The use of 18O as a groundwater tracer in the Marlborough Region

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Stewart, M.K. 2006 The use of 18O as a groundwater tracer in the Marlborough Region. Lower Hutt, N.Z.: GNS Science. GNS Science report 2003/03. 8 p.

Abstract: The usefulness of 18O for monitoring groundwater wells in the Wairau River coastal plain has been investigated by conducting a survey of the wells used for monitoring. The delta18O values of the Wairau Aquifer well waters show trends along the lines of flow depending on distance and depth. Near the head of the valley and close to the river, the groundwater has delta18O values like that of the Wairau River. With distance from the river and towards the coast, the delta18O values of shallow groundwater in the unconfined aquifer become more positive as land surface infiltrating recharge with higher delta18O values contributes to the flow. The infiltrating water also carries nitrate and other chemicals to the groundwater. At deeper levels, the groundwater is not affected by water infiltrating through the surface, and retains its Wairau River delta18O signature when it rises towards the surface near the coast. However, this conclusion is regarded as preliminary because of the possible temporal variability of the groundwater delta18O values. Regular 3-monthly 18O measurements should be carried out on 8 key wells to substantiate this conclusion. The delta18O values of the southern valley aquifer waters indicate that they are sourced from local streams and surface recharge. The Rarangi and Tuamarina aquifers are sourced from local rainfall with delta18O of -6.2 permille. (auth)