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Potential hazard due to Microcystis aeruginosa in Tenmile Lakes
August 31, 2001
Public health officials at the Oregon Department of Human Services and Coos County Health Department are advising lake front property owners, visitors and other interested persons of a significant bloom of Microcystis aeruginosa in Tenmile Lakes, located approximately 10 miles north of Coos Bay on the Oregon coast.
The organism is a blue-green algae capable of producing a dangerous liver toxin called microcystin. The toxin poses danger to humans or to animals using the water. Ingestion of water is the most significant hazard, but direct contact with the water may also pose some danger to humans and animals.
Samples collected on August 21 showed this particular algae at levels ranging from 77 to over 11,000 cells per mililiter of water. Available guidance for direct water contact suggests that cell counts of 15,000 to 20,000 pose hazards to swimmers. Since current cell counts do not necessarily represent the highest levels in the lakes, officials are advising that water contact be avoided.
Algae at current levels may produce toxin levels at or above 1 mg/liter, which is considered the maximum safe level for drinking water. Samples of the water have been submitted for laboratory analysis to measure the kind and amount of toxin. Until further notice health agencies advise:
- That swimming or other water contact activities be avoided in any area in which the water has a green or blue-green appearance or where patches of algae are visible.
- That pets and other animals be kept from access to the lake water until the algae bloom has declined,
- That persons living on the lakefront and using water from the lake for household purposes avoid using the water for drinking, food preparation or other ingestion purposes unless they are certain it is being treated effectively as recommended below. This treatment guidance is the same as that issued in 1997 and 2000.
Drinking water treatment guidance:
- Treatment systems should consist of sand filtration followed by chlorination, followed by activated charcoal filtration. It is essential that sand filtration be done before disinfection to remove as many algal cells as possible without killing or rupturing them.
- Chlorination systems should be capable of maintaining at least 1 ppm of chlorine residual for at least 20 minutes contact time before the water enters the activated charcoal system.
- The final step in the process should be effective activated charcoal treatment to remove toxin remaining after the sand filtration and disinfection processes.
- All treatment equipment used should meet NSF standard 53, and should be adequately sized to treat the maximum amount of water that you use. Treatment equipment needs regular monitoring and servicing to assure that it functions properly.
- Ideally all water entering the home should be treated as recommended. It is possible to treat only water used in the kitchen, but this increases chances that animals or pets would inadvertently drink untreated water.
As more monitoring is done and toxin levels are measured this advisory may be altered. The advisory is to remain in effect until specifically changed or lifted by county and state health officials.
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