This chemical turns polluted green lakes clear. Is it safe?

Aluminum sulfate, or alum, is increasingly being used to fight algae blooms spurred by an over abundance of phosphorus from human activity.

Green algae blooms are seen on Lake Okeechobee in Port Mayaca, Florida, in July 2018. Water managers are increasingly turning to a chemical weapon in the fight against algae.

Autumn sunlight streams across Lotus Lake in southern Minnesota, animating a chattering kingfisher. The spunky bird flits among drooping willow branches, whose yellowing fingers tickle the water’s surface. Behind this idyllic setting, however, something is amiss in Lotus Lake.

“It’s pretty green,” remarks local resident Greg Fletcher, backing his truck down the boat launch to haul out the family jetski for winter. Indeed, the sickly stained water evokes the cheeks of an acutely nauseous cartoon character. And Lotus Lake is not alone.

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. These “alum” treatments can be an ecological switch, flipping lakes from grimy to glorious almost instantly—but only if used in the right environmental context. When successful, alum is making swimming safer nationwide and could one day stem the red tide that plagues Florida’s coast.

A murky legacy

A self-described “lake lover,” John Holz spent 23 years at the University of Nebraska studying algae blooms. He explains that algae are simple aquatic organisms that thrive on sunlight and dissolved nutrients like phosphorus. Algae can “bloom,” growing in high densities when nutrients are plentiful.

And Holz says humans have spiked phosphorus inputs to lakes by over-fertilizing crops and lawns. Rainwater plucks phosphorus-rich residue from these cultivated surfaces and trickles into waterways, serving up a treat for algae. The resulting blooms shade out native plants that anchor in the sediment and form fish habitat. Some algae, like cyanobacteria, can produce toxins that trigger rashes or flu-like symptoms for swimmers and have proven fatal to dogs.

Holz sought a solution to this cascade of algal agony. But he quickly found that dialing down phosphorus inputs to lakes, by culling fertilization and infiltrating stormwater into soil, was not enough to halt the blooms. The issue, he says, was hiding beneath the water itself.

Lake-bottom sediment can grow saturated with phosphorus over time. “As a society we’ve been overloading our lakes with phosphorus for decades,” says Holz, who is concerned that waterways have “reached a tipping point.” Even when new phosphorus is kept out of the lake, “legacy phosphorus” already banked in the sediment can rise back into the water under low-oxygen conditions, haunting the lake with algae blooms for years. At Nebraska, Holz learned about a possible fix—a chemical that lays the ghost of legacy phosphorus to rest in the sediment.

Aluminum sulfate, or alum, has an affinity for phosphorus. In water, alum assumes a cotton-candy-like form, “a nice fluffy floc,” according to Spokane-based environmental engineer Shannon Brattebo. 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.

Sea Lion Suffers Seizure From Toxic Algae Bloom

“There is an immediate impact,” says Brattebo. But quick clarity is just the beginning. Alum floc rests in the sediment and continues to bind phosphorus for years, even decades. Legacy phosphorus stays stuck in the sediment, starving potentially harmful algae in the lake above.

Holz seized on the predictable chemistry of alum, focusing his research on its ability to restore polluted lakes. He finetuned a process to deliver a precise amount of alum to the right part of the lake at the optimal time of year. Holz recalls that managers eager to flip their lakes “would contact us and say ‘that sounds good. Let’s do it.’” But there was a problem—nobody provided the rigorous, science-guided treatments that Holz was studying.

So in 2010 Holz scrapped the job security of his research post and dove into a risky fledgling industry, founding his own alum treatment company. “It was scary at first,” he says. He purchased two barges customized with an array of hoses to dispense pre-calculated alum doses along GPS-controlled tracks. His investment paid off quickly. Demand for alum swelled, as did his fleet of customized barges.

“We’ve been fortunate,” says Holz. “We’ve had steady growth over the eight years we’ve been doing this.”

A clear future

Dosing lakes with alum is not a new practice—it was first tested in Sweden half a century ago. But it is gaining popularity as water managers battle the specter of legacy phosphorus fueling algae blooms. More than 250 alum-treated lakes worldwide support a growing body of evidence for the strategy’s efficacy. The science of alum has “advanced substantially and continues to advance every year,” says Harvey Harper, a Florida-based environmental engineer with 60 treatments under his belt.

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. After a parks department in Washington state shelled out a typical $100,000 for an alum treatment in 2008, a botched application killed hundreds of fish.

The sun sets over Lower Lewis River Falls in Washington's Gifford Pinchot National Forest. The falls mark a wild and scenic stretch of the river, but other sections of the Lewis, which drains the state's mighty Cascade Range, boast large dam and reservoir systems.

Hydroelectric plants produce power, but they've changed the river’s natural character—to the special detriment of migratory fish like salmon. Utilitiesy concerns have agreed to begin trucking fish around the Lewis damns in an effort to restore fish access-open some 170 miles (270 kilometers) of prime habitat upstream.

The river flows 1,470 miles (2.366 kilometers) from the Rocky Mountains to the Gulf of California's Sea of Cortez—or did when its waters were more plentiful. Reduced rainfall and the growing water demands of some 30 million thirsty westerners have sucked some of the life from the Colorado, and these days its delta is often dry.


Some scientists warn that changing weather patterns and unchecked human use could dry up the river’s reservoirs within 50 years.


The Rio Grande flows under the 1,500 foot-high (458 meter-high) cliffs of Santa Elena Canyon in the wild realms of Big Bend National Park.

The west Texas waterway travels through some of the Southwest’s most remote wilderness areas. But the Rio Grande isn’t immune to the impacts of humans.


In the past century, dams have stopped flooding that once created wooded banks for wildlife habitat. Reduced water levels have dried up some sections in recent summers and helped concentrate toxins, which enter the river upstream, into levels dangerous for wildlife.


The mighty Mississippi River cuts a graceful "S" curve through the city of New Orleans, not far from its delta in the Gulf of Mexico.

The Mississippi rolls on for some 2,350 miles (3,782 kilometers) from Lake Itasca, Minnesota to its Louisiana delta at the Gulf of Mexico, second-largest in the United States behind only the Missouri River.

Americans have built some 40,000 dams and levees along its length during the past century to aid navigation and help control deadly flooding.

However, they also stop sediments from flowing downstream, which is causing the delta to erode so rapidly that some scientists fear an area nearly the size of Connecticut could disappear by 2100.


Hindus visit a Ganges River to bathe in the "mother river." The faithful believe her waters are cleansing, but science tells a different story.

Industrial pollution from cities like Kanpur, untreated waste from riverside communities, and even islands of floating trash soil the Ganges' sacred waters. Efforts are underway to clean India’s iconic river, but the Ganges has a long way to go.


A lonely, icy tree stands sentinel in the mists of Quebec’s Richelieu River. The river, which flows north from Vermont’s Lake Champlain to the St. Lawrence, was an important transportation route for Native Americans and, later, French explorers.


In places the Richelieu can seem unchanged by time, but America’s waters are far less rich than they were in times past. Forty percent of the continent’s fish species are at risk of extinction.


The Colorado River flows freely through this stretch of the American west, but elsewhere human hands have altered her path for their own ends—and the Colorado is not alone.


Today large damns intercept some 35 percent of all the world’s river flows. Shifting river flows to serve human needs has played havoc with the inhabitants of many freshwater ecosystems. Fish and other aquatic species have lost critical habitat, as well as the seasonal cues, like floods, that help sustain their lifestyles.


The walls of Glen Canyon seem a bit taller when Lake Powell’s water levels are low, as evidenced by a telltale “bathtub ring” rising from the surface.


The enormous lake’s levels fluctuated with rainfall and snowmelt totals. D, and droughts during much of the 21st Century’s first decade reduced it to nearly half capacity. But even at low levels the nation’s second-largest reservoir is enormousholds a tremendous amount of water—Lake Powell contains a staggering 27 million acre-feet of water when full, even at low water levels.


When Glen Canyon Dam first began to pool the Colorado River’s waters it took 17 years to fill the lake.


Indigenous peoples paddle the waters of the Rio Negro, in Brazil’s Amazon Basin. The rivers and streams of this enormous basin, larger than Texas, comprise an aquatic ecosystem like no other—and hold an amazing 20 percent of the planet’s available fresh water.


Each year the rivers, home to more than 3,000 fish species, flood during the rainy season to create an amazing ecological mix of aquatic and terrestrial habitats known as “flooded forests.”


Since 1966 Glen Canyon dam has dramatically altered the Colorado River ecosystem by trapping enormous quantities of silt.


A 2008 experiment unleashed "high flow" waters from the dam to mimic natural flooding that once moved silt and built up sandbars and beaches downstream in the Grand Canyon.


The natural floods once created crucial habitat for plants and animals, and the artificial version shows some promise at doing the same—but USGS scientists say the sandbars created in 2008 vanished in just six months


A management regime of simulated floods might rebuild the Colorado's ecosystem, but must be balanced against lost power production from the dam.


Not all river ecosystems appear as bucolic as this sunrise scene. Today, large dams intercept some 35 percent of all global river flows, primarily for flood control and hydropower, and some riverbeds are short of water.

The United States is particularly thirsty for freshwater. It takes 1,320 gallons (5,000 liters) of water to produce the daily diet of an average American, and U.S. homes use more than half of their water—some 60 percent—just to keep their lawns green.


The setting sun silhouettes riverside trees along the banks of the Mississippi in Nauvoo, Illinois.


The Mississippi rolls on for some 2,350 miles (3,782 kilometers) from Lake Itasca, Minnesota to its Louisiana delta at the Gulf of Mexico. The river nourishes America’s great farmlands, with 92 percent of U.S. agricultural exports cominge from the Mississippi Bbasin. It also takes their bounty to market—60 percent of U.S. export grain travels downriver and heads to sea through Louisiana ports. In addition, birds travel the Mississippi as well—60 percent of all North American species use the basin as a migratory flyway.


A 700 foot-long (213 meter-long) cable and pipe footbridge stretches across the mighty Zambezi River near Chinyingi, Zambia.


The bridge, one of the few over the river, was built by a Catholic priest turned amateur engineer after five people lost their lives in a nighttime boating accident in these same waters.


In less developed regions, the Zambezi is still a main mode of transportation where roads are impassible or nonexistent.

Tap images for captions

Brattebo also emphasizes that while alum smothers legacy phosphorus in the sediment, it cannot halt algae blooms feasting on new sources of phosphorus. A nutrient-laden stream flowing into a lake can carry plenty of algal nourishment no matter how much alum floc is resting in the lakebed below.

But under suitable environmental conditions, alum treatment holds key advantages compared to techniques like dredging. “You can get it done quickly and it’s safe,” says Brattebo. Plus, alum removes “more pounds of phosphorus per dollar,” she adds.

Most importantly, if applied properly alum works. Suppressing rampant algae growth “really opens up the lake for recreation,” says Brattebo. While it is difficult to quantify such benefits, experts estimate that alum treatments generally pay for themselves within a decade. Improved water clarity also allows native submerged plants to reestablish.

“That is super beneficial for fish habitat and the overall health of the water body,” she notes. Harper speculates that more alum use in Florida lakes could mitigate coastal red tide by slashing phosphorus flowing from land to sea.

At Lotus Lake, Holz’s team prepares to inject 100,000 gallons of alum solution during the next five days. Lotus marks the 76th lake Holz has exorcized of legacy phosphorus—business is booming. Holz claims he has never had an unhappy customer.

Greg Fletcher looks forward to clearer water when he returns with the family jetski next spring. He also plans to swim across the lake with his two dogs, finally with a view of the fish and aquatic vegetation below.

“It’s been a battle,” Fletcher says of the lake’s struggle with algae. “It will be nice to be able to see better.”

Daniel Ackerman is a science and environmental reporter based in Minneapolis. Follow him on Twitter @BurOak5.

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