LA officials used 400,000 black balls to help save the LA water reservoir.

Los Angeles had a huge problem with its water reservoir. It contained high levels of bromate and chlorine. The two chemicals in and of themselves were not a problem as neither is particularly hazardous on its own. The problem was that the two chemicals could be reacted by the heat of the sun forming a chemical that could be cancer causing over time. 

Many different people worked on finding a solution to the problem. The best way to save the water seemed to be to build something to shade the water to prevent reactions occurring between the bromate and chlorine. The problem was that any plan to shade that much water was highly expensive and labor intensive. 

Finally, a biologist in the area came up with a financially practical idea. His plan was to drop 400,000 little, black plastic balls into the reservoirs. All these balls would cover the surface of the water, shading it and preventing the reactions in the water. These thousands of balls were poured into the water by workers, officials, and community activists.

(Source)