In August 2015, Los Angeles released 96 million black plastic balls onto the surface of its Sylmar reservoir. The project covered about 70 hectares and cost $34.5 million, with officials expecting it to prevent the loss of more than 1.1 billion litres of water a year during California’s prolonged drought, which lasted from 2011 to 2017.
The balls were intended chiefly to protect water quality, rather than simply reduce evaporation. Sunlight can help convert bromide in the reservoir into bromate, a potentially cancer-causing byproduct regulated in drinking water. The spheres blocked about 95% of sunlight, also limiting algae growth and reducing the need for chlorine treatment.
Each ball was about 10 centimetres across and made from high-density polyethylene suitable for contact with drinking water. Carbon black helped shield the plastic from ultraviolet radiation, while water inside each sphere kept it from being blown away. The approach was far less expensive than a proposed system of dividing the reservoir and installing large floating covers, initially estimated to cost at least $300 million.
A 2018 study by researchers from MIT, Imperial College London and the University of Twente examined a cost not visible from the reservoir’s edge: the water used to make the balls. Taking into account the petroleum, natural gas and electricity involved in producing polyethylene, the researchers estimated manufacturing-related water consumption at between about 250 million and 2.9 billion litres, depending on production conditions and the plastic’s characteristics.
In the study’s reference scenario, producing the balls required about 2.9 million cubic metres of water, while the estimated savings over the drought period examined amounted to 1.7 million cubic metres. The researchers calculated that it would take around two and a half years of comparable savings to offset the manufacturing water footprint. The payback could take longer outside drought conditions, when evaporation is lower.
The study did not conclude that the balls were ineffective. Rather, it showed that judging their environmental benefit requires looking beyond the reservoir itself and accounting for the resources used in production.
The balls were expected to last at least ten years. The Los Angeles Department of Water and Power warned that they could deteriorate, split at the seams or break, and said they could be removed, recycled and periodically replaced. As of 2026, the available information does not confirm how many have deteriorated or been replaced, or whether there is an updated schedule for replacing the full installation.
Los Angeles had previously used the balls at the Ivanhoe, Elysian and Upper Stone Canyon reservoirs, where they were considered temporary measures while other infrastructure was developed. The Sylmar reservoir posed a different challenge because of its size and the high cost of conventional covers. The department continues to describe the balls as a way to protect water quality, alongside ultraviolet disinfection, but no official information specifies whether Los Angeles plans to replace them with floating solar panels or another system.
Sources: Xataka.