Potential recreational and drinking water hazard in Tenmile Lakes

July 06, 2002

Public health officials at the Oregon Department of Human Services (DHS) and Coos County Health Department are advising lakefront property owners, visitors and other interested persons of another bloom of Microcystis aeruginosa in Tenmile Lakes, located about 10 miles north of Coos Bay on the Oregon coast. Similar advisories were issued for these same lakes in 1997 and 2001 for recreational contact. Drinking water advisories have been made for the lakes each year since 1997.

Microcystis aeruginosa is a blue-green algae capable of producing a dangerous liver toxin called microcystin. The toxin poses danger to humans or animals using the water. Ingestion of the water is the most significant hazard, but direct contact with algae may also pose some toxin hazard as well as possible skin reactions.

Samples of the water collected on June 19 contained this algal species at levels ranging from 1888 cells per milliliter of water to over 7000 cells per milliliter. Current guidance for the toxin in water suggests that cell concentrations greater than 2000 may be hazardous to users of the water. Since samples and cell counts do not necessarily represent the highest concentrations in the lakes, officials are advising that direct water contact be avoided at any point where there is visible algae or where the water has a bluish or bluegreen color.

Water from the lakes should not be used for domestic purposes unless it has first been carefully treated as recommended below.

At current cell densities, it is very likely that toxin levels exceed 1 microgram of toxin per liter of water, the maximum level considered tolerable in drinking water. Future samples found to have greater than 15,000 cells per milliliter will be tested for toxin.

Until further notice from DHS and Coos County Health Department, it is advised that:

• Swimming, wading or other skin contact with lake water be avoided in any area where there is visible algae in the water or where the water has a bluish or blue-green color;

• Pets and animals should be kept away from the water, and should not be allowed to drink water from the lakes;

• Persons living on the lakefront and using water from the lakes for household purposes should not use the water for drinking, food preparation or any other ingestion purpose, unless it has been treated effectively as recommended below.

• Drinking Water Treatment Guidelines:

• Treatment must consist of sand filtration followed by chlorination, followed by activated charcoal filtration. It is essential that the sand filtration be done before disinfection, in order to remove as many of the algal cells as possible without killing or damaging the cells;

• Chlorination systems must be capable of maintaining at least 1 ppm free chlorine residual in the water for at least 20 minutes before the water enters the charcoal filtration system;

• The final step in the treatment process must be activated charcoal treatment to remove toxin remaining from the earlier treatment steps;

• All treatment equipment must meet NSF standard 53 and needs to be sized adequately to treat the maximum amount of water needed. Treatment equipment must be regularly monitored and maintained to assure that the water is adequately treated at all times.

• Ideally all water entering the home should be treated as recommended. It is possible to treat water used only in the kitchen but this greatly increases the chances that people (particularly elderly and children), pets and animals will inadvertently drink untreated water. It also fails to protect humans from harm due to skin absorption and skin reactions.