Catchment-scale hydrological and hydrogeological investigations commonly conclude by finding that particular stream reaches are either gaining or losing; they also often assume that the influence of bedrock aquifers on catchment water balances and water quality is insignificant. However, in many cases, such broad findings are likely to oversimplify the spatial and temporal complexity of the connections between the different hydrological system components, particularly in regions dominated by cycles of droughts and flooding. From a modelling perspective, such oversimplifications can have serious implications on the process of identifying the magnitude and direction of the exchange fluxes between the surface and groundwater systems. In this study, we use 3D geological modelling and historic water chemistry and hydraulic records to identify the origins of groundwater at different locations in the alluvium and along the course of streams in the Lockyer Valley (Queensland, Australia), a catchment impacted by a severe drought (‘Millennium Drought’) from 1998 to 2009, followed by extensive flooding in 2011. We also demonstrate how discharge from the sub-alluvial regional-scale volcanic and sedimentary bedrock influences the water balance and water quality of the alluvium and streams. The investigation of aquifer geometry via development of a three-dimensional geological model combined with an assessment of hydraulic data provided important insights on groundwater flow paths and helped to identify areas where bedrock aquifers interact with shallow alluvial aquifers and streams. Multivariate statistical techniques were then applied as an additional line of evidence to groundwater and surface water hydrochemical data from large historical datasets. This confirmed that most sub-catchments within the Lockyer Valley have distinct water chemistry patterns, which result from mixing of different water sources, including discharge from the sub-alluvial bedrock. Importantly, in addition to the observed spatial variability, time-series hydrochemical groundwater and surface water data further demonstrated that the hydraulic connection between alluvial aquifers, streams and sub-alluvial bedrock aquifers is temporally dynamic with very significant changes occurring at the transition from normal to drought conditions and following flooding, affecting both catchment water quality and water balances. <b>Citation:</b> M. Raiber, S. Lewis, D.I. Cendón, T. Cui, M.E. Cox, M. Gilfedder, D.W. Rassam, Significance of the connection between bedrock, alluvium and streams: A spatial and temporal hydrogeological and hydrogeochemical assessment from Queensland, Australia, <i>Journal of Hydrology</i>, Volume 569, 2019, ISSN 0022-1694, https://doi.org/10.1016/j.jhydrol.2018.12.020.

<div>Cooper Creek is a dryland river system that extends from the western Great Dividing Range in Central Queensland to Lake Eyre in South Australia. The middle course of the Cooper Creek is characterised by anabranching river channels across a wide floodplain that flow intermittently due to monsoonal flooding event higher in the catchment. As floodwaters recede, freshwater stagnates within numerous deeper segments of river channels forming ‘waterholes’ which support ecosystems with significant ecological and cultural value. However, there is little evidence that shallow groundwater discharges into these surface water bodies and the link between surface water and groundwater is not well understood. This study aims to demonstrate how airborne electromagnetics (AEM) and other geoscientific data can be integrated to identify recharge within shallow saline groundwater systems, which are so common in arid inland Australia.</div><div> The regional water table underneath the floodplain is shallow (<10m) and highly saline (>38,000 TDS), with a chemical signature suggesting salts were concentrated by evapotranspiration. Surface swelling clays likely limits the amount of recharge that occurs through the floodplain itself. However, a detailed study by Cendón et al (2010) found that during high flow events, floodwater scoured the base of the waterholes allowing freshwater to recharges into the shallow groundwater system forming chemically distinct freshwater lenses.</div><div> AEM is a geophysical technique capable of estimating bulk conductivity for the top few hundred metres of the subsurface. Part of the AusAEM Eastern Resource Corridor survey (Ley-Cooper 2021) crossed the Cooper Creek floodplain with a 20km line spacing. The bulk conductivity models delivered as part of this survey resolved the top of the saline water table regionally. In several locations, we identified resistive lenses sitting on the shallow water table which coincide with river channels that are frequently inundated.</div><div><br></div><div>Cendón, D.I., Larsen, J.R., Jones, B.G., Nanson, G.C., Rickleman, D., Hankin, S.I., Pueyo, J.J. and Maroulis, J., 2010. Freshwater recharge into a shallow saline groundwater system, Cooper Creek floodplain, Queensland, Australia.&nbsp;<em>Journal of Hydrology</em>,&nbsp;<em>392</em>(3-4), pp.150-163.</div><div>LeyCooper, Y. 2021. Exploring for the Future AusAEM Eastern Resources Corridor: 2021 Airborne Electromagnetic Survey TEMPEST® airborne electromagnetic data and GALEI inversion conductivity estimates. Geoscience Australia, Canberra.</div> This Abstract was submitted/presented to the 2022 Australasian Groundwater Conference 21-23 November (https://www.aig.org.au/events/australasian-groundwater-conference-2022/)

<div> Airborne electromagnetic (AEM) data has been acquired at 20km line spacing across much of the Australian continent and conductivity models generated by inverting these data are freely available. Despite the wide line spacing these data are suitable for imaging the near surface and better understanding groundwater systems. Twenty-kilometre spaced AEM data acquired over the Cooper Creek floodplain using a fixed-wing towed system were inverted using deterministic and probabilistic methods. The Cooper Creek is an anabranching ephemeral river system in arid eastern central Australia. We integrated conductivity data with a range of surface and subsurface data to characterise the hydrogeology of the region and infer groundwater salinity from the shallow alluvial aquifer across a more than 14,000 km2 Cooper Creek floodplain. The conductivity data also revealed several examples of focused recharge through a river channel forming a freshwater lens within the more regional shallow saline groundwater system.</div><div>&nbsp;</div><div>This work demonstrates that regional AEM conductivity data can be a valuable tool for understanding groundwater processes at various scales with implications for how to responsibly manage water resources. This work is especially important in the Australian context where high quality borehole data is typically sparse, but high-quality geophysical and satellite data are often accessible.</div><div> </div> This presentation was given to the 8th International Airborne Electromagnetics Workshop (AEM2023) (https://www.aseg.org.au/news/aem-2023)

The Upper Burdekin Chloride Mass Balance Recharge web service depicts the recharge rates have been estimated at borehole locations in the Nulla and McBride basalt provinces. Using rainfall rates, rainfall chemistry and groundwater chemistry, the recharge rates have been estimated through the Chloride Mass Balance approach.

The Upper Burdekin Chloride Mass Balance Recharge web service depicts the recharge rates have been estimated at borehole locations in the Nulla and McBride basalt provinces. Using rainfall rates, rainfall chemistry and groundwater chemistry, the recharge rates have been estimated through the Chloride Mass Balance approach.

<div>As part of the Exploring for the Future (EFTF) programme, the groundwater team undertook an in-depth investigation into characterising surface water -- groundwater interaction in the Cooper Creek floodplain using airborne electromagnetics (AEM). This work is to be released as part of the Lake Eyre Basin detailed inventory and as an EFTF extended abstract. As part of Geoscience Australia's commitment to transparent science, the scientific workflows that underpinned a large component of this investigation are to be released as a jupyter notebook. This notebook contains python code, figures and explanatory text that the reader can use to understand how the AEM data were processed, visualised, integrated with other data and interpreted.</div>