Fugitive CH4 emissions, and their source, detected from dissolved fluvial CH4 concentration profiling
There is both considerable opposition and support for unconventional hydrocarbon extraction, commonly known as 'fracking'. Opposition and hesitancy focuses on two primary issues: 1) aquifer contamination and 2) the increased greenhouse gas (GHG) footprint associated with such activity, particularly the fugitive CH4 emissions, but also CO2 associated with the CH4 can also be released. The occurrence of aquifer contamination is likely to be heavily-monitored as significant regulation generally exists to protect water bodies, particularly those from which we abstract drinking water. However, fugitive emissions by their nature are difficult to detect and quantify and so this atmospheric emission remains a concern.
A challenge in identifying fugitive GHG emissions is that, unless there has been background monitoring of an area prior to any abstraction, one cannot be certain that the fugitive emission detected is associated with the unconventional hydrocarbon extraction. Indeed, there has been little consideration of what other fugitive CH4 emissions exist, and if present, from what source they are derived. The problem is that, unless the temporal variability of other unplanned and unregulated methane emissions are known, and those which are currently hidden are detected and quantified, it will be impossible provide robust answers to industry and regulators about the GHG impact of a new energy extraction process.
The lack of knowledge of whether there are fugitive CH4 emissions and their location reflects the difficulty in detecting these. Aircraft survey independently, or in conjunction with roving vehicles, both fitted with CH4 detectors, can identify fugitive emissions and provide estimates of flux strength. However this is expensive fieldwork, so is generally used to refine fluxes where higher atmospheric CH4 concentrations have already been identified, for example by remote-sensing. An eddy covariance system with a large footprint could characterise land surface CH4 fluxes and so detect sporadic fluxes, but this is equipment is not portable and can only be deployed on flat landscapes. However, rivers integrate processes at the landscape scale and CH4 is highly insoluble in water, with concentrations further reduced by rivers degassing as they flow. Unless there is a fugitive CH4 emission close by, CH4 concentrations in rivers should be close to background. Consequently, we set out to use catchment scale river CH4 concentration profiling to looking for locations of increased dissolved CH4 concentrations. Our premise was that where above background CH4 concentrations are observed, there must be a nearby source of fugitive CH4 emissions contributing to this increased concentration.
Here we shall present the results of three years of sampling of a drainage system where catchment scale sampling found CH4 present at concentrations above background in the river water and allowed us to identify nearby sources of fugitive CH4 emissions. The river drains a catchment which has abandoned underground and open cast coal mining and the fugitive CH4 efflux is released to the atmosphere from springs that have arisen from hydrological rebound as a result of flooded mining systems. This presentation will discuss the spatial and temporal variability of the fugitive CH4 emissions (and associated significant CO2) efflux, and our understanding of where we think the source is of the emission. Crucially, the water chemistry analysis, in particular 14C-dating of the CH4 and dissolved inorganic C pool, does not support the interpretation the methane and carbon dioxide are derived from abandoned mine degassing. This tool has value for all fugitive source identification.
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Prof SUSAN WALDRON (UNIVERSITY OF GLASGOW)
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