A history of pollution is sparking a colorfully devastating scourge in lakes across the country: algae blooms. These events can turn pristine waterways to pea soup, choking out wildlife and toxifying water. Increasingly, scientists are fighting back with chemical warfare, injecting aluminum sulfate into lakes to neutralize the pollutants that fuel the blooms.
Aluminum sulfate, or alum, has an affinity for phosphorus. In water, alum assumes a cotton-candy-like form, “a nice fluffy floc.” This floc (short for flocculation) grabs phosphorus and other particles as it settles to the bottom of the lake, flipping the water from cloudy to clear.
Like any method of ecosystem restoration, dumping thousands of gallons of alum into a lake is not risk-free. If pH plummets during an alum treatment, the usually benign chemical—often used to purify drinking water—can turn toxic for wildlife.