Small, dark-colored plastic spheres floating on the surface of large reservoirs, known as shade balls, have captured public attention due to their unusual appearance. These compact, hollow spheres, typically about four inches in diameter, are deployed en masse onto open-air drinking water sources, creating a dense, floating cover. This simple physical barrier serves multiple goals focused on maintaining the quality and quantity of a public water supply.
Shielding Water Quality from Sunlight
The primary purpose of the dark-colored shade balls is to shield the water from solar ultraviolet (UV) radiation. Drinking water reservoirs are typically disinfected with chlorine to eliminate harmful pathogens. However, when chlorine encounters naturally occurring bromide salts in the water and is exposed to intense sunlight, an undesirable photochemical reaction is triggered.
This reaction leads to the formation of bromate, a disinfection byproduct classified as a probable human carcinogen. Its presence can cause a reservoir to fall out of compliance with federal water quality standards. The opaque, black surface of the shade balls creates a dense physical barrier that prevents UV light from penetrating the water column. By blocking the light, the balls effectively halt the chemical reaction that produces bromate, maintaining the water’s safety without needing costly treatment methods.
The black color maximizes light absorption, creating a near-total block of solar radiation. This simple, non-chemical intervention addresses a specific regulatory challenge faced by water utilities operating large, uncovered reservoirs. Reducing sunlight exposure also limits the growth of algae, which thrive in warm, illuminated water. Reducing algae growth allows the utility to reduce the amount of chlorine required for disinfection, minimizing the potential for the formation of other chemical byproducts.
The Role in Water Conservation
While maintaining water quality is the technical driver for deployment, the floating cover also significantly reduces water loss. The collective layer of shade balls acts as a physical blanket over the surface, minimizing the area directly exposed to the atmosphere. This barrier dramatically cuts down on the two main external factors that cause evaporation: direct solar heating and wind action.
In drought-prone regions, this function provides a substantial conservation benefit. Studies indicate that a fully covered reservoir can reduce evaporation rates by up to 80 to 90 percent. For instance, large-scale deployments in Los Angeles reservoirs were estimated to save hundreds of millions of gallons of water annually. The layer of balls also contributes to conservation by keeping the water temperature lower, as reduced solar absorption slows the molecular movement that drives the evaporation process.
Implementation and Material Considerations
Shade balls are manufactured from high-density polyethylene (HDPE), a plastic chosen for its durability and resistance to breakdown. This material is widely used in the food and beverage industry and is considered food-grade and non-toxic. The plastic is stabilized with UV inhibitors, such as carbon black, which makes the balls opaque and protects the material from degrading under prolonged sun exposure.
The use of plastic in a public drinking water source often raises concern about potential leaching or environmental impact. Manufacturers address this by ensuring the material is certified by third-party organizations for contact with potable water. The balls are designed for a long service life, after which the HDPE material is recyclable. Deploying millions of these spheres is a massive logistical undertaking, but the relatively low cost per unit makes it a far more economical solution compared to constructing large, permanent floating covers.