In the Broken Hill Managed Aquifer Recharge (BHMAR) project, a large (>50) multi-disciplinary group of geoscientists (geomorphologists, sedimentologists, regolith geoscientists, hydrogeologists, hydrogeophysicists, hydrogeochemists, geospatial analysts, remote sensing specialists, geochronologists, groundwater modelers and structural geologists), worked in a team to understand the hydrogeological system and identify MAR and potential groundwater resource targets. A trans-disciplinary approach was critical to successful completion of MAR pre-commissioning maximal and residual risk assessments. Initially, a number of different (Laterally and Spatially Constrained) inversions of the AEM data were carried out, with refinements made as additional information on vertical and lateral constraints became available. However, hydrostratigraphic mapping using all of these inversions was unable to resolve fundamental aspects of the hydrogeological system, particularly in the near-surface (top 20m), where hydrodynamic data indicated a connection between the major rivers and the underlying aquifers, either through incision and/or through faults (and bypass flow). To resolve these issues, a trans-disciplinary approach was used to investigate all underlying assumptions, including the regularization used in the AEM inversion. In modern laterally-correlated inversion of AEM data, the usefulness of the resulting inversion models depends critically on an optimal choice of the vertical and horizontal regularization of the inversion. Set the constraints too tight, and the resulting models will become overly smooth and potential resolution is lost. Set the constraints too loose, and spurious model details will appear that have no bearing on the hydrogeology. In this study we used a pragmatic approach to optimizing the constraints by an iterative procedure involving all available geological, hydrogeological, geochemical, hydraulic and morphological data and understanding. This was accomplished using a Wave Number Domain Approximate (WANDA) Inversion procedure with a 1D multi-layer model and constraints in 3D. This inversion procedure only takes days to run, enabling the rapid trialing to select the most appropriate vertical and horizontal constraints. In this approach, in a process of both confirming and negating established interpretations and underlying assumptions, the inversion results are judged by their ability to support a coherent conceptual model based on all available information. This approach is dependent on integrating a team of scientists, where all facets of data and interpretation are considered and questioned in a trans-disciplinary analysis of the hydrogeological system. Necessary elements for this approach to succeed are the experience and professional insights of the scientists involved and a willingness and ability of scientists from diverse areas to establish a dialogue that will question and refine the inversion constraints and the quality of the final hydrogeological conceptual model. This approach has been essential to the identification and assessment of MAR and groundwater extraction options in the Broken Hill Managed Aquifer Recharge project. The resultant improved 3D conductivity model revealed details of the hydrostratigraphy and Neogene-to-Present tectonics. Prior to the mapping of near-surface hydrostratigraphy and structural features, it had not been possible to explain apparently contradictory data, nor develop a plausible hydrogeological conceptual model. In summary, a pragmatic approach to optimizing the constraints was achieved using an iterative procedure involving all available geological, hydrogeological, geochemical, hydraulic and morphological data and understanding.


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