 homes with lake intakes, but none in homes with wells.
  4. Emergency Measures: A “Do Not Drink” advisory was issued in September 2021 for private lake water systems, with free drinking water provided to affected residents.
  5. Future Strategies: The report emphasizes the need for frequent monitoring, public education, and long-term strategies like transitioning homes to public water systems to mitigate risks from HABs.

Original Article Author and Citation

Corresponding Author

Gina M. Solomon,

Suggested Citation

Solomon GM, Stanton B, Ryan S, Little A, Carpenter C, Paulukonis S. Notes from the Field: Harmful Algal Bloom Affecting Private Drinking Water Intakes — Clear Lake, California, June–November 2021. MMWR Morb Mortal Wkly Rep 2022;71:1306–1307. DOI:


During 2021, Clear Lake in California experienced significant harmful algal blooms (HABs) caused by cyanobacteria. Microcystin levels, a cyanotoxin, frequently surpassed the state’s health advisory limit, posing risks to private drinking water systems.


The California Water: Assessment of Toxins for Community Health (Cal-WATCH) project conducted a study involving sampling and analysis of tap water from homes using private lake water intakes and wells close to the shore.


The study revealed microcystin presence in 22 out of 31 homes with lake water intakes. The emergency measures included a “Do Not Drink” advisory and provision of free drinking water. Frequent monitoring and public education were identified as key in managing such events.


The increasing frequency of HABs, potentially exacerbated by climate change, necessitates robust public health responses, including transitioning homes to public water systems and long-term environmental strategies.


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