Hydrochemical classification of groundwater and river state of the environment monitoring sites in the Greater Wellington region

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Daughney, C.J. 2010 Hydrochemical classification of groundwater and river state of the environment monitoring sites in the Greater Wellington region. Lower Hutt, N.Z.: GNS Science. GNS Science report 2009/68 60 p. + 1 CD

Abstract: This investigation employed several complementary multivariate statistical techniques to provide insight into the spatial and seasonal variations at State of the Environment (SOE) river and groundwater quality monitoring sites across the Wellington region. The aim of this study was to assess, compare and classify the hydrochemical composition of river and groundwater from the SOE sites across the region, to provide input into possible future refinement or redesign of the river and groundwater quality SOE networks operated by Greater Wellington Regional Council (GWRC). This study complements two previous assessments of hydrochemistry in the Wairarapa Valley. Overall, this investigation has shown that multivariate statistical analysis of river and groundwater chemistry can be valuable for the conceptualisation of hydrology and hydrogeology at the regional scale. The results of the multivariate analysis show that the SOE river and groundwater quality monitoring sites can be grouped into two, four or eight hydrochemical clusters. These clusters are differentiated by total dissolved solids concentration, redox potential and concentration ratio of the major ions. The natural controls on hydrochemistry include degree of water-rock interaction, recharge source, and mixing of groundwater and surface water, all of which are influenced by geology, land forms and rock type. Human activities also affect water quality, for example where land use patterns lead to above-background concentrations of nitrate in rainfall-recharged groundwater. Both natural and anthropogenic drivers of water quality show seasonal patterns that relate to variations in rainfall and river flow. The observed spatial and seasonal variations in hydrochemistry at the SOE sites provide insight into the hydrology of the Wellington region. For example, differences in catchment geology and altitude cause the rivers draining the Tararua Range to have markedly different hydrochemistry compared to the rivers draining the eastern hills, and these hydrochemical signatures can be traced into the aquifers in the vicinity of losing river reaches. Shallow rainfall-recharged groundwaters are found throughout the Wellington region, generally where alluvial fan deposits are mapped at the surface. Deeper groundwaters are commonly oxygen-poor, especially in the Lower Wairarapa Valley. (auth)