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By Robert Ferris

Reports of mysterious "shade balls" in Los Angeles reservoirs have been bobbing to the surface.

But the black plastic spheres are there for a reason. Partially filled with water, they are now floating in Los Angeles reservoirs by the millions as part of a $34 million project to protect the region's water supply from contaminants and evaporation.

The plan is expected to save the city $250 million, compared with other available plans, according to a press release.

The idea was developed by a former biologist with the Los Angeles Department of Water and Power, who was inspired by the use of similar balls to keep birds from landing in ponds along airplane runways. The balls cost 36 cents each to make and are now floating on the surface of four reservoirs in the Los Angeles area, including 96 million on the Los Angeles Reservoir alone.

Pure black carbon gives the balls their color after being added to the plastic to absorb UV rays. That prevents sunlight from penetrating the plastic and getting to either the water inside the balls or beneath them. A lighter-colored ball may allow sunlight to pass through into the water, rendering it ineffective, said Sydney Chase, who owns shade ball manufacturer XavierC, one of the suppliers involved in the project.

Some 96 million plastic "shade balls" have been delployed on the Los Angeles Reservoir in the Sylmar area of Los Angeles to protect water quality. The 4-inch-diameter plastic balls block sunlight from penetrating the 175-acre surface of the reservoir, preventing chemical reactions that can cause algae blooms and other problems.Damian Dovarganes / AP

"Carbon black has been utilized for decades in rugged outdoor environments" precisely because it can help materials withstand sunlight, Chase said.

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The Los Angeles Department of Water and Power began blanketing local reservoirs with the balls to prevent the UV rays from catalyzing certain chemical reactions that could produce algae or harmful chemicals, such as bromate.

Though they originally were intended to protect reservoir water from contaminants catalyzed by sunlight, an added benefit is that the shade balls will prevent the evaporation of 300 million gallons of water, enough to supply drinking water for more than 8,000 people for a full year, according to the press release.