Linkwater catchment groundwater residence time, flow pattern, and hydrochemistry trends

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Morgenstern, U.; Brown, L.J.; Begg, J.G.; Daughney, C.J.; Davidson, P. 2009 Linkwater catchment groundwater residence time, flow pattern, and hydrochemistry trends. Lower Hutt, N.Z.: GNS Science. GNS Science report 2009/08 47 p.

Abstract: Water demand in the Marlborough Sounds Linkwater catchment is increasing due to pasture irrigation by dairy farmers. Little is known about the geology and the hydrology of the area. Resource management decisions are being made without a sound understanding of sustainable limits. Groundwater is the main potential source for irrigation water at Linkwater. Marlborough District Council requires a baseline hydrological assessment of the flow characteristics of the Linkwater catchment to guide day-to-day allocation practice and advice to Council. While this may be regarded as a baseline study, according to Marlborough District Council consent records, a significant amount of water (around 10,000 m3/day) has already been allocated at Linkwater. It is assumed that Linkwater catchment water is derived predominantly from local rainfall, either directly onto the flats or as surface run-off from the adjacent ranges via Cullens Creek and other smaller creeks. The variation in well depth at Linkwater suggests that there are different water-bearing formations, ranging from shallow flow through the alluvial gravels recharged by seepage from Cullens Creek or recent rainfall, to medium and deep aquifers that may contain older water. The objective of this study is to establish the hydrogeology of the Linkwater area, and groundwater flow patterns from the isotopic and chemical signature of the water, including the sources of catchment recharge and mean residence times of the water in the catchment. Tritium, chloroflurocarbons (CFCs) and sulphur hexafluoride (SF6) are used for age dating, and delta18O, N2, Ar, CH4 and a full chemical assessment at all sites (including arsenic, nutrients and heavy metals) are used along with the age data to identify flow characteristics. (auth)