NPS ready to battle grimy biofilm coating on Jefferson Memorial

The National Park Service has posted this sign outside the Jefferson Memorial last August explaining to the public that a microbial substance is growing on the 73-year-old landmark, turning the marble black. (WTOP/Nick Iannelli)
This photo shows the black growth that began coating the sides of the Jefferson Memorial more than a decade ago. The biofilm - a combination of bacteria, fungi and algae, has spread more rapidly in recent years and this week the National Park Service applied various chemical agents to a section of the marble to see if any would remove the film. (WTOP/Nick Iannelli)
Biofilm has blackened sections of the rotunda and the foundation wall of the Jefferson Memorial. The National Park Service describes the film as a combination of bacteria, fungi and algae growing on the marble of the 73-year-old landmark. (WTOP/Nick Iannelli)
A biofilm has blackened sections of the roof line of the Jefferson Memorial. The National Park Service describes the film as a combination of bacteria, fungi and algae growing on the marble of the landmark. (WTOP/Nick Iannelli)
Blackened areas of the Jefferson Memorial are caused by a microbial growth, the National Park Service said. Testing began this week to see if any chemical agents will remove the biofilm. (WTOP/Nick Iannelli)
Chalk marks denote different chemicals applied to the side of the Jefferson Memorial to see if any would remove a microbial growth that is slowly turning the marble black. The tests began this week, the National Park Service said. (WTOP/Nick Iannelli)
A National Park Service employee dusts off the statute of Thomas Jefferson as the sun streaks through the pillars at the Jefferson Memorial in Washington, early in the morning on Friday, July 8, 2016. (AP File Photo/J. David Ake)
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WASHINGTON — Visitors who walk up to the iconic Thomas Jefferson Memorial in D.C. might see the monument and think it is dirty, but what they are actually looking at is a collection of microorganisms known as a biofilm.

The 73-year-old landmark is covered with the biological growth, making it appear black, filthy and grimy in spots.

“This is a really important, but very challenging topic,” said Gay Vietzke, superintendent of the National Mall and Memorial Parks. “The challenge we have is not just figuring out a solution to address it, but also helping the public understand what’s going on.”

The biofilm was first visible on the monument in 2006 and has become much more pronounced in the last decade. Since 2014, the National Park Service has been studying options to safely remove it. And just this past week, testing began to see what chemicals would work best.

“Treatment is difficult as there is no known permanent method of biofilm removal,” said Catherine Dewey, chief of resource management for the National Mall and Memorial Parks.

Using a power washer on the biofilm is not an option because it would cause severe damage to the marble, Dewey said.

“Ultimately we will choose the treatment option that is the least damaging to the stone, safe for the environment and visitors and cost effective.”

There is no timeline yet for how long the cleanup project might take to complete.

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