So why deploy these balls — which are typically used by airports to prevent bird congregation on runways — in particular? Some of the other alternatives, such as a large tarp or metal cover, were considered too costly or impractical. The balls, on the other hand, are (relatively) cheap — costing 40 cents each — and are safe for drinking water; black is also the only color able to deflect UV rays.

The DWP has ordered 6.5 million of these balls, 3 million of which it plans on using to blanket the Ivanhoe and Elysian reservoirs. So, yeah, this probably isn’t the best solution for the city’s water woes but, given the circumstances, maybe the only “realistic” option in the short-term.

The water needs to be shaded because when sunlight mixes with the bromide and chlorine in Ivanhoe’s water, the carcinogen bromate forms, said Pankaj Parekh, DWP’s director for water quality compliance. Bromide is naturally present in groundwater and chlorine is used to kill bacteria, he said, but sunlight is the final ingredient in the potentially harmful mix.