A reassessment of the Lower Namoi Catchment aquifer architecture and hydraulic connectivity with reference to climate drivers |
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| Authors: | B F J Kelly W Timms T J Ralph B M S Giambastiani A Comunian A M McCallum |
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| Institution: | 1. Connected Waters Initiative Research Centre;2. National Centre for Groundwater Research and Training;3. Biological Earth and Environmental Sciences, The University of New South Wales, UNSW Sydney, NSW 2052, Australiabryce.kelly@unsw.edu.au;5. School of Mining Engineering, The University of New South Wales, NSW 2052, Australia;6. Department of Environment and Geography, Macquarie University, NSW 2109, Australia;7. Physics and Earth Sciences Department, University of Ferrara, via Saragat, 1–44122 Ferrara, Italy;8. Biological Earth and Environmental Sciences, The University of New South Wales, UNSW Sydney, NSW 2052, Australia;9. School and Civil and Environmental Engineering, The University of New South Wales, NSW 2052, Australia |
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| Abstract: | We demonstrate the need for better representations of aquifer architecture to understand hydraulic connectivity and manage groundwater allocations for the ～140 m-thick alluvial sequences in the Lower Namoi Catchment, Australia. In the 1980s, an analysis of palynological and groundwater hydrograph data resulted in a simple three-layer stratigraphic/hydrostratigraphic representation for the aquifer system, consisting of an unconfined aquifer overlying two semi-confined aquifers. We present an analysis of 278 borehole lithological logs within the catchment and show that the stratigraphy is far more complex. The architectural features and the net-to-gross line-plot of the valley-filling sequence are best represented by a distributive fluvial system, where the avulsion frequency increases at a slower rate than the aggradation rate.We also show that an improved understanding of past climates contextualises the architectural features observable in the valley-filling sequence, and that the lithofacies distribution captures information about the impact of climate change during the Neogene and Quaternary. We demonstrate the correlation between climate and the vertical lithological succession by correlating the sediment net-to-gross ratio line-plot with the marine benthic oxygen isotope line-plot – a climate change proxy. Pollens indicate that there was a transition from a relatively wet climate in the mid–late Miocene to a drier climate in the Pleistocene, with a continuing drying trend until present. Groundwater is currently extracted from the sand and gravel belts associated with the high-energy wetter climate. However, some of these channel belts are disconnected from the modern river and flood zone. We show that the cutoff between the hydraulically well- and poorly connected portions of the valley-filling sequence matches the connectivity threshold expected from a fluvial system. |
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| Keywords: | groundwater hydrostratigraphy global climate change Neogene distributive fluvial system Namoi Catchment |
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