A team of scientists at UNSW have tested a new method for detecting if methane from coal seam gas extraction sites is leaking into nearby groundwater.
One of the greatest engineering challenges in coal seam gas extraction is managing concerns about the contamination or breaching of groundwater deep underground.
The team tested their technique at the Walloon Coal Measures, near Dalby in the south east of Queensland. Gas extraction operations in this region have expanded rapidly in the past decade, raising concerns among farmers that the groundwater level in the Condamine River Alluvial Aquifer will fall due to the coal seam gas production.
Project leader Associate Professor Bryce Kelly of the UNSW Connected Waters Initiative Research Centre (CWI) said the team used the technique to test 19 irrigation bores, discovering four locations where the Walloon Coal Measures appear to be hydraulically connected to the Condamine River Alluvial Aquifer. It is not yet known whether the hydraulic connections that allow methane from the coal seam to enter the aquifer are natural, such as a geological fault in the rock, or man-made, such as a leaky abandoned coal exploration well.
However, the tests showed that the risk of any short-term impact from CSG production on groundwater resources used by the irrigation sector remains low.
“Farmers should not be concerned about the current scale of gas extraction to the north west of Cecil Plains,” Professor Kelly said. “But if the industry were to expand considerably, more work would be needed to determine the possible impacts of gas extraction on the aquifer in the decades ahead.”
The technique is based on an analysis of air and groundwater chemistry. The researchers first analysed methane in the air near a coal seam gas water holding pond, in order to determine the isotopic carbon “fingerprint” of the methane that was coming from the Walloon Coal Measures.
Methane from different sources tend to have a different isotopic carbon fingerprint. For example, methane from coal beds tend to differ from the methane produced by other processes, such as microbial activity.
The team identified the isotopic fingerprint of the methane the found in the groundwater pumped from the Condamine Alluvium at 19 locations. They could then compare the two methane types side by side, to see if the methane in the groundwater is the same as the methane from the coal seam.
Two additional tests had to be passed to establish connectivity between the coal seam and groundwater.
Firstly, the groundwater had to be old water containing no tritium, a radioactive form of hydrogen which was introduced into the water supply in the 20th century due to atomic bomb testing
Secondly, the groundwater had to contain high levels of dissolved organic carbon from the coal.
Via their new technique, the researchers concluded that the methane in the water most likely came from the Walloon Coal Measures at only four locations, indicating only a low hydraulic connectivity between the coal seam and the aquifer.
“Our new workflow, based on an analysis of air and groundwater chemistry, could be applied around the world to help safeguard groundwater resources by identifying any underground connections between coal seams and aquifers,” said Professor Kelly.