Follow the money.
LA has done this before:
Ivanhoe Reservoir Covered With 400,000 Black Plastic Balls
In 2007, the Department of Water Protection in Los Angeles detected high levels of bromate, a carcinogen that forms when bromide and chlorine react with sunlight, in Los Angeles’s Ivanhoe Reservoir. Bromide is naturally present in groundwater and chlorine is used to kill bacteria, but sunlight is the final ingredient in the potentially harmful mix. The 102-year-old facility serves about 600,000 customers downtown and in South Los Angeles. When the Department of Water Protection realized the problem, they began construction of a new underground reservoir in Griffith Park, but while the new facility was being built they had to determine a way to keep the sunlight out of the water.
The possibility of tarps and metal coverings were explored but they were either too expensive or will take too long to install. So one of the DWP’s biologists, Brian White, suggested “bird balls,” commonly used by airports to prevent birds from congregating in wet areas alongside runways. The balls are made of polyethylene and cost only 40 cents each. The coating contains carbon and black is the only color strong enough to deflect ultraviolet rays.
400,000 balls were dropped into the reservoir on June 2008, where they will remain for the next four to five years until the new underground reservoir is completed.