An Othello well with fluoride levels more than four times higher than the federal limit should be sealed by the end of the week.
City officials expect sealing Well 6 about 980 feet down will decrease the fluoride and allow the well to be used again.
Fluoride in the well's water has been more than 15 parts per million at times, said Othello City Administrator Ehman Sheldon.
Recent tests showed the water at 5 parts per million, which still is above the 4 parts per million limit set by the Environmental Protection Agency.
EPA says exposure to higher fluoride levels can increase the likelihood of adult bone fractures and can children's chances of developing pits in tooth enamel.
The state Department of Health said that fluoride in water can help prevent tooth decay and dental disease.
However, recommended levels are between 0.8 and 1.3 parts per million.
The fluoride appears to be naturally occurring in the inflow to the well system, Sheldon said.
The well is one of eight city wells. But one well has been decommissioned, and Well 6 has been off-line because of the high fluoride.
Work began Monday to seal the lower Grande Ronde aquifer from supplying that well.
Plugging the bottom will mean that the well will pull water only from the Wanapum aquifer, Sheldon said.
The 1,400-foot well was providing about 3,000 gallons of water per minute.
After the well is plugged, he said, the city will be lucky to get 1,100 gallons of water per minute.
After the seal is complete, a smaller pump will be installed, and the city will test to see how much water remains and the quality of the water, he said.
Capping wells is common to prevent aquifers from commingling, said Othello's city engineer Larry Julius from the firm of Gray and Osborne in Yakima.
The Department of Ecology does not want different aquifers mixing together, he said.
The water from each aquifer has a different chemical makeup.
Even after the well is sealed, Sheldon said fluoride levels could remain slightly high.
That's why the city will consider mixing water from that well with another one to lower the fluoride level.
Othello's contractor for the project, Holt Services of Lakewood, will use a tube about 2 to 3 inches in diameter to feed cement into the well, Julius said.
The cement will displace the water and seal it.
A $312,000 state Department of Ecology grant is paying for the project.