Nitrate levels in the lake sometimes become higher than the legal limit.
It happens through run-off from chemicals in fields.
Nitrates are especially harmful for babies under 6 months old.
Their bodies aren't developed enough to fight off the nitrate if it's consumed in high levels.
The Nature Conservancy is working with the City of Bloomington on constructing wetlands to naturally dissolve some of the nitrogen.
"That allows you to have two benefits to increase water quality for human consumption and the other to increase quality for fish and wildlife in the river," says Director of Conservation at the Nature Conservancy, Bob Moseley.
The project could also save the city millions of dollars by solving the problem up-stream instead of at the water treatment plant.