Stable Isotopes of Lithium as Indicators of Coal Seam Gas-bearing Aquifers

Abstract

In this study lithium isotopes were used in combination with hydrochemistry to investigate interactions between coal-seam-gas bearing sedimentary bedrock aquifers and surrounding basalt and alluvial aquifers in a large catchment in eastern Australia. Understanding groundwater transport and aquifer connectivity is critical to managing coal seam gas (or coal bed methane) developments, because large volumes of water need to be extracted in order to release the sorbed gas; however, to date lithium isotopes have not been applied to coal seam gas groundwater management problems and no information on the δ7Li of coal or coal-seam groundwater is available. Li/Cl and Li/Na ratios in the coal-bearing and sedimentary bedrock aquifers are distinct (>0.0001) from alluvial and basalt aquifers (<0.0001). Preliminary δ7Li results for coal measure samples are typically between 7 and 11‰; many of these samples also contain methane, and can therefore be expected to be influenced by coal and the early stages methanogenesis. Interestingly the coal measure with lowest δ7Li value occurs in an area where the coal measures outcrop and direct recharge is likely, with nearby basalt groundwater having much higher δ7Li values (δ7Li>18‰). Preliminary lithium isotope results show that δ7Li may be effective in distinguishing groundwater flow paths in the coal-bearing aquifer from basalt aquifers, and from a transitional zone between the alluvium and underlying coal measures. Further lithium isotope analysis is being carried out to: a) compare the δ7Li between alluvial, basalt and coal-bearing aquifers to further investigate aquifer connectivity; b) to describe δ7Li for CSG production waters with low- and high-methane groundwater in the coal-bearing aquifer; c) to describe the δ7Li from coal and basalt leachate.