Environmental experts said the runoff is feeding algae in Geist Reservoir, curbing summer fun in the water, 6News' Ericka Flye reported.

"They want to kill the weeds, make their yards perfect," said Rose Hughes, of Rosie's Gardens and Nursery.

There's a tendency among some homeowners in the Geist areas to have their lawn services do whatever it takes to have the greenest yard in the land, but that is the source of the problem.

"I would say in general, if you have an exceedingly green yard, you're probably over fertilizing," said John South, of the Hamilton County Soil and Water District.

Hamilton County officials sent out fliers asking residents to choose fertilizer with little or no phosphorous and use it sparingly.

The phosphorous runoff can flow directly into the groundwater. The same situation led to a water advisory at the reservoir in 2007, when unsafe levels of blue-green algae were found.

"Those nutrients are the food that the algae takes to live and grow," South said.

Toxic bacteria in the water will significantly cut the types of activities people can do this summer. Officials hope the fliers will keep the water warning from being issued this year.

Hughes said she received one of the letters at her home, but she already knew what to do to help keep the water clean.

"I don't use phosphorous in my fertilizers since I learned about that a while ago," she said. "You'd hate to see one of our nice lakes be polluted."

Experts said algae is blooming rapidly in Geist already because of high levels of nutrients.