Among the many impacts of this past summer’s prolonged drought – a weather pattern we unfortunately can probably expect to see recur for years to come due to climate change – was a drop in groundwater levels throughout much of the state.
In Boston, that means a possible increased threat to wood piles supporting thousands of residences, offices and retail properties in neighborhoods including the Back Bay, East Boston, Fenway, Longwood, Lower Roxbury, the North and South Ends and parts of South Boston along the Fort Point Channel.
Across large swaths of Boston, mud flats were filled with gravel in the 18th and 19th centuries. Now many thousands of homes, buildings, churches, and other structures are supported on wooden pilings that, when groundwater levels drop, can begin to rot as they are exposed to air. These include some our city’s most famous landmarks and hundreds of residences, offices and retail properties.
In all, Garrett Dash Nelson, president of the Norman B. Leventhal Map & Education Center at the Boston Public Library, has estimated that properties assessed in value at more than $36 billion as of 2020 are situated on filled-in former mud flats, almost all of them buildings dependent on wooden pilings for support. When you count buildings straddling both fill and original pre-1630 lands, that valuation number rises to $54 billion, according to Nelson.
As long as wooden pilings remain submerged in groundwater, they can last for centuries. But with this summer’s drought, many of the city’s 810 groundwater monitoring wells overseen by the Boston Groundwater Trust – an organization established by the city of Boston in 1986 with a mission to stabilize groundwater at proper levels – showed groundwater levels falling, in some cases to levels that, over time, could lead to piles beginning to rot. If this happens, it will cause the structures they support to settle and sag.
But rainfall, or lack thereof, is not the only thing impacting groundwater levels. In fact, in our experience as geotechnical engineers, we’ve seen that a much bigger influence on groundwater levels than annual rainfall totals is the condition of underground utilities and transportation infrastructure. When sewer lines or tunnels develop cracks and gaps, they can begin to function like drains sucking down the level of groundwater. And unlike the weather, much can be done and is being done to address these causes of lowering groundwater levels.
Our Work Is Paying Off
For decades, the Boston Water and Sewer Commission has been vigilant and proactive about repairing, replacing, and relining old sewer lines to reduce pipe exterior inflow and infiltration, keeping groundwater where it should be under city neighborhoods instead of having it delivered to the Deer Island sewage treatment plant.
In concert with the Boston Groundwater Trust, governmental agencies, including the MBTA and Massachusetts Department of Transportation, have done an effective job of identifying locations where their infrastructure has developed inflow and infiltration issues. City regulations require new construction and large renovations to install stormwater infiltration systems in the neighborhoods where wooden pilings are located. The Groundwater Trust provides authoritative real-time information about groundwater conditions and functions as a convener and source of information about best practices for managing this issue.
At a comprehensive Groundwater Trust Forum held at the Boston Public Library on Sept. 20, Mayor Michelle Wu noted that while Boston recently experienced its worst drought since the summer of 2016, “groundwater levels are higher now than they were in 2016. That just goes to show that all of the parcel-by-parcel property intervention, the citywide focus, the collaboration and the constant perseverance by [the Groundwater Trust and its supporters] to keep this on everyone’s radar has paid off and has made a difference. We have a lot more work to do – but we can already see just how much it is working.”
I couldn’t agree more strongly with the mayor. As we all know, none of us can control whether it rains or not. But protecting Boston homes and buildings from the challenge of lowered groundwater levels is well within our control with the use of stormwater infiltration systems and leaking utility repairs – so long as we do the work and keep investing the funding in the many proven practices, technologies and solutions that are available.
Bruce Fairless is a principal with GZA GeoEnvironmental, a 10-year member of the Boston Groundwater Trust Technical Advisory Group and a 35-year veteran of the geotechnical engineering field.