Strontium isotopes in the atmosphere, geosphere and hydrosphere: Developing a systematic "fingerprinting" framework of rocks and water in sedimentary basins in eastern Australia

Abstract

Understanding the connection between aquifers, aquitards, and groundwater-dependant ecosystems remains a key challenge when developing a conceptual hydrogeological model. The aim of this study was to develop a systematic strontium isotope (⁸⁷Sr/⁸⁶Sr) fingerprinting framework of rocks and water within the sedimentary Surat and Clarence-Moreton basins (SCM basins) in eastern Australia - an area of extensive coal seam gas development and high potential for aquifer and groundwater-surface water connectivity. To do this, new groundwater samples (n = 298) were collected, analyzed and integrated with published data (n = 154) from the basins' major sedimentary, volcanic and alluvial aquifers, including the major coal seam gas target, the Walloon Coal Measures. Samples were also analyzed from rainfall (n = 2) and surface water (n = 40). In addition, rock core samples (n = 39) from exploration and stratigraphic wells were analyzed to determine the range of Sr isotope composition from host rocks. The analyses of cores demonstrate a distinct and systematic contrast in ⁸⁷Sr/⁸⁶Sr between different hydrogeological units. This confirms that all major hydrogeological units have a narrow range with unique ⁸⁷Sr/⁸⁶Sr population characteristics that are useful for guiding conceptual model development. Comparison with selected hydrochemical and groundwater age tracers (¹⁴C and ³⁶Cl) suggests only limited changes of ⁸⁷Sr/⁸⁶Sr from recharge beds to the deeper parts of the basins or with a decrease in natural ¹⁴C and ³⁶Cl tracer content along flow paths. Stream sampling during baseflow conditions confirms that ⁸⁷Sr/⁸⁶Sr in surface waters are similar to those of the underlying bedrock formations. We demonstrated that ⁸⁷Sr/⁸⁶Sr analyses of rocks and water provide a powerful hydrostratigraphic and chemostratigraphic fingerprinting framework in the SCM basins, enabling reliable assessments of plausible aquifer and groundwater-surface water interconnectivity pathways. Applied in other complex multi-aquifer sedimentary basins in Australia, and globally, a similar approach can help to constrain conceptual hydrogeological models and facilitate improved water resource management.

Keywords: Aquifer connection; Clarence-Moreton Basin; Great Artesian Basin (GAB); Groundwater-dependent ecosystems (GDEs); Strontium isotopes; Surat Basin.