Significant Environmental Impact (SEI) Field Trial Report

Report of the 2018 SEI Field Trial – Utilities Enacting the Leak Extent Method

Executive Summary

Massachusetts has the second oldest natural gas infrastructure in the country. Old pipes leak methane, a greenhouse gas 84 times more potent than carbon dioxide over the first 20 years in the atmosphere.[1]

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.[2] The cost of this wasted gas is passed on to the customers, estimated to be over $11 million per year.[3]  In addition to polluting the air, methane suffocates street trees as it seeps into their root zones, depriving them of oxygen. 

Research by Boston University and Gas Safety Inc. in 2016 showed that just 7% of the greater Boston distribution system leaks emit half of all the gas by volume, creating a clear policy opportunity.[4] Later the same year, the Massachusetts Legislature enacted a law requiring that these leaks of significant environmental impact (SEIs) be repaired, since doing so would cut methane emissions in half for the least cost to the utilities and the least disruption to cities and towns. 

However, given that gas companies had always been mandated to focus on the explosive potential of a leak and not emissions, they had no reliable and accurate method to identify these largest leaks that have a significant environmental impact. In 2017, HEET coordinated a large collaborative study working with Columbia Gas MA, Eversource Gas, and National Grid Gas, together with Gas Safety Inc., Mothers Out Front and other stakeholders. This research team field tested multiple methods and found the leak extent method[5] was a quick, effective and low cost solution.[6]

This report documents the progress of this first-in-the-nation program and reports on the use of this new identification protocol – the leak extent method – to identify and repair SEIs in the 2018 dig season in Columbia Gas, Eversource Gas, and National Grid Gas territories.  HEET independently verified the results, with Gas Safety Inc., and provided analysis.

Massachusetts is the first to enact legislation to identify environmentally significant leaks, the first to determine an SEI protocol, and the first to test it widely in the field across multiple gas companies. We hope to report in coming years that we are also first in the nation to cut in half our methane emissions from the gas distribution system. 

Main Results of SEI Pilot Year

  • Gas companies were able to use the leak extent method to identify SEIs in the field. The measurements of individual leaks were relatively consistent over a work season, even when measured by different personnel from different organizations with varying weather conditions.
  • A top-down analysis shows a fast 14-month return on investment for repairing SEIs.
  • All three gas companies appear to be under-identifying some SEIs, potentially because the protocol was new.  A mobile CRDS (cavity ring-down spectrometer) survey also showed promise in identifying SEIs.
  • The FluxBar, a tool for comparing and confirming the emissions of leaks through a proxy measure of flux, needs data in 2019 to determine efficacy and refine the protocol.
  • Leak repairs do not appear to always be successful and the success rate should be further evaluated to maximize emissions savings per dollar.
  • There is potential for refinement of the leak extent method, helping to identify and repair SEIs faster and for less cost. This will require more information sharing.

In summary, the leak extent method is working to identify the worst leaks so they can be repaired.

Download the full report here.

[1] IPCC Climate Change Report, “Climate Change 2013: The Physical Science Basis,” Table 8.7

[2] See page 5 in the full report

[3] See Appendix 4 in the full report: Annual Total Cost from the Distribution System for more information. Reference is calculated using findings from McKain et al, Methane emissions from natural gas infrastructure and use in the urban region of Boston, Massachusetts, https//

[4]  Hendrick et al, “Fugitive methane emissions from leak-prone natural gas distribution infrastructure in urban environments”,

[5] Originally suggested by Bob Ackley of Gas Safety Inc.

[6] Magavi, Z., Ackley, R., Hendrick, M., Salgado, E., Schulman, A., Phillips, N., “A Method of Identifying Large Volume Leaks in Natural Gas Distribution Systems”, publication pending.