Uncounted and understudied: methane from CSG ponds

Understanding methane emissions is required to meet Australia’s obligations under the Global Methane Pledge. Coal seam gas (CSG) holding ponds, widely used by industry for storage of produced water and brine, are not currently included in methane emission inventories but represent a potentially understudied source. A synthesised literature on methane fluxes from natural waterbodies was undertaken and data were applied to estimate emissions from CSG ponds, initially to understand potential controls on rates. Increased temperature and shallower water depth were positively correlated with increased levels of emissions. Methane emissions from natural waterbodies are in the order of 10 s of mg/m2/day; however, the three emissions measurements found in published data from CSG ponds are ~2–4 orders of magnitude higher. CSG ponds may generate more methane than many natural waterbodies chiefly because they tend to be shallower, have elevated inorganic and organic carbon, and in the case of brine ponds, higher nutrients. To accurately estimate CSG pond emissions, three measured values are critically insufficient to represent overall CSG pond emissions and their overall contribution to a national inventory. Numerous engineering, chemical and biological controls are available to mitigate pond emissions, but their implementation is contingent upon improved understanding of methane fluxes within ponds and whether they are material. The drivers of these fluxes remain poorly understood, particularly for microbial methanogenesis and methanotrophy, organic versus inorganic carbon pools, and spatial and depth heterogeneity within CSG ponds. New data collection presented here aims to reduce this uncertainty.