Soil oxidation of methane influenced by natural gas leaks

Methane leaking from buried natural gas distribution and transmission systems can be oxidized at least in part by the soil microbial community (mainly by methanotrophs). Two sites near Washington State University (Pullman, WA) have been used since summer 1992 to determine the potential of subsurface microbes to oxizide methane over a wide range of simulated leak rates and to determine the influence of soil parameters on oxidation rates. Sulfur hexafluoride (SF6) tracer is being used to quantify the subsurface diffusion associated with gas leakage. Our preliminary flux measurement results indicate that at low leakage rates (1 ml/min.) and under favorable environmental conditions about 80 to 90% of methane can be oxidized. Rates of methane consumption are strongly dependent on soil temperature and moisture content. The results also indicate that methane oxidation is accompanied by oxygen consumption and production of carbon dioxide. Soil gas samples from subsurface layers indicate that methane oxidation is predominant in the top 6 to 12 inches of subsurface horizon. During the winter period when the ground was frozen oxidation rates declined dramatically