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  • A multi-disciplinary, hydrogeological systems mapping approach has been developed to guide development of new geological and hydrogeological conceptual models, and provide a framework for understanding complex hydrogeological and hydrogeochemical processes. Integration of the 3D mapping with hydrochemical and hydrodynamic data provides critical new insights into surface-groundwater interactions and groundwater flow. Using this approach, it has been possible to develop a new understanding of recharge processes, and identify potential recharge and groundwater flow pathways. The new datasets, knowledge and hydrogeological conceptual models provide a reliable basis for the identification, characterisation and initial assessment of groundwater resources and MAR options. To meet the challenge of rapid identification and assessment of potential MAR targets and groundwater resources over the relatively large study area (7,541.5 sq km) within relatively short timeframes (18 months), the only cost-effective method with the ability to resolve key features of the hydrogeological system in the 0-150m depth range was airborne electromagnetics (AEM). The SkyTEM system is a high-resolution helicopter-borne time-domain electromagnetic system, and was developed specifically for high-resolution groundwater and environmental investigations. The SkyTEM survey, validated by borehole and ground geophysics and drilling, successfully delineated the key functional elements of the Darling Floodplain hydrogeological system, and identified potential groundwater resources, zones of river leakage, and a large number of potential MAR targets. The survey revealed significant heterogeneity in the sub-surface electrical conductivity structure, reflecting a complex geology. The survey mapped heterogeneity (and 'holes') within the near-surface aquifers and confining aquitards, while conductivity variations validated by drilling enabled five hydraulic classes (based on grain size) to be mapped within the main aquifers, as well as groundwater salinities. Locally, pump and slug tests, and NMR data were integrated with the AEM data to produce maps of interpreted hydraulic conductivity and aquifer transmissivity. Previously unrecognised faults, and landscape warping and tilting are observed to disrupt hydrostratigraphic units. These data necessitated development of a completely new hydrogeological conceptual model for the study area. This model shows the importance of faulting and erosional 'holes' in aquitards for recharge models. Discrete vertical fault offsets up to 20m produce localised inter-aquifer leakage. Sampling of rainfall, river, lake, groundwater and pore fluids has provided a comprehensive hydrochemical dataset for the alluvial aquifers of the Darling River floodplain. Major ion chemistry highlighted a mixing signature between river waters, the shallow unconfined aquifer and the underlying semi-confined target Calivil aquifer. Hydrochemical analysis including fuzzy-k means (FCM) cluster analysis, integrated with conventional hydrochemical and hydrodynamic analysis also provides invaluable new insights into groundwater processes. Recharge is dominated by river leakage during high flows, when scouring of riverbank mud veneers allows infiltration. In summary, the new hydrogeological conceptual model of the study area has enabled a number of MAR options to be identified and assessed. The integrated, multi-disciplinary approach provides critical insights for developing appropriate conceptual models for groundwater processes and dynamics. This approach provides an invaluable tool for the rapid identification and assessment of MAR options, particularly in shallow sedimentary systems. *Note: corresponding author is Ken Lawrie, as Ross S. Brodie is currently on leave until February.

  • Multiple lines of evidence were used to understand recharge processes in shallow (<100m) unconsolidated alluvial sediments of the Darling River floodplain, NSW. Major-ion chemistry highlighted a mixing signature between river waters, the shallow unconfined aquifer and the underlying semi-confined Pliocene aquifers. The hydrostratigraphy and groundwater salinities were mapped using airborne electromagnetics (AEM), validated by drilling. The fresh near-river shallow groundwater has a modern carbon signature. The mounding of groundwater levels near the river indicates the regional significance of losing river conditions. Stable isotope data show that recharge is episodic and linked to high-flow flood events rather than river leakage being continuous. This is also evident when groundwater chemistry was compared with river chemistry under different flow conditions. Critically, rapid and significant groundwater level responses were measured during flood events. Continuation of rising trends after the flood peak receded suggests that this is an actual recharge response rather than hydraulic loading. Mud veneers and mineral precipitates are evident along the Darling River channel bank when river flows are low. During low flow conditions these act as impediments to river leakage. During floods, high flow velocities scour these deposits, revealing lateral-accretion surfaces in the shallow scroll plain sediments. This scouring allows lateral bank recharge to the shallow aquifer. During flood recession, mud veneers are re-deposited while return flows from bank storage results in carbonate precipitation in river banks. Recharge to the underlying Pliocene aquifer occurs through mapped faults and via erosional 'holes' in the confining aquitard. Mapped depressions in the river bed ('cod holes'), are floored by indurated clays, and do not provide preferential connectivity to the underlying aquifer. Such flow-dependent recharge has implications for groundwater assessment and management. For example, an analysis of historic river flows suggests that active recharge to the groundwater system would only occur for about 17% of the time when flow exceeds about 9,000 ML/d. Recharge would be negligible with groundwater extraction during low-flow conditions.

  • Airborne electromagnetic (AEM) methods for near-surface hydrogeological investigations have undergone significant improvements in the past 10-15 years, particularly in the development of calibrated systems designed for high-resolution groundwater and environmental investigations (Sorensen & Auken, 2004; Auken et al., 2006). Important advances have been also been made in the development of rapid computational methods for AEM inversion (e.g. Christensen, 2002; Christensen et al., 2009), enabling conductivity models to be available for integration into real-time, short-term hydrogeological investigations (Lawrie et al., 2012). The processing of AEM data and the presentation of electrical conductivity data as maps and sections is now routine, and particularly effective for regional mapping in shallow-dipping sedimentary environments (Lane, 2002; Lawrie et al., 2009, 2012). In Australia, the application of electromagnetic (EM) methods for hydrogeological investigation is made more complex by the highly salinized nature of many landscapes, which can also be often deeply and variably weathered. In many instances, the electrical conductivity distribution does not equate with formation (and/or hydrogeological) boundaries, but instead to a combination of groundwater salinity and formation composition and texture (Lawrie et al., 2000, 2009). Despite these additional challenges, AEM is the only broadacre technique that can detect and resolve key functional elements of near-surface groundwater systems (Spies & Woodgate, 2005). The complex electrical structure of Australia's near-surface landscapes and the presence of conductive layers and basement in many regolith terrains has necessitated the development of constrained inversion approaches that utilise a priori geological, hydrogeological and hydrogeophysical data (Green & Munday, 2004; Lane et al., 2004; Lawrie et al., 2012). Constrained inversions, combined with rigorous technology selection, and appropriate calibration and validation procedures have enabled the successful mapping of potential groundwater resources and salinity hazards in several floodplain environments (Walker et al., 2004; Lawrie et al., 2009, 2012; Christensen & Lawrie, 2012). Importantly, studies in Australia have also demonstrated that the benefits from new AEM technologies and constrained inversion modeling are maximised when these technologies are employed within multi-disciplinary, systems-based approaches to the analysis of problems (George et al., 2003). Systems-based approaches incorporate an understanding of landscape evolution and scale, utilise modern investigative approaches to the conceptualisation of groundwater systems, and incorporate data on mineralogy, petrophysics, hydrology, ecology, topography, hydrogeochemistry and hydrodynamics. Within this multi-disciplinary research framework, the power and long-term value of AEM-based datasets for groundwater management lies largely in providing stakeholders with a range of customized interpretation products derived from the integration of electrical conductivity data with other hydrogeological, hydrogeophysical and hydrochemical datasets (George et al., 2003; Lawrie et al., 2000, 2009, 2012). The Broken Hill Managed Aquifer Recharge (BHMAR) project, in western N.S.W., Australia, has built significantly on the principles, methodologies, experience and products developed for salinity mapping and management in Australia. This included, a staged approach to technology selection and survey design, and the use of a 4D systems approach to integrate

  • In the southern half of Australia, recent droughts and predictions of a drier future under a number of climate change scenarios have led to the search for innovative strategies to identify more secure water supplies for regional communities and industries. This study was commissioned to investigate groundwater options for increasing the drought security for the city of Broken Hill. Investigations involved the assessment of a number of aquifers across a broad region, followed by the rapid mapping and assessment of potential MAR and/or groundwater extraction sites over a large data-poor area (>7,500 km2), of the Darling floodplain. Data acquisition included an airborne electromagnetics (AEM) survey (31,834 line km), a 7.5 km drilling program (100 sonic and rotary mud holes), and complementary field and laboratory measurements. Integrated studies found that surface infiltration approaches were not viable MAR options in this area due to the ubiquitous presence of thick near-surface aquitards. However, 3D mapping validated by drilling and complementary hydrogeological investigations identified >30 potential Aquifer Storage and Recovery (ASR) targets where Pliocene aquifers contain significant volumes of fresh groundwater and are sandwiched between confining aquitards. A pre-commissioning semi-quantitative residual risk assessment was carried out for a priority site (Jimargil), located within 20 km of existing power and surface water infrastructure at Menindee. Using national MAR guidelines, assessment of 12 hazard types included hydrogeological modelling, laboratory column clogging studies and geochemical assessment to identify source water treatment requirements. The study found that the residual scientific/technical risks for ASR at Jimargil are low. Key to project success was the development of new rapid mapping and assessment methodologies and workflows. It is our understanding that this is the first use of AEM as part of multi-disciplinary mapping and assessment of MAR targets. The investigations in this study also completely revised our understanding of the age, stratigraphy, structure and mode of deposition of the Darling floodplain sediments, with practical implications for the hydrogeological conceptual model underpinning the assessment of groundwater resources and MAR options.

  • The discovery of significant volumes of good-quality groundwater resources in the BHMAR project study area, near Menindee, NSW, highlights the likelihood of similar opportunities further upstream in the Darling-Barwon system, and in other data-poor river systems within the Murray-Darling Basin (MDB) and elsewhere. The project identified the importance in the Darling floodplain of river leakage for recharge, especially during high-flow events. Similarities in geomorphology, stratigraphy and tectonics between the study area and the Upper Darling indicate that groundwater resources could occur where recharge pathways through the upper confining aquitard connect scroll-plain tracts to suitable (Calivil Formation equivalent) aquifer cells. Based on project findings, groundwater investigations in the Darling upstream of Menindee along coincident, separate and particularly intersecting scroll-plain tracts should acquire fundamental data to characterise confining aquitards, semi-confined and surficial unconfined aquifers and zones of preferential river leakage. In the Upper Darling, hydrogeological similarities with the BHMAR study area are likely to also provide opportunities for managed aquifer recharge (MAR). The new understanding of recharge mechanisms during flood events has broader implications for the modelling and assessment of surface-groundwater interaction in many Basin rivers. This includes the need to vary stream bed conductance under different stream flow regimes. It is also recommended that estimates of groundwater extraction limits for relevant aquifers should focus on recharge from flood-based episodic river leakage. In the BHMAR study, the integrated use of airborne electromagnetics (AEM), ground electrical methods, sonic drilling and borehole nuclear magnetic resonance (NMR), enabled the rapid characterisation of complex hydrogeological systems, including key groundwater parameters necessary for assessing groundwater resources and MAR options. This methodology has the potential for application in many Australian landscapes (and more broadly). The new geological, geophysical, geochemical and hydrogeological datasets and understanding acquired in this project also have broader implications for fundamental geological studies and mineral exploration.

  • The Broken Hill Managed Aquifer Recharge (BHMAR) project is part of a larger strategic effort aimed at securing Broken Hill's water supply and identifying significant water-saving measures for the Darling River system. In this study, airborne electromagnetics (AEM) mapping validated by drilling, field and laboratory measurements has identified significant volumes of fresh to acceptable quality groundwater stored beneath the Darling Floodplain. These potential resources were identified in 14 discrete targets within Pliocene aquifers (Calivil Formation and Loxton-Parilla Sands) at depths of 25-120m. The Calivil Formation occurs predominantly within structurally-controlled palaeovalleys. Aquifer quality is best where thick (30-50m), high-yielding zones (test flows > 25 L/s) occur in palaeochannels at the confluence of palaeo-river systems. Here, the hydraulic properties make the Calivil Formation aquifer best suited for groundwater extraction (and/or MAR injection), with excellent recovery efficiencies predicted where ambient salinities are low. The aquifer is sandwiched between variably thick clay aquitards, and is confined to semi-confined. Indicative groundwater volumes have been calculated using groundwater salinity and texture mapping derived for the AEM depth slices, combined with porosity statistics derived from laboratory measurements and borehole nuclear magnetic resonance (NMR) logging. In most of the targets, further investigation is required to quantify natural recharge and discharge processes, identify the negative impacts associated with groundwater pumping (particularly the potential for saline groundwater ingress), delineate the more transmissive parts of the formation, and assess the economics and logistics of borefield and water supply design. Calibrated, transient numerical groundwater flow and solute transport models are also needed to determine appropriate groundwater extraction rates. The multi-disciplinary systems-based methodology used in this project has enabled rapid identification and assessment of largely unknown potential groundwater resources and aquifer storage. These have the potential to provide drought security for regional communities and industries, and to assist with regional development.

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