Rai-Pelorus groundwater baseline study : residence time, recharge source, rate and volume, resurgence into rivers, and impact of high intensity land use activities on groundwater quality

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Morgenstern, U.; Townsend, D.B.; Davidson, P.; Martindale, H. 2014 Rai-Pelorus groundwater baseline study : residence time, recharge source, rate and volume, resurgence into rivers, and impact of high intensity land use activities on groundwater quality. Lower Hutt, N.Z.: GNS Science. GNS Science report 2014/31 41 p.

Abstract: A reconnaissance assessment of groundwater resources underlying the Rai-Pelorus River floodplain was undertaken using groundwater chemistry and isotope methods. Water samples from eight groundwater wells and seven river/stream sites were investigated to help inform a general understanding of groundwater flow and recharge patterns, reservoir size, rates of recharge and groundwater flow, evolution of hydrochemistry and contaminants, baseline groundwater quality and impacts by land use activities. Identification of the recharge source of the groundwater was facilitated by the stable isotope compositions; groundwater age; argon, nitrogen and methane concentrations; and hydrochemistry. All the applied indicators resulted in consistent interpretations in regard to recharge source. A recharge source from rivers or streams is indicated for three of the groundwater wells, while local rain recharge is indicated as the source of the groundwater for the other five groundwater wells. One production well next to the coast contains old brackish groundwater, and the water from the deep Marlborough District Council (MDC) monitoring well contained old connate seawater shortly after drilling but this is now replaced by younger water from a highly anoxic environment. Most of the groundwaters are less than 8 years old, except the two very old waters that are impacted by geological processes and time scales. (auth)