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 above-ground 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.