Natural gas leaks and tree death: A first-look case-control study of urban trees in Chelsea, MA USA

Schollaert, C., Ackley, R. C., DeSantis, A., Polka, E., & Scammell, M. K. (2020). Natural gas leaks and tree death: A first-look case-control study of urban trees in Chelsea, MA USA. Environmental Pollution263.

Urban trees and green space provide people with a variety of mental and physical health benefits. But tree density across the US is declining, as urban trees suffer from drought, excess moisture, and construction (e.g., sprawl). Another threat to urban trees is natural gas leaks. Gas leaks from pipelines can cause a significant increase in background methane concentrations, and are a particular problem in Massachusetts, which has one of the oldest gas infrastructure systems in the US. Fugitive emissions not only pose as threat to consumers wallets (as we pay for increased utility cost), and contribute to the greenhouse effect, but they also pose as a threat to local vegetation. Methane gas can make soil conditions anaerobic (oxygen-deficient), suffocating and causing damage to tree root systems. Methane may also increase the tree susceptibility to other dangers such as pests and fungi.

Residents of Chelsea, MA were concerned about these gas leaks and reached out to BU Professor Nathan Phillips to map ambient street leaks. Simultaneously, the Center for Research on Environmental and Social Stressors in Housing Across the Life Course (CRESSH) Community Engagement Coordinator, Claire Schollaert, was working with Green Roots, Inc. a community partner in Chelsea, to monitor tree health in the City. Initially, she and Prof. Scammell (CEC Lead and EH Faculty) sought to assess the effects of gas leaks, identified via ambient gas measures, on tree health. After a great deal of effort and pilot studies, assessing historic and present tree data in the City, the team of researchers set out to quantify the relationship between subsurface exposure to natural gas and tree deaths in in Chelsea. They used a case control study design to determine the odds of soil gas exposure among dead or dying trees in residential and commercial zones of the cities, compared with healthy trees.

All 84 cases (dead or very close to dead) in the city were ascertained by a visual assessment of every tree in the city. Prof. Scammell did this on her bicycle. Trees that appeared to have died from other causes, physical trauma, improper pruning, infestation, or powerlines above were excluded. Healthy comparison trees (controls) were selected by Ms. Schollaert independent of Prof. Scammell, using data from a 2016 city-wide tree inventory. She selected 97 trees roughly matching controls on size, age and species. Each tree was visited and visually inspected and separately categorized as a case or control by the retired arborist for the City of Chelsea, Mr. Andy DeSantis. Subsurface gas measurements were collected around each tree (led by GasSafety USA founder, Bob Ackley), using a plunger bar that punctured sampling holes of 6-10 inches deep to captures gas measurements around the root zone. A total of eight holes were punctured around the tree truck, two on each side, to measure methane and oxygen concentrations.

The study found that dead tress had 30 times the odds of having measurable methane concentrations in their pits compared with control trees.  Trees in commercial zones had significantly higher methane than in residential zones, and mean oxygen was significantly lower in tree pits with dead or dying trees. Soil methane concentrations were also highest in puncture holes closest to the street, suggesting leaky pipelines as the source.

These findings suggest that fugitive gas emissions may be contributing to urban street tree decline. The City of Chelsea currently spends around $50,000 a year on tree removals, and $500 to plant a single new tree. Many gas leaks go undetected and unrepaired unless they pose as a health risk to nearby residents. Repairing old and leaky natural gas infrastructure could not only save cities a lot of money and trees, but could also help decrease background methane emission levels, helping to slow the rate of rising temperatures and global warming.

This project was supported with funding from the JPB Environmental Health Fellowship, funded by the JPB Foundation and managed through Harvard T.H. Chan School of Public Health.