Understanding Wellesley's Natural Gas Leaks

Important Note About the Gas Leaks Data:

Gas leaks are identified by street address but they are located in public streets — NOT inside buildings.

If you smell gas inside, call 911 immediately.

The Gas Leaks on Our Maps

The leaks shown in maps on this page are known to National Grid, are regularly surveyed, and considered not likely to explode. However, if you smell gas outside, please call the gas company to make sure the company knows about that leak: 1-800-233-5325. Please also call if you notice a change in an existing leak—if the smell is stronger or has moved.

 Every year, Massachusetts gas utilities are required to report the locations of all of their outstanding and repaired gas leaks. HEET (a nonprofit based in Cambridge) extracts, cleans, and geocodes leak data from these reports and shares it under a Creative Commons license. The Town of Wellesley Information Technology Department produced this map showing gas leak locations in Wellesley based on recently available data. 

 Due to National Grid reporting standards, the data represented on the map may not include all gas leaks in Wellesley during 2019. The map represents a snapshot of the status of gas leaks as of December 2019. Since then, some leaks labeled "unrepaired" may have been repaired and new leaks may have appeared.

 Gas leaks are graded according to a Gas Leak Grading system: 

  •  Grade 1 leaks are very hazardous and require a 24-hour response and repair window. This includes leaks in or near a contained space, such as a building or manhole, where the gas could build up enough to potentially explode.
  • Grade 2 leaks are non-hazardous and must be monitored every six months and scheduled for repair within a year.
  • Grade 3 leaks are not considered potentially explosive because they are not in a contained space or near the foundation of a building. However, they can still be large leaks. 
  • Grade 3 SEI (Significant Environmental Impact) leaks are large volume leaks that are not considered potentially explosive but emit enough gas to be in the top 10% of gas leaks in terms of emissions.

repaired leaks

Total estimated emissions from Wellesley’s current 256 gas leaks
CO2: 8,437 metric tons
Methane: 98 metric tons
Cost of leaked gas (residential rate): $92,663
Equivalent emissions from US passenger cars: 1,834


Leaks Reported By National Grid 2015-2019                                                                         

The Problem with Natural Gas

The natural gas distribution infrastructure in Massachusetts is the second oldest and most leak-prone in the country. Gas leaks cause explosions, kill trees, and emit methane, which is an extraordinarily potent greenhouse gas. The leaks are also expensive since gas customers pay for the lost gas in their bills. At the end of 2019, approximately 16,000 unrepaired leaks and approximately 11,500 repaired leaks were reported in the Commonwealth by gas utilities.

Gas Leaks Contribute to Climate Change

Natural gas is mainly composed of methane, a greenhouse gas that’s about 86 times more potent than carbon dioxide over the first 20 years in the atmosphere. The amount of gas leaked annually from the Commonwealth’s aging gas distribution system is equivalent to the emissions of all of the state’s stores and businesses combined. In addition to polluting the atmosphere, methane suffocates street trees as it seeps into their root zones, depriving them of oxygen.

Gas Leaks Affect Public Health and Safety

Leaked gas can accumulate in underground spaces, such as electric and sewer structures and manholes. When gas accumulates, it can ignite, causing fires and explosions. 

 Cooking with gas, without opening the window or turning on a fan is associated with an increase in asthmatic events in children. It can also  cause spikes in emissions that would exceed the outdoor air pollution standards. Homes with gas stoves can have nitrogen dioxide concentrations 50-400% higher than homes with electric stoves and increase the risk of asthma in children by 24-42%. These public health hazards are described in more detail in the Health Effects From Gas Stove Pollution report put out by the Physicians for Social Responsibility.

Gas Leaks Kill Trees

Decades of scientific research have shown that trees are adversely affected by gas that escapes from pipes under our streets and infiltrates the soil around tree roots. As the Tree Warden for the Town of Wellesley, the Natural Resources Commission is working to protect our public shade trees – and our entire tree canopy – from the devastating and costly effects of dozens of persistent gas leaks throughout the town.

Watch this video to find out more:

Watch: “How Gas Leaks Affect Our Trees: The science, the signs, and what we can do”

INSERT VIDEO HERE:  5-28-2020 Webinar: “How Gas Leaks Affect Our Trees: The science, the signs, and what we can do” 

This webinar was held on June 2020. It was moderated by Arlington Town Manager Adam Chapdelaine, with expert panel and topics:

  • Dr. Nathan Phillips, Boston University: "How do methane leaks affect trees?" 
  • Bob Ackley, Gas Safety USA: "What does the impact of methane on trees look like in our towns?” 
  • Brandon Schmitt, Director, Wellesley Natural Resources Commission: "How are some municipalities addressing this problem and what more needs to be done?
Gas Leak Infographic

What’s Being Done


Fixing the largest leaks 

Until 2016, utilities were not required to fix any leak - no matter how large - unless it was considered potentially explosive. Then researchers found that just 7% of all gas leaks are much bigger than the rest, emitting half of all the gas. A law was passed mandating that gas utilities fix these leaks, called “Significant Environmental Impact (SEI)” leaks. Fixing these SEI leaks would cut emissions from gas leaks in half. However, gas utilities did not know how to identify these leaks since they had previously only been required to consider safety, not emissions. HEET and Mothers Out Front persuaded Columbia Gas, Eversource, and National Grid to participate in a study to find an effective method for identifying SEI leaks. The “leak extent method” was selected and the three utilites agreed on a shared action plan to deal with these leaks. As part of the plan, HEET checks that the utilities are identifying the right leaks and repairing them. This work is currently ongoing through 2023.


Pipeline Replacement Programs 

Approximately a quarter of the gas pipes under the ground in Massachusetts are considered “leak-prone.” Each year, gas utilities submit gas system enhancement plans (GSEPs) to state regulators to accelerate the replacement of this leak-prone infrastructure with new plastic pipes.  At current rates, it’s estimated that replacing all leak-prone pipes in Massachusetts with plastic ones will cost $17 billion. This cost will be paid for by gas customers through increased gas prices. At the current rate, gas customers will be paying for them until 2107, well past the Commonwealth’s 2050 deadline to reduce emissions. Thus by the time all gas pipes are replaced, the gas system itself will be defunct, leaving the new pipes abandoned in the ground while our children and grandchildren pay for them. 



What Can You Do?


Smell Gas? Call 1-800-233-5325 or 911


Natural gas is almost pure methane, which does not have any odor. For safety purposes, gas companies must add a chemical odor similar to rotten eggs so that it may be detected by scent. 

If you smell gas and suspect there is a gas leak in or around your home, please follow these steps (from massgov.com):

  • Evacuate the building as quickly as possible and call 911 immediately
  • Do not turn off any appliances or lights
  • Do not open or close any windows
  • Do not attempt to shut off the gas

Fire and/or Police will come and conduct a leak determination reading and will contact the gas company 


You Can Be a Gas Leaks Detective

[info here from the Worcester Mothers Out Front group?]



This page was prepared with content and support from HEET. HEET is a nonprofit based in Cambridge, Massachusetts, working to cut carbon emissions. HEET acts as a watchdog, checking that the utilities identify and repair the largest gas leaks to  cut emissions for the least cost and disruption. At the same time, HEET works to transition Massachusetts off gas to renewable energy solutions. HEET is solely funded by foundations and individual donors and never accepts money from gas companies.