Now, as a prolonged climate-fueled drought afflicts the region, groundwater levels have dropped to alarming levels, in some cases to record lows, triggering worries that buildings across large swaths of the city could be put at risk as pilings are exposed to air and begin to decay. There are nearly 10,000 row houses and other buildings in nearly a dozen neighborhoods that rely on wood pilings for support, from the North End to the Back Bay and Fenway. Some of the city’s most historic landmarks, including Trinity Church, Custom House Tower, and Old South Church, are supported by the pilings, which typically extend 15 to 20 feet below the surface.
Experts said rotting halts when groundwater levels rise again, but will resume whenever pilings are re-exposed, a prospect made increasingly possible by the likelihood of more frequent and long-lasting droughts.
“The more prolonged periods of drought, the more frequent we have them, the more sustained they are, the bigger risk it is to the buildings that are supported on pilings,” said Christian Simonelli, executive director of the Boston Groundwater Trust, an organization established by the Boston City Council to monitor groundwater levels in threatened parts of the city and make recommendations.
This summer, as drought in Boston went from mild to significant to critical, the trust has observed drops in many of its 813 monitoring wells across the city, with 31 at their lowest level on record, Simonelli said.
“I’ll be very clear: We need rain. We can’t go another three or four months like this.”
It’s a potentially expensive problem. In a 2021 analysis, Garrett Dash Nelson, president of the Norman B. Leventhal Map & Education Center at the Boston Public Library, found that more than $36 billion of assessed property in Boston lies on former mud flats that were filled in with sand and gravel. Nearly all the buildings on those former mud flats that were constructed in the early part of the 20th century and earlier are supported by wood pilings. “And there are many ways in which that estimate really is a floor,” Nelson said at a recent meeting of the Groundwater Trust, noting that his estimate excludes properties not listed in the tax assessor’s database.
Giuliana Zelada-Tumialan, a structural engineer with Simpson Gumpertz & Heger, said taking into account the current high costs in the construction industry, underpinning a typical Boston row house — in which steel and concrete supports replace the exposed tops of rotted woodpiles — runs about $400,000.
A short period of decay to pilings isn’t necessarily an immediate threat to a building, experts said. But significant problems can begin to arise in as little as three years. Rotted pilings can cause a building to settle and crack — making it hard or impossible to open windows and doors and leaving jagged scars across the facade — or, in the worst case, make it unsafe to occupy.
Worries about groundwater have dogged Boston for a century, having nothing to do with climate. Historically, the biggest threat has been underground infrastructure such as subway tunnels, sewers, and basements, which can crack and allow water to quickly siphon away. Before regular monitoring began in 1999, sometimes such leaks would only be discovered when the foundation of a building cracked.
In 1929, for instance, cracks began to appear in the walls of the Boston Public Library’s main building in Copley Square, caused by rotted woodpiles below ground. More than 50 years later, in 1984, the woodpiles under nearly 20 row houses on Brimmer Street on Beacon Hill were discovered to be rotted, reigniting concerns about Boston’s groundwater levels.
The Groundwater Trust was…
Read More: ‘We need rain’: Drought threatens the very foundation of some Boston buildings