A severe algae bloom clogged equipment at one of the treatment facilities providing drinking water in the Washington region, forcing officials to declare a boil-water advisory on the night of July 3—as thousands of visitors arrived to celebrate Independence Day.

The advisory was lifted the morning of July 4. But the incident was an ominous sign of how warming water temperatures caused by climate change can disrupt essential civic services.

The algal blooms caused a drop in water supply at the Dalecarlia Water Treatment Plant on the Maryland-D.C. border. All water treatment operations were switched to the McMillan Treatment Plant in Northwest D.C. to ensure adequate supply of water, the District of Columbia Water and Sewer Authority (DC Water) said in its July 3 advisory.

The disruption in water supplies affected the entire District of Columbia and parts of Arlington, Virginia, including the Pentagon, Arlington National Cemetery and Reagan National Airport. Among the blooms’ impact: increased turbidity, a measure of cloudiness in water.  

The U.S. Army Corp of Engineers (USACE) operates the treatment plants located in D.C. and supplied by the Washington Aqueduct, which collects, treats and pumps drinking water for nearly 1 million customers in Washington, Arlington County and other areas in northern Virginia.

The algae intruded at a time when demand for water was particularly high due to the influx of visitors and possible firefighting activities related to the annual fireworks display on the National Mall.

“DC Water issued a precautionary boil water advisory to protect public health and safety due to a sharp reduction in the volume of water being supplied by the Army Corps of Engineers’ Washington Aqueduct and due to the Aqueduct expressing concerns that they might be unable to comply with strict U.S. EPA water quality standards while simultaneously attempting to increase water supply volumes to levels adequate to meet DC Water’s customer demands,” said Sherri Lewis, senior manager of communications at DC Water, in emailed remarks. “The combined output of treated water from both of the Aqueduct’s plants was insufficient to meet DC Water’s water consumption demands.”  

Lewis said the week of the July 4th holiday has historically been one of DC Water’s highest water demand days of the year. 

Affected customers scrambled to stock up on bottled water supplies after the advisory was announced, quickly emptying out aisles in supermarkets and local stores.    

“We had floating algae mats along the top of our sedimentation basin at our Dalecarlia Treatment Plant, which then washed into the filter building, clogging filters in the process,” said Cynthia Mitchell, a public affairs specialist with the U.S. Army Corps of Engineers. 

The sedimentation portion of the water filtration process removes suspended particulates in water. In emailed comments to Inside Climate News, Mitchell said the situation at Dalecarlia led to a decrease in supply, while the McMillan Treatment Plant continued to operate under normal conditions.

“Our recent algae bloom was not a cyanobacterial harmful algae bloom—we had green algae which does not pose a risk to human health,” Mitchell added.

The U.S. Environmental Protection Agency EPA, warns that “blooms of red tides, blue-green algae, or cyanobacteria can result in severe impacts on water quality, human health, aquatic ecosystems, and the economy.” 

In the case of the July 3 event, it was the sheer amount of algae that caused problems. The region’s record-high temperatures are driving growth, and climate change is expected to worsen the situation, Mitchell said.

“Washington Aqueduct staff that have served for decades, including General Manager Rudy Chow with 40 years of experience in the water utility industry, agree the severity of algae blooms this summer is unprecedented,” Mitchell said.

DC Water’s Lewis said several other utilities that use the Potomac River as one of their water supply sources, such as Fairfax Water in Virginia and the Washington Suburban Sanitary Commission in Maryland, encountered and successfully treated these same algal blooms. 

Nitrogen, key fuel for algae, flows into water bodies from sewage overflows and runoff. Bill Dennison, a professor and vice president at the University of Maryland Center for Environmental Science, said the Potomac River has historically had high levels of pollution from sewage but now agriculture and stormwater runoff is the biggest source of pollution in the Chesapeake Bay watershed. 

“Generally, both cyanobacteria and green algae form in the Potomac River,” Dennison said. “Fortunately, green algae don’t tend to be as toxic as cyanobacteria. But they’re not a pleasant addition to the environment and can clog the waterways … and produce bad taste in drinking water.” 

Climate change contributes a one-two punch. More rain instead of snow in the winter leads to extra pollution runoff earlier in the season, Dennison said. And warming temperatures allow algae to bloom earlier in the summer than before.

DC Water officials said they are reviewing their actions and communications to the public about the July 3 event to determine what can be improved. 

Lewis said that unlike the majority of other public water utilities, DC Water does not have a second source of water and is fully dependent on the Aqueduct to supply its needs. “It is also extremely unusual for a water utility serving a large metropolitan city not to also have direct responsibility for water supply and water treatment. DC Water will be reviewing the Aqueduct’s actions to determine if any changes are necessary to ensure proper notification steps are taken in a timely manner,” she said.  

While greater D.C. avoided major calamity with this algae bloom, other cities haven’t been as lucky. In 2014, the water supply of Toledo, Ohio, had to be shut down because of a harmful algal bloom in Lake Erie, and the toxin associated with that algae could not be destroyed by boiling. Half a million people could not use water supplies for days. Agricultural runoff was later declared the cause of the ordeal.