Letter Cite This: Environ. Sci. Technol. Lett. 2019, 6, 401−406
pubs.acs.org/journal/estlcu
Natural Gas Emissions from Underground Pipelines and Implications for Leak Detection Bridget A. Ulrich,† Melissa Mitton,† Emily Lachenmeyer,‡ Arsineh Hecobian,‡ Daniel Zimmerle,§ and Kathleen M. Smits*,†,∥ †
Department of Civil and Environmental Engineering, Colorado School of Mines, Golden, Colorado 80401, United States Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado 80521, United States § Energy Institute and Department of Mechanical Engineering, Colorado State University, Fort Collins, Colorado 80523, United States ∥ Department of Civil Engineering, University of Texas Arlington, Arlington, Texas 76019, United States
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‡
S Supporting Information *
ABSTRACT: Underground natural gas (NG) leaks pose an urgent safety threat, motivating ongoing efforts to improve leak detection methods. The objectives of this study were to investigate how realistic environmental conditions affect methane concentration distributions near leaking underground NG distribution pipelines and ultimately to inform protocols for leak detection by walking surveys. In the first study to do so to date, subsurface and atmospheric methane concentrations were measured at high spatial resolution at a field-scale testbed configured to allow controlled release of NG from an underground source. Our findings demonstrate the importance of considering the effects of subsurface processes with respect to aboveground methane concentrations measured in walking surveys. While subsurface methane concentrations from a large leak (0.52 kg/h of NG, 0.44 kg/h of methane) exceeded 80 vol % 20 cm below the ground, atmospheric concentrations dropped below 100 ppmv (0.01 vol %) within the first 10 cm above the ground when the average wind speed was >2 m/s, demonstrating substantial atmospheric dilution in a narrow boundary layer above the surface under moderate wind conditions. Our analysis indicates that detectors with minimum detection limits on the order of 10 ppmv may be required to detect large underground leaks under certain environmental conditions. While efforts to assess a broader range of leak rates and environmental conditions are ongoing, the findings of this study provide critical insight to practitioners regarding detector performance and placement requirements for walking surveys.
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million in property damage affecting private citizens.4 Such
INTRODUCTION
incidents have demonstrated methane can migrate farther
Fugitive emissions of methane (a potent greenhouse gas) from natural gas (NG) production and distribution are associated with urgent safety concerns.1 Though there is evidence for declining emissions from production facilities,2 leaks from underground distribution pipelines continue to threaten public safety.3 In 2017, 301 reported pipeline incidents classified as “significant” resulted in five fatalities and more than $20 © 2019 American Chemical Society
through actual subsurface environments than has been Received: Revised: Accepted: Published: 401
May 12, 2019 June 24, 2019 June 27, 2019 June 27, 2019 DOI: 10.1021/acs.estlett.9b00291 Environ. Sci. Technol. Lett. 2019, 6, 401−406
Letter
Environmental Science & Technology Letters Table 1. Weather Conditions during Atmospheric Measurement Collection for Each Experimenta experiment
experimental conditions
weather conditions
1, CTL
control experiment
overcast, high of 29 °C
2, OBS
wind obstruction
sunny, high of 35 °C
3, WET 4, IMP
moist top layer impermeable surface cover
overcast, high of 6 °C partially cloudy, high of 20 °C
atmospheric condition reference no. 1-1 1-2 2-1 2-2 3-1 4-1 4-2
average wind speed [m/s] 3.2 2.9 3.6 5.0 2.5 1.4 5.3
(0.8) (0.8) (1.0) (0.9) (0.5) (0.7) (1.1)
stability class
average wind direction [°]
D D B C D B C
37 76 102 132 86 133 321
a Standard deviations are shown in parentheses. Wind direction is reported in north azimuth degrees (i.e., 0° for wind from due north). The Pasquill stability class during each set of atmospheric measurements was assigned according to recommended guidelines (B, moderately unstable; C, slightly unstable; D, neutrally stable).23
trench backfill (typically natural soil). Overall, additional field studies under realistic conditions are needed to better account for the effects of subsurface processes in leak detection surveys.21 The objectives of this study were to establish a benchmark understanding of above-ground methane concentrations associated with underground NG leaks under realistic conditions and ultimately to inform protocols for leak detection surveys. We applied a novel approach utilizing a field-scale testbed configured to release NG at a controlled rate from an underground source to assess how realistic environmental conditions affect resulting methane concentration distributions. We simulated steady state leaks from 0.9 m below the ground and collected subsurface, surface, and aboveground methane concentration data at high spatial resolutions over an area within 5 m from the leak, such that our results could inform sensor placement and performance requirements for walking surveys. We further assessed (i) how ground cover (i.e., partial impermeable surface cover and moisture in the upper soil layer) affects methane concentrations in the subsurface and at the surface and (ii) how wind speed affects atmospheric methane concentrations in the near-surface boundary layer (i.e., the first 50 cm above the ground surface). A 0.52 kg/h leak of NG (0.44 kg/h of methane) was simulated for most experiments to determine the detector minimum detection limit (MDL) required to detect a large leak during walking surveys.11 Our findings demonstrate that even relatively large leaks may be missed by walking surveys under certain conditions if critical detector placement and performance requirements are not met, revealing an urgent need for additional field studies to assess atmospheric methane concentrations that arise from underground sources.
observed in controlled experiments (2−10 m).5−7 Thus, there is an urgent need for an improved understanding of how realistic environmental conditions affect the migration and emission of methane from underground NG leaks. Improved leak detection methods could prevent incidents that cause loss of lives and property; however, the efficacy of widely varying leak detection methods (often consisting of mobile surveys that deploy detectors by aerial, vehicle, or walking patrol) remains a critical knowledge gap.8,9 Recent work revealed that walking surveys failed to detect >60% of urban NG leaks that could be identified by vehicle surveys,10 potentially due to inadequate detector sensitivity. Underground leaks vary widely in magnitude (i.e., from approximately 0.05 to 2 kg/h of methane),2 and previous work has predicted that >80% of urban leaks are
