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The Floating Black Paradox: Why Heat-Absorbing Spheres are Saving California's Water
It seems illogical to fight evaporation with millions of black, heat-absorbing plastic balls. But their dark color is a crucial design choice, providing the UV protection needed for longevity while blocking the sunlight that creates both algae and harmful carcinogens.
An Ocean of Black Plastic
In the summer of 2015, the Los Angeles Reservoir became a surreal landscape. Ninety-six million black plastic spheres were tipped down its concrete banks, forming a vast, shimmering blanket across 175 acres of water. The sight was captivating, but it begged an immediate and obvious question. In a state scorched by drought, where the goal was to prevent solar-driven evaporation, why cover a reservoir with millions of tiny, black, heat-absorbing objects?
A Problem of Sun and Chemistry
The LADWP's challenge was twofold. First was the relentless evaporation that cost the city billions of gallons of water each year. The second, more insidious problem was chemical. Reservoirs contain naturally occurring bromide. When combined with the chlorine used for disinfection and then exposed to sunlight, a chemical reaction can produce bromate, a suspected human carcinogen. Federal regulations demanded that the water be protected from the very sunlight that defined Southern California.
An Idea from the Airport
The solution came from a retired LADWP biologist, Dr. Brian White. He had learned of “bird balls,” small plastic spheres used in ponds near airport runways to deter birds from settling and interfering with air traffic. The concept was simple: a floating physical cover. Dr. White saw the potential to apply this idea on a massive scale, not for birds, but for water conservation and quality. These repurposed objects were renamed “shade balls.” But the critical design choice remained: what color should they be?
The Counterintuitive Color
Logic suggests the balls should be white, or even reflective, to bounce sunlight away and keep the water as cool as possible. The decision to make them black appears to defy the project's primary purpose. However, the engineers were not focused on surface-level thermodynamics; they were concerned with material science and long-term viability.
The choice was not about reflecting heat; it was about surviving the sun.
The Secret Ingredient: Carbon Black
The shade balls are made from high-density polyethylene (HDPE), the same durable plastic used for milk jugs and piping. Left to its own devices, however, even this tough material will break down under years of relentless UV radiation. The plastic would become brittle, crack, and shed microplastics into the very water it was meant to protect. The solution was to infuse the plastic with a crucial additive: carbon black. This pigment is one of the most powerful UV light stabilizers available. It absorbs the UV radiation and dissipates it as heat, preventing it from severing the polymer chains within the plastic. This protection gives each ball an expected lifespan of over a decade. The minor heat gain was a negligible trade-off for ensuring the structural integrity of the entire system.
More Than a Lid
The carbon black additive delivered a second, equally vital benefit. By making the balls completely opaque, it created a total barrier to sunlight penetrating the water. This accomplished two critical goals simultaneously.
- Algae Annihilation: With no access to sunlight, photosynthetic algae cannot bloom. This prevents the growth of massive green tides that can choke water systems and impair quality.
- Carcinogen Prevention: Blocking the sun's rays completely halts the chemical reaction that forms bromate. The shade balls solved the reservoir's compliance issue with a simple, physical solution.
A Lesson in Practicality
The story of the shade balls is a masterclass in pragmatic engineering. It reveals that the most elegant solution is rarely the most obvious one. By looking beyond a single variable—heat reflection—and considering the entire system of material longevity, chemical reactions, and operational costs, a counterintuitive choice proved to be the most effective. This floating sea of black plastic is more than just a quirky visual; it’s a monument to solving complex problems not with what seems right, but with what truly works.