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Abstract
Springs in the Christchurch region were sampled and geochemically analysed with the aim of understanding their sources and subsurface flowpaths. As well as the chemistry and water stable isotopes, a range of age tracers including tritium, chlorofluorocarbon, sulphur hexafluoride and Halon-1301 concentrations were examined. The chemical compositions and stable isotopes showed that the waters are dominated by seepage from the Waimakariri River with variable additions of infiltrated rainfall. Tritium and the gas concentrations showed that the four springs emerging on the western side of Christchurch are young (2 to 14 years), and likely sourced from the first confined aquifer (Aquifer 1). The two eastern Christchurch springs are very old (>180 and 150 years) and are likely to represent leakage from deeper aquifers.
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Introduction
Springs, wetlands and streams have been prominent features of the Christchurch area from the earliest times (e.g., White et al., 2007). Springs are fed by the groundwater system underneath Christchurch. While several studies have investigated the groundwater ages in the various aquifers via wells to obtain information on the timescales of the water flow (Taylor et al., 1989; Stewart, 2012), there has been no systematic study to understand the transit times of the water through the usually shallow aquifers to the springs, and to investigate leakage of old water, potentially from deeper aquifers, to the surface.
Christchurch is located on a Quaternary floodplain of interleaved alluvial gravels and low-permeability marine sediments resulting in a 'layer cake' of artesian aquifers below the city (Brown and Weeber, 2001). At least five aquifers have been identified to depths of about 200m, but the uppermost confined aquifer (Aquifer 1) is considered to play the key role in sustaining baseflows to the springs and resulting lowland streams in Christchurch (Figs. 1 and 2). This artesian aquifer provides groundwater pressure that enables the springs to flow and produces seepage from soils and streambeds. The Styx, Avon/Otākaro and Heathcote Rivers drain the area involved (Fig. 1).
Near Banks Peninsula, two aquifer systems meet (Brown and Weeber, 1994). The first is the northern Canterbury Plains coastal, confined, upper Quaternary, fluvial gravel aquifers, with groundwater recharge derived mainly from influent seepage from the bed of the Waimakariri River to the northwest of Christchurch. The...