Physical Controls on Methane Ebullition from Reservoirs and Lakes

Understanding the nature and extent of methane production and flux in aquatic sediments has important geochemical, geotechnical, and global climate change implications. Quantifying these processes is difficult, because much of the methane flux in shallow sediments occurs via ebullition (bubbling). Direct observation of bubble formation is not possible, and bubbling is episodic and dependent upon a number of factors. Whereas previous studies have correlated methane flux with surface wind intensity, detailed study of Lake Gatun in Panama and Lago Loiza in Puerto Rico suggest that methane flux is more closely correlated with the shear stress in sediments caused by bottom currents. Bottom currents in turn are a complex function of wind, internal pressure gradients, and lake bathymetry. A simple physical model of bottom currents and sediments in these lakes suggests that most methane ebullition originated from the upper 10–20 cm of the sediment column. Our data reaffirm previous studies showing that ebullitive methane flux is minor in water deeper than ~5 m.