Isotopic signatures of major methane sources in the coal seam gas fields and adjacent agricultural districts, Queensland, Australia

In regions where there are multiple sources of methane (CH4) in close proximity, it can be difficult to apportion the CH4 measured in the atmosphere to the appropriate sources. In the Surat Basin, Queensland, Australia, coal seam gas (CSG) developments are surrounded by cattle feedlots, grazing cattle, piggeries, coal mines, urban centres and natural sources of CH4. The characterization of carbon (ffi 13C) and hydrogen (ffi D) stable isotopic composition of CH4 can help distinguish between specific emitters of CH4. However, in Australia there is a paucity of data on the various isotopic signatures of the different source types. This research examines whether dual isotopic signatures of CH4 can be used to distinguish between sources of CH4 in the Surat Basin. We also highlight the benefits of sampling at nighttime. During two campaigns in 2018 and 2019, a mobile CH4 monitoring system was used to detect CH4 plumes. Sixteen plumes immediately downwind from known CH4 sources (or individual facilities) were sampled and analysed for their CH4 mole fraction and ffi 13CCH4 and ffiDCH4 signatures. The isotopic signatures of the CH4 sources were determined using the Keeling plot method. These new source signatures were then compared to values documented in reports and peer-reviewed journal articles. In the Surat Basin, CSG sources have ffi13CCH4 signatures between 55 :6 parts per thousand and 50 :9 parts per thousand and ffiDCH4 signatures between 207 :1 parts per thousand and 193 :8 parts per thousand. Emissions from an open-cut coal mine have ffi 13CCH4 and ffi DCH4 signa- tures of 60 :0 similar to 0 :6 parts per thousand and 209 :7 similar to 1 :8 parts per thousand respectively. Emissions from two ground seeps (abandoned coal exploration wells) have ffi 13CCH4 signatures of 59 :9 similar to 0 :3 parts per thousand and 60 :5 similar to 0 :2 parts per thousand and ffi DCH4 signatures of 185 :0 similar to 3 :1 parts per thousand and 190 :2 similar to 1 :4 parts per thousand. A river seep had a ffi 13CCH4 signature of 61:2 similar to 1:4 parts per thousand and a ffiDCH4 signature of 225:1 similar to 2:9 parts per thousand. Three dominant agricultural sources were analysed. The ffi13CCH4 and ffi DCH4 signatures of a cattle feedlot are 62:9 similar to 1:3 parts per thousand and 310:5 similar to 4:6 parts per thousand respectively, grazing (pasture) cattle have ffi13CCH4 and ffi DCH4 signatures of 59:7 similar to 1:0 parts per thousand and 290:5 similar to 3:1 parts per thousand respectively, and a piggery sampled had ffi 13CCH4 and ffiDCH4 signatures of 47:6 similar to 0:2 parts per thousand and 300:1 similar to 2:6% respectively, which reflects emissions from animal waste. An export abattoir (meat works and processing) had ffi13CCH4 and ffiDCH4 signatures of 44:5 similar to 0:2 parts per thousand and 314:6 similar to 1:8 parts per thousand respectively. A plume from a wastewater treatment plant had ffi 13CCH4 and ffi DCH4 signatures of 47:6 similar to 0:2 parts per thousand and 177:3 similar to 2:3 parts per thousand respectively. In the Surat Basin, source attribution is possible when both ffi13CCH4 and ffiDCH4 are measured for the key categories of CSG, cattle, waste from feedlots and piggeries, and water treatment plants. Under most field situations using ffi13CCH4 alone will not enable clear source attribution. It is common in the Surat Basin for CSG and feedlot facilities to be co-located. Measurement of both ffi13CCH4 and ffi DCH4 will assist in source apportionment where the plumes from two such sources are mixed.

Automated summary

This research examines the use of dual isotopic signatures of CH4 to distinguish between sources in the Surat Basin. The study found distinct isotopic signatures for various sources and highlighted the benefit of nighttime sampling for accurate source attribution.