Quantifying groundwater discharge into New Zealand rivers using radon and concurrent flow gauging

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Martindale, H.; van der Raaij, R.W.; Knowling, M.J.; Morgenstern, U. 2018 Quantifying groundwater discharge into New Zealand rivers using radon and concurrent flow gauging. Lower Hutt, N.Z.: GNS Science. GNS Science report 2018/25. 39 p.; doi: 10.21420/G2093B

Abstract: Radon has been applied in New Zealand groundwater – surface water interaction studies since 2014 to identify areas of groundwater discharge. To date, these groundwater – surface water interaction studies have been used in an analytical approach to qualitatively assess where groundwater discharge is occurring. In this report, a radon mass-balance modelling approach was applied to two case study sites, the Shag River, Otago and the Te Arai River, Poverty Bay, to obtain insight into groundwater and river water interaction. In both case study rivers, the radon mass-balance model was found to reliably estimate groundwater discharge in some gaining river reaches. Furthermore, the mass balance approach was also able to identify potential areas of significant parafluvial flow. However, in both case studies, the uncertainty of the calculated groundwater discharge is large. Given that the present analytical approach appears to provide a basis for rapid, albeit approximate, assessment of groundwater discharge, it is recommended that it is used as an indicative assessment, in the first instance. Where results indicate more accurate and targeted techniques are required, the more sophisticated but also more expensive approaches for quantify groundwater discharge, such as fibre optic distributed temperature sensing, can be applied. (auth)