In 2015, Los Angeles dropped 96 million black balls into a drinking-water reservoir. 10 years later, the bizarre experiment solved two major problems

Los Angeles did something back in 2015 that looked almost impossible to explain with simple logic. They released 96 million black plastic balls onto the surface of one of its drinking-water reservoirs, turning a huge stretch of water into what looked like a giant black carpet. The unusual sight quickly attracted attention far beyond California. Pictures and videos of millions of balls being poured into the Los Angeles Reservoir spread online, with many people wondering why a city would deliberately cover its drinking water with so much plastic.

The reason, however, was linked to a problem California had been struggling with for years. This project was not meant as a publicity stunt or some unusual experiment for the sake of it. The balls, known as shade balls, were part of a larger effort by the Los Angeles Department of Water and Power (LADWP) to protect the city's water supply.

It started with a serious water shortage

The timing of the project was important as California was in the middle of a severe drought in 2015, and water conservation had become a major concern across the state. Residents were being asked to cut down on water use, while authorities were looking for ways to protect the water already stored in reservoirs.


The Los Angeles Reservoir in Sylmar covers about 175 acres and can hold billions of gallons of water. Leaving such a large body of water exposed to the hot California sun meant that some of it would naturally disappear through evaporation.

Officials estimated that covering the surface with shade balls could prevent the loss of more than 300 million gallons of water every year, as per LADWP.

The balls were around four inches in diameter and made from high-density polyethylene, a type of plastic also used in products such as water pipes and milk bottles. They were manufactured for use around drinking water.

But conserving water was only one reason for covering the reservoir.

The black balls had another job

There was a less visible problem happening beneath the surface.

The water stored in the reservoir contained naturally occurring bromide. During the treatment process, bromide can react with ozone, and sunlight can contribute to the formation of bromate. Bromate is a regulated drinking-water contaminant and is considered a potential health concern at elevated levels.

The simplest way to reduce the sunlight reaching the water was to cover the surface and that is where the millions of black spheres came in.

The balls floated across the reservoir and blocked sunlight from reaching much of the water below. They also reduced conditions that encourage algae growth, which could otherwise create additional problems for water treatment.

Richard Harasick of LADWP explained the idea at the time in comments reported by BBC Mundo.

“These balls block sunlight, preventing algae from growing. When the water leaves the reservoir, it goes through an ultraviolet treatment plant that eliminates pathogens,” Harasick said.

The project therefore addressed two different concerns at once — keeping more water in the reservoir and reducing problems associated with sunlight.

Why were there 96 million of them?

Covering roughly 175 acres of water required an enormous number of individual balls.

The spheres were designed to float freely rather than being attached to one another. Each one contained some water, which gave it additional weight and helped prevent it from being blown away.

Their black colour also served a purpose. Carbon black was added to the plastic to protect it from ultraviolet radiation and slow down damage caused by long-term exposure to sunlight.

Workers did not have to place the balls individually. They were delivered to the reservoir in large quantities and released into the water, where they spread across the surface.

It was a relatively simple solution to a very large infrastructure problem.

The entire project cost around $34 million, with the balls costing roughly 36 cents each. Building a large permanent cover over a reservoir of this size would have been considerably more expensive.

The strange solution had one environmental catch

The shade balls did not completely settle the debate around water conservation.

A team of researchers from MIT, Imperial College London and the University of Twente later looked at another part of the story,the water needed to manufacture the plastic balls in the first place.

As reported by PBS, the researchers estimated that producing the 96 million spheres could require between 66 million and 766 million gallons of water. The wide range was linked partly to differences in manufacturing processes and the thickness of the plastic.

This meant that the balls had to remain in the reservoir for a period of time before the water saved through reduced evaporation outweighed the water used to manufacture them.

The researchers estimated that this could take between about one and 2.5 years.

That did not mean the project had failed. Instead, it showed that even a water-saving technology can have an environmental cost elsewhere in its manufacturing process.

What changed after a decade?

The original shade balls were not designed to last forever. Officials had estimated a useful life of around 10 years, after which the plastic would gradually begin to deteriorate. But by then, Los Angeles had also developed another part of its water-treatment system.

The city completed a major ultraviolet treatment facility for the reservoir in 2022. The facility treats water as it leaves the reservoir and provides another layer of protection against contaminants and microorganisms.

The shade balls also had other practical benefits during their years on the reservoir. Their presence discouraged birds from landing on the water and reduced the amount of sunlight available for algae growth. The floating layer could also limit some wind-blown material from reaching the water.

The balls were therefore doing considerably more than simply making the reservoir look unusual.