The Washington Monument is seen at sunrise from the Tidal Basin. (Oliver Contreras for The Washington Post)

The reopening of the Washington Monument, scheduled for this spring, has been delayed because of the presence of possibly contaminated underground soil, the National Park Service said Monday.

The soil “is below the ground surface and poses no risk to public health,” spokesman Mike Litterst said in a statement.

“The soil in question was likely introduced in the 1880s as the monument was being completed,” he said. “Due to the necessary mitigation efforts, the reopening of the Washington Monument is now expected to take place in August.”

Officials could not immediately say what the possible contamination might be.

The monument has been closed since August 2016 for the construction of a new visitor screening facility and modernization of the elevator, which takes visitors to the 500-foot observation level.

One of the tallest free-standing masonry structures in the world, the monument is also perhaps the most recognized of American structures.

It was damaged during the 2011 earthquake, reopened and then closed again for elevator replacement and construction of the visitor facility.

The monument’s cornerstone was laid July 4, 1848, at a ceremony attended by President James K. Polk and Rep. Abraham Lincoln. Work was halted from 1858 to 1878 because of a lack of funds.

In December 1884, a 3,300-pound marble capstone was placed atop the monument and capped with a pyramid of aluminum.

The following Feb. 21, on a sunny, frigid day, the monument was dedicated.

But several years later, officials deposited 250,000 cubic yards of soil to create the knoll that surrounds the monument, Litterst said in an interview.

For the construction of the visitors facility, workers planned to dig 16 geothermal wells for heating and cooling of the building.

But Litterst said the District’s Department of Energy and Environment informed the Park Service that the area where the well “field” is planned might have contaminated soil.

Litterst said the Park Service planned to include special metal sleeves around the wells to prevent leaching of any contamination into the wells.

It is the latest problem with a national landmark that has had a troubled history.

The original site, which was supposed to be on a north-south line with the White House and an east-west line with the Capitol, had to be moved because it was on unstable ground near the bank of the Potomac River, according to a study by the Army Corps of Engineers.

Thus, the current site is about 370 feet east of the White House axis and 123 feet south of the Capitol axis, the Corps said.

Later, the monument foundation had to be bolstered for fear it would not hold the weight of the structure.

And in 1887, a pond called Babcock Lake just north of the monument was filled in because it was seen as a threat to the stability of the monument.