Transmission and Distribution Pipeline Leak Identification and Characterization by Walking Survey and Soil Flux Measurements

We identified fugitive methane (CH₄) leaks within natural gas transmission and distribution pipeline rights of way (ROWs) around Pittsburgh, PA, and Baltimore, MD, by means of a walking survey while measuring ambient methane and ethane (C₂ H₆) mixing ratios. We used the methane time series to determine discrete leaks using a simple algorithm and verified that the methane was fossil in origin via the methane-to-ethane ratio. For transmission ROWs, we found an average of 23 leaks (range of 12 to 47) over 20.1 total km, corresponding to an activity factor (AF) of 1.1 leaks/km (range 0.60 to 2.3 leaks/km). We also quantified total methane emissions for a subset (N = 5) of the identified leaks using a soil flux measurement chamber. The mean leak emission rate (ER) was 172 g/h/leak (range 17 to 452 g/h/leak). Our AF is higher than the Environmental Protection Agency’s Greenhouse Gas Inventory (GHGI) estimate for transmission pipelines, which is 0.02 leaks/km. Our mean ER is also larger than the GHGI estimate for protected steel pipelines (44 g/h/leak). This study provides a model for making AF and ER measurements in vegetated environments with difficult terrain and suggests fugitive emissions from transmission pipelines may be a more significant source of atmospheric methane than is currently outlined in the GHGI.

This study presents activity (leaks per km) and emission rates (grams per hour per leak) measurements from natural gas transmission pipelines, which have received very little attention as a source of fugitive methane compared to distribution and gathering pipelines. Our results indicate that transmission pipelines may be a more substantial source of fugitive methane than is suggested by current inventories.