County Health Services Urges Discovery Bay Residents to Be Cautious for Blue-Green Algae


Contra Costa Health Services received several calls in June from Discovery Bay residents concerned about blue-green algae in the water near their homes and its possible impact to their health and the health of their animals.

Results from water samples taken in the southwestern portion of Discovery Bay found levels of toxin above the state guideline. In those areas, we advise residents to stay out of the water and keep their pets out of the water.

Water in Discovery Bay is actively pumped from one place to the next and thus water that is in one location today is in a different location tomorrow. Thus we also advise all Discovery Bay residents that it is prudent to be cautious and to stay out of the water if they see scum or algae blooms.

We need your help getting the word out. In 2016, in at least one instance a child became sick after swimming in Discovery Bay.

We are urging anyone who has swam in an area where blue-green algae is present and who has developed symptoms to talk to their health care provider right away or call California Poison Control at 1-800-222-1222.

Updated information about blue-green algae in Discovery Bay including a map of the areas that were sampled is available on our website at

Via the County Website:


Is it safe for me to go in the water in Discovery Bay?

It depends where you are and if there are advisories posted in the area.

A CAUTION sign means:

  • Do not swim or wade near algae or scum
  • Keep your children away from algae in the water or on the shore
  • Do not drink the water or use it for cooking
  • Do not let pets or livestock go into or drink the water or eat scum on the shoreline
  • Do not eat shellfish from the water


A WARNING sign means:

  • No swimming
  • Stay away from scum, and cloudy or discolored water
  • Do not use these waters for drinking or cooking
  • Do not let pets or livestock go into or drink the water, or go near the scum
  • Do not eat shellfish from these waters
  • For fish caught here, throw away guts and clean fillets with tap water or bottled water before cooking


A DANGER sign means:

  • Stay out of the water until further notice. Do not touch scum in the water or on shore.
  • Do not let pets or livestock drink or go into the water or go near the scum
  • Do not eat fish or shellfish from these waters
  • Do not use the water for drinking or cooking. Boiling or filtering will not make the water safe.


Exposure to blue-green algae can cause rashes, skin and eye irritation, allergic reactions, gastrointestinal upset, and other effects. At high levels, exposure can result in serious illness or death, according to the California Department of Public Health.

What about my pets?

Keep pets out of water with blue-green algae blooms. Dogs are especially vulnerable to getting sick and there have been reports through the years of dogs dying following exposure associated with drinking the water and licking algae from their fur after wading/swimming in water with blue-green algae blooms.

Is there an issue with drinking water?

No, contamination only affects recreational water areas, not drinking water from the tap in Discovery Bay.

Who should I contact if I swam in the water and now have symptoms?

If you think you or someone else is displaying symptoms of blue-green algae poisoning, contact your health care provider or the California Poison Center Help Hotline at 1-800-222-1222.

What can I do to get rid of blue-green algae in Discovery Bay?

We do not recommend attempting to treat or remove the algae. Blue-green algae blooms are natural to the environment’s food chain, and eventually dissipate on their own. Chemical treatment is often ineffective and can result in more toxins being released into the water as algae cells die, as well as unintended effects on other species in the area.

Algae blooms result from changes in water conditions that encourage sudden growth of the species. The best way to reduce and prevent blooms is to reduce water pollution, particularly from runoff containing fertilizers or pesticides. Pick up trash dumped in waterways, and make sure all household sewer systems are working properly.

Treating aquariums with a hydrogen peroxide solution is a common way to get rid of some kinds of algae. But the Delta is not a closed system like an aquarium, so the results would be unpredictable, both in terms of efficacy and impact on the environment.