Assessment of groundwater and surface water chemistry in the upper and lower Wairarapa Valley

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Daughney, C.J.; Guggenmos, M.; McAlister, D.; Begg, J.G.; Jackson, B.M. 2009 Assessment of groundwater and surface water chemistry in the upper and lower Wairarapa Valley. Lower Hutt, N.Z.: GNS Science. GNS Science report 2009/21 29 p.

Abstract: This investigation employed hierarchical cluster analysis (HCA) to provide insight into the groundwater chemistry in the Upper Wairarapa Valley (the area north of the Waingawa River) and the Lower Wairarapa Valley (the area south of the Waiohine River). The aim was to assist with development and substantiation of the transient groundwater flow models for the Upper and Lower Wairarapa Valley currently being developed by Greater Wellington Regional Council. This study complements the assessment of hydrochemistry in the Middle Wairarapa Valley (the area between the Waingawa and Waiohine Rivers) made by Daughney (2007). HCA conducted using all available surface water and groundwater chemistry data for the entire Wairarapa Valley allowed for the definition of two, six or thirteen hydrochemical clusters, depending on the separation threshold employed. The hydrochemistry and spatial positioning of sites assigned to these various clusters provides insight into the hydrology of the Wairarapa Valley. For example, differences in catchment geology cause the rivers draining the Tararua Ranges 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 Wairarapa Valley, generally where alluvial fan deposits are mapped at the surface. Deeper groundwaters are commonly oxygen-poor, especially in the Lower Valley. Lake Wairarapa has a hydrochemical signature that suggests that it receives some inflow from deeper groundwater in addition to surface drainage. Overall, this investigation has shown that HCA can be valuable for the conceptualisation of hydrology and hydrogeology at the basin scale. (auth)