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Ten Mile Lake Health Advisory Lifted
December 1, 1997
(PORTLAND) The Oregon Health Services, in coordination with the Coos County Health Department, today lifted a health advisory for Tenmile Lakes. The advisory was issued on October 3, 1997, following the identification of the toxic blue-green algae Microcystis aeruginosa in the lakes.
Microcystis aeruginosa is known to produce toxins called microcystins which can cause liver damage and other health effects in animals or humans. The advisory recommended that ingestion of water from Tenmile Lakes be discontinued until further notice.
Lake water samples were collected in October and November and analyzed for the presence of Microcystis and microcystin toxins. Recent tests indicate that the Microcystis bloom has dissipated and toxin concentrations throughout the lakes are well below 1 microgram per liter (1 part-per-billion), a level that is generally considered safe.
The Health Services's primary concern is for persons who take drinking water from Ten Mile Lakes, utilizing home water treatment systems. To begin to evaluate the effectiveness of these systems, the Health Services collected and analyzed five paired lake water/tap water samples. Home treatment appeared to be very effective at microcystin removal in two of the five systems tested and moderately effective in two others; treatment was ineffective in one system.
Although historic information is limited, it is likely that Microcystis blooms are not uncommon in Tenmile Lakes. The Health Services recommends that persons drawing water from the lakes for drinking and cooking purposes ensure that their home treatment systems are adequate for the removal of microcystin toxins. If properly maintained, a configuration of sand filtration followed by chlorination followed by activated carbon filtration appears to be effective for microcystin removal. It is important that sand filtration be the initial step in the treatment process, as this removes intact algal cells. Exposure of Microcystis cells to chlorine before filtration can cause the cells to die and release their toxins.
The Health Services is continuing to evaluate treatment methods and will be issuing specific recommendations in the near future.
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