Characterizing the impacts of fugitive methane emissions from natural gas distribution infrastructure in urban ecosystems

Background/Question/Methods

Methane is a potent greenhouse gas and the primary constituent of the fossil fuel, natural gas. As the US develops its sizeable shale gas reserves, it is important to characterize the social and environmental ramifications of the expanding natural gas industry. Recent studies have begun to characterize the impacts of upstream stages of the natural gas industry in rural environments. As increased demand for natural gas places strain on already aged and “leak-prone” distribution infrastructure in older US cities, we must also assess the downstream impacts of fugitive methane emissions from gas leaks in urban ecosystems. We conducted this study in Boston, Massachusetts, one of the oldest cities in the US with more than 3,300 identified gas leaks. We addressed four questions: [1] How much methane is lost from leaks in aged distribution infrastructure? [2] What is the distribution of leak sizes? [3] Is there seasonal variation in urban fugitive methane emissions? [4] How many leak sites pose a risk for explosion? Methane flux at leak sites is measured using closed dynamic chambers. Winter and summer field surveys of leak sites characterize the distribution of leak sizes, seasonal variation in emissions, and the frequency of leaks with explosive potential.

Results/Conclusions

Preliminary winter survey results show that methane emissions from individual natural gas leaks range from 0.2 to 655.5 cubic feet per day. The distribution of leak sizes is not normal, as previously assumed, but is log-normal with a long right hand tail. Preliminary winter survey results reveal a high frequency of leaks with small flux and small number of large leaks, or “super emitters”, that contribute the majority of fugitive methane emissions. Total emissions measured during the preliminary winter survey are lower than the national average, most likely due to the reduced diffusivity of urban winter soils. Preliminary winter survey results show that at 13.3% of leak sites visited, natural gas had accumulated in voids under roadways to explosive concentrations. Summer survey results are forthcoming. Improved estimates of fugitive methane emissions in urban ecosystems will inform sustainable management practices. Stemming “super emitter” leaks will reduce urban greenhouse gas emissions and ameliorate contributions to global climate change. However, even small gas leaks reduce survivorship of urban vegetation and their associated ecosystem services, as well as pose a risk to human safety via explosions. Coordinated management of buried infrastructure will facilitate replacement of “leak-prone” pipes in urban ecosystems.