GAB | Refined understanding of groundwater in heterogeneous units through identification of vertical stratification of hydrochemistry as identified in the Walloon subgroup of Australia’s Great Artesian Basin

• Daniel Owen
• St.John Herbert

Groundwater management in the Great Artesian Basin (Australia) is challenging due to the large areas and heterogeneity of sequences involved. This challenge has often been addressed with up-scaled conceptual models; however, where there is significant heterogeneity or local competing groundwater uses, more detail is required. When good understanding of heterogeneity within sequences is not available, the upscaled approach can lead to a disparity between model predictions based on homogenisation inherent in upscaling and the observed behaviour of heterogeneous systems, leading to bias in results. This requires improved conceptual resolution of these heterogeneous systems. Geological data are often insufficient to achieve the required resolution. This paper reports on the use of hydrochemical data from existing bores in the Walloon subgroup of the Surat Basin, using compositional data analysis techniques to refine the unit description. The approach moves beyond simple hydrochemical characterisations of water type via conventional hydrochemical interpretations to use a combination of centred- and isometric-log ratios to describe discrete end members. The results show that, despite similar water types, the upper Juandah and lower Taroom coal seams of Walloon subgroup are distinguished by an inverse relationship between ions Cl−, Ca2+, Mg2+, Sr2+, Li− and H+ and ions HCO3−, F− and B3+, and by an isometric log ratio that describes the balance between the activities of Cl, H+ and HCO3. These relationships suggest that higher-resolution conceptual models based on flow paths mapped from hydrochemical data can form the basis for refined groundwater models that address both heterogeneity and regional model.