Here's something you could spend an entire career studying without ever quite believing: 96 million black plastic balls covering an LA reservoir. The number alone is absurd. Derek Muller visits the largest collection of these spheres anywhere on earth at LA Reservoir, and what he uncovers is a story about sunlight, chemistry, and one accidental scientific discovery that changed how cities treat drinking water.
The Bromate Problem
Muller explains the core issue with a clarity that makes the science easy to follow. "The problem all started with bromide," he writes. "Bromide is a naturally associated with salt water and so normally places like the california aqueduct that comes down from the delta you get some salt water intrusion she has some bromide in the water." This sounds like a minor footnote, but it's not. Bromide is harmless on its own — until it meets chlorine under sunlight. "When you disinfect the water with ozone that bromide becomes bromate and bromate's carcinogenic," Muller explains. The regulations around year 2000 required treatment plants using ozone to watch their bromate formation carefully.
But then an unusual call came in from a beverage company in LA. They had some really high levels of bromate showing up in their product. The utility was confused because between the filtration plant and the customer was the reservoir — and they measured nothing unusual there. Almost immediately upon coming into this open reservoir, the bromate levels jumped.
It turned out that bromide with chlorine which was supposed to be safe in bright sunlight formed bromate even more than ozone did.
This is the moment where everything shifted. The discovery was "unfortunate" — a scientific surprise that wasn't part of any regulatory scheme. They had three inputs: water source with bromide, disinfectant (chlorine), and open sunlight. The only choice was to remove sunlight.
Testing the Solution
The shade balls weren't originally designed for reservoirs. They were called bird balls and used on mine tailing ponds and around airports — anywhere they wanted to deter wildlife from landing on water. But when deployed at LA Reservoir, something remarkable happened: "once we deployed these balls all the birds were gone." They used to hang out everywhere — at the curbs, down at the outlet tower — but after the shade balls, no more.
The key test involved three kiddie pools filled with reservoir water. One sat in sunlight, one had a tarp, and one had shade balls. "Amazingly, the shade balls knocked out the problem immediately." The black pigment blocks all light from reaching the water and triggering the bromate reaction.
Why Black?
This is where Muller gets particularly interesting. The balls are black for a reason — not aesthetic preference or brand identity. They're made of high-density polyethylene, the same material as milk jugs, but they contain carbon black to make them last at least 10 years in the sunlight. "The dyes were so unstable," Muller explains. "They said we can't guarantee it's gonna last more than a year." It's that carbon black — this magic powder — that really makes the product last.
Critics might note: what about heat? Won't black balls absorb energy and warm the water, leading to faster evaporation? The answer is no. The top of the balls gets hot but the bottom stays cool. The balls contain mostly air, which acts as a thermal insulator. It's almost like a double pane window — you get that air gap in there, and the air acts as an insulator so the sun never hits the water directly.
The Evaporation Question
Here's what surprised me: these black plastic spheres actually reduce evaporation by 80 to 90 percent. In an open reservoir, there's more exposed surface area where water molecules can escape into the air plus greater airflow over the water surface continually removing the layer of moist air and replacing it with drier air. The shade balls do absorb more energy and get hotter on top, but not much heat is transferred through to the water beneath.
We've actually done some measurements and it's actually cooler under the shade balls even though they're black than it is without the balls just with the sun itself.
The balls have a slight cooling effect. For Los Angeles, a dry climate where evaporation matters, this is significant.
The Cost
At roughly 33 to 34 cents per ball, these are remarkably cheap. They have salvage value — recyclable material — and over the life of the balls, between the savings in chlorination and evaporation, at least half the cost will be paid for. "The water quality benefit is immeasurable," Muller says.
Bottom Line
This piece works because it takes something absurd on the surface — a reservoir covered in 96 million black balls — and reveals genuinely important science underneath. The bromate problem was real, the solution was accidental, and the result is cleaner drinking water for LA. The strongest part of this argument is how it shows that sometimes the most unusual-looking solutions are the most effective. The biggest vulnerability: we never hear what happens when these balls eventually need to be replaced or what environmental questions remain about microplastic leaching over a decade-plus in direct sunlight.
What matters next: as reservoirs across drought-prone regions consider this approach, we'll need to see more data on long-term performance and whether the cooling effect holds up under sustained observation.