Hydrochemistry of the Southland region

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Daughney, C.J.; Rissmann, C.; Friedel, M.J.; Morgenstern, U.; Hodson, R.; van der Raaij, R.W.; Rodway, E.; Martindale, H.; Pearson, L.; Townsend, D.B.; Kees, L.; Moreau, M.; Millar, R.; Horton, T. 2015 Hydrochemistry of the Southland region. Lower Hutt, N.Z.: GNS Science. GNS Science report 2015/24 202 p.

Abstract: The purpose of this study is to characterise the chemistry of the groundwater and surface water resources of the Southland region. This characterisation of hydrochemistry is intended to assist with the conceptualisation of the regional groundwater-surface water system and inform development of groundwater flow and transport models. The ultimate objective is to aid land and water management in Southland under the National Policy Statement for Freshwater Management (NPS-FM) 2014. This study has been jointly undertaken and delivered by Environment Southland and GNS Science. This work is a component of the Water and Land 2020 & Beyond (WAL2020) programme, Environment Southland’s response to the NPS-FM. Specifically, this work fits within the Fluxes and Flows objective, the component of the WAL2020 programme that aims to determine the origin of water and pathways and fluxes of contaminants through Southland’s groundwaters and surface waters and into receiving water bodies. This work is also a component of the GNS Science-led Ministry of Business, Innovation and Employment-funded research programme Tracer Validation of Hydrological Systems, which aims to provide resource managers with means of using hydrochemical tracers to develop and calibrate numerical groundwater-surface water models. The general approach in this study is to use the hydrochemistry of specific samples and sites for ‘fingerprinting’ the origin and flow paths of water and contaminants through the region’s groundwater-surface water system. This study has made use of an existing hydrochemical dataset of over 26,000 water samples collected from over 1,000 locations across the region between January 2010 and December 2014, each of which was analysed for up to 50 parameters. This study also involved collection and analysis of new samples for determination of water age based on concentrations of tritium, chlorofluorocarbons and sulphur hexafluoride, and identification of groundwater-surface water interaction based on concentrations of radon. One notable advance in this study was to apply a modified version of the self-organising map (SOM) technique to estimate the values of every hydrochemical parameter that had not been specifically measured in the lab or the field. This approach greatly expanded the amount of information that could be used for the hydrochemical evaluations. A series of quality assurance and quality control checks confirmed that these hydrochemical datasets are fit for the purpose of this investigation and would permit a high level of analysis. This study has also made use of existing regional GIS layers for catchment boundaries, soil types and characteristics, and surface and subsurface geology. (auth)