Groundwater systems in northern Australia – are they suitable for a northern food bowl: evidence from residence times and geochemical analyses of ground and surface waters in the Lawn Hill region, northwest Queensland
Geological Society of Australia
Water resources in the northern regions of Australia have become increasingly important with the possible development of a northern ‘food-bowl’. Understanding the chemistry, flow systems, and mean residence time (MRT) of groundwater systems in this region is therefore essential. The Lawn Hill region of northwest Queensland is subjected to a semi-arid monsoonal climate with an average of 542 mm/a – the majority falling in the warmer months through November–March. Due to the polarity of rainfall, most streams run dry during the dry season. However, four major perennial streams are maintained by groundwater discharge, highlighting the importance of groundwater–surface water interaction and our understanding of such systems. The regional geology is dominated by the Barkly Tableland, an expansive Cambrian carbonate plateau, or siliciclastic formations of the Proterozoic Mount Isa Inlier. The carbonate and siliciclastic lithologies exhibit variable influences on groundwater chemistry and flow. There are distinct differences in chemical signatures whereby the carbonate-hosted groundwater was found to be strongly influenced by carbonate dissolution with little evidence of evapotranspirative enrichment as indicated by both Cl concentrations and stable water isotopes (SWI). Conversely, major ion chemistry and SWI composition of the siliciclastic-hosted groundwaters suggest they are strongly influenced by evapotranspirative enrichment and less by later water–rock interactions (though these do impart a signature on groundwater chemistry). Importantly, the limited influence of evapotranspiration on carbonate-hosted groundwater with TDS values ranging 540–611 mg/L means these waters are classified as freshwater and represent a low–medium irrigation salinity risk. Comparatively, the siliciclastic-hosted groundwaters have much higher TDS values (599–7204 mg/L), which spans the fresh–brackish–saline classifications and represents a medium–high salinity risk. These differences highlight the fact that suitability of groundwater for irrigation purposes greatly depends on the geological controls through water–rock interactions and influence on groundwater infiltration. The mean residence time (MRT) of groundwater is an important indicator for groundwater sustainability. Again, there is a clear distinction between carbonate and siliciclastic hosted groundwaters. Both tritium and radiocarbon analyses of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) indicate a modern age for carbonate groundwaters with fast recharge indicating the carbonate groundwater is less susceptible to depletion but more susceptible to anthropogenic influences. Comparatively, measurable tritium concentrations in siliciclastic groundwaters indicate a modern system; however, DIC and DOC radiocarbon analyses indicate a much older groundwater up to ca 10 000 a (depending on the flow model applied). The differences between tritium and radiocarbon MRT may indicate more complex mixing between young and old groundwater and slower recharge, suggesting siliciclastic groundwaters may be susceptible to depletion.
Water, Australia, Ground water, Rain water, Chemistry, Carbonates
Van der Ley, M., Cendón, D. I., & Graham, I. T. ( 2014). Groundwater systems in northern Australia – are they suitable for a northern food bowl: evidence from residence times and geochemical analyses of ground and surface waters in the Lawn Hill region, northwest Queensland. Presentation to the Australian Earth Sciences Convention 2014 (AESC 2014), 22nd Geological Convention, Newcastle NSW, 7-10 July 2014, (pp. 4-5). Retrieved from: http://aesc2014.gsa.org.au/assets/Various-reg-partner-opp-workshop-summ-/AESC-Abstract-Proceedings.pdf