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Title: Use of multi-isotope surveys to identify bedrock-alluvium interactions, Cressbrook Creek Catchment, southeast Queensland
Authors: King, AC
Raiber, M
Cendón, DI
Cox, ME
Keywords: Carbon 14
Ground water
Strontium 87
Strontium isotopes
Stable isotopes
Oxygen isotopes
Metamorphic rocks
Issue Date: 19-Sep-2013
Publisher: International Association of Hydrogeologists
Citation: King, A., Raiber, M., Cendón, D. & Cox, M. (2013). Use of multi-isotope surveys to identify bedrock-alluvium interactions, Cressbrook Creek Catchment, southeast Queensland. Paper presented to IAH 2013, Perth, Australia : "Solving the groundwater challenges of the 21st century", International Association of Hydrogeologists, 40th International Conference, Perth, Western Australia, 15-20 September 2013. (p. 162).
Abstract: Radiocarbon (14C) is commonly used to study groundwater residence times, but the interpretation of results is often subject to a high degree of uncertainty due to interaction with modern and ‘dead carbon’, especially for relatively young groundwater or groundwater that has interacted with organic material. To address this concern, 87Sr/86Sr ratios, δ2H and δ18O, combined with tritium and radiocarbon are used to identify zones where older bedrock water recharges the alluvial aquifer of the Cressbrook Creek catchment in southeast Queensland. Cressbrook Creek is an intermittent stream that is primarily recharged by groundwater in the upper catchment. The alluvial system overlies variable bedrock with metamorphic rocks, rhyolites and granites in the headwaters, and sedimentary sequences (mostly sandstones) downstream. The catchment has largely been dry during a decade of drought, but has owed continuously since 2010 and experienced severe flooding in January 2011. Groundwater samples collected from alluvial and bedrock aquifers in June 2011 were analysed for a range of environmental tracers. Six alluvial waters were analysed for 14C; of these, four are modern. The other two samples have pMC values of 88.0 and 81.1 (uncorrected ages of 1,045 and 1,680 years; Sites A and B, respectively) whereas tritium analyses indicate an age of less than 100 years for the same samples. This disparity in groundwater ages may have been caused by: 1) seepage of older bedrock groundwater into the alluvium; or 2) carbonate dissolution processes. Therefore, evidence from other tracers, including 87Sr/86Sr, was assessed as an independent constraint to support the conceptual understanding of aquifer interactions. Alluvial groundwater from Site A has an enriched 87Sr/86Sr signature, indicating that it has probably received recharge from the underlying granite aquifer. Groundwater from Site B has a lower 87Sr/86Sr ratio than the other alluvial groundwaters, which indicates this site probably received recharge from the underlying sandstone aquifer.
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