Scientists Know How to Fix DC's Green Reflecting Pool

When the Lincoln Memorial Reflecting Pool reopened in June after a $14.7 million makeover, officials hoped the freshly painted "American flag blue" bottom would dazzle visitors for the nation's 250th anniversary. Instead, the 6.5 million gallon pool turned bright green with algae within days, creating an online spectacle and a headache for the National Park Service.

But here's the good news: water treatment experts know exactly how to fix it.

Dr. Ashley Bair, a senior research developer who earned her PhD studying algae, says the green bloom was predictable and preventable. The culprit isn't the blue paint or even the heat, despite speculation online. It's phosphates in the water acting as algae food.

The pool fills from the Tidal Basin, which draws from the Potomac River. That natural water contains phosphates from agricultural runoff and other pollution sources. Add June sunshine and warmth, and you've got the perfect recipe for an algae bloom.

The same thing happened back in 2012 when the pool was refilled, long before any blue paint touched the bottom. That's proof the paint isn't the problem.

The solution is straightforward: treat the water with metal-based coagulants before it enters the pool. These chemicals bind to phosphates and organics, causing them to sink to the bottom where they can be vacuumed out. Without phosphates to feed on, algae can't bloom.

Crews have been pouring hydrogen peroxide around the pool's edges to kill existing algae. Bair says peroxide works beautifully as an algicide, breaking down safely into oxygen and water. The challenge is covering all 2,000 feet of the pool evenly, especially since the water needs to stay still for that perfect mirror reflection.

The Bright Side

This green pool moment has sparked a valuable conversation about water treatment and maintenance. The National Park Service now has clear guidance from experts on preventing future blooms with simple, proven methods.

The fix doesn't require another multi-million dollar renovation or draining the pool again. It just needs proper water treatment before filling, the kind of coagulation process water treatment facilities use every day across America.

Washington DC summers will always bring heat and sunshine. The Potomac will always carry some phosphates. But with the right treatment approach, the Reflecting Pool can stay crystal clear and blue, ready to inspire millions of visitors celebrating America's history and future.

The iconic monument can shine exactly as intended, proving that sometimes the best solutions are the ones scientists have been using all along.