Oral Presentation NCGRT/IAH Australasian Groundwater Conference 2019

Strontium isotopes as tracers to assess inter-aquifer and groundwater-surface water exchanges in sedimentary basins: an example from the Surat and Clarence-Moreton basins in Australia (504)

Matthias Raiber 1 , Andrew Feitz 2 , Steven Flook 3 , Jorge Martinez 1 , Gerhard Schoening 3 , Axel Suckow 4
  1. CSIRO Land and Water, Dutton Park, QLD, Australia
  2. Geoscience Australia, Canberra, ACT, Australia
  3. Office of Groundwater Impact Assessment (OGIA), DNRME, Brisbane, QLD, Australia
  4. CSIRO Land and Water, Adelaide, SA, Australia

Understanding connectivity between aquifers, aquitards and groundwater dependant ecosystems (e.g. streams or springs) remains a challenge and a key component of conceptual hydrogeological model development, particularly in large sedimentary basins potentially affected by coal or gas resource developments.

This presentation will demonstrate how Sr isotope (87Sr/86Sr) analysis can help to identify and characterise interactions between different aquifer systems and overlying surface water features within the geologically linked Surat and Clarence-Moreton basins.

For this, samples (>200) were collected from the major aquifers (e.g. Gubberamunda, Hutton, Precipice and Springbok sandstones), from the coal seam gas target formation (Walloon Coal Measures) as well as rainfall, shallow alluvium, streams and springs. In addition, sub-samples of core  from exploration and stratigraphic wells within these two sedimentary basins were analysed to provide end-member strontium compositions from host rocks. 

The analysis of cores demonstrated that there is a distinct and regular vertical contrast in 87Sr/86Sr ratios between different hydrogeological units. The analysis of groundwater from sedimentary and volcanic bedrock showed that all major hydrogeological units have a distinct  and relatively narrow range of Sr isotope ratios, confirming the benefit of this tracer as a fingerprinting tool to characterise  connectivity between aquifers in these basins.  This was further confirmed by Sr isotope analyses in shallow groundwater and surface water.  A synoptic stream sampling campaign under baseflow conditions confirmed that Sr isotope signatures in creek water are similar to those of the underlying bedrock formations, allowing to identify the regions where aquifers and surface water systems may interact.

Although each study requires screening of the regional geology and mineralogy to determine whether Sr isotopes are useful as an isotopic fingerprinting tool, the results of this study suggest that a similar approach is likely to be useful for refining the conceptualisation of hydrogeological systems in other sedimentary basins in Australia (e.g. other sub-basins of the Great Artesian Basin) and globally.