2019 Groundwater residence time assessment of Hawke’s Bay municipal water supply wells in the context of the Drinking-water Standards for New Zealand

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Morgenstern U, van der Raaij RW. 2019. 2019 Groundwater residence time assessment of Hawke’s Bay municipal water supply wells in the context of the Drinking-water Standards for New Zealand. Lower Hutt (NZ): GNS Science. 48 p. (GNS Science report; 2019/76). doi:10.21420/8KKH-4W33.

This report provides new context regarding groundwater dating in the Hawke’s Bay region and the results of age-tracer analyses and modelled mean residence times (MRT), together with piston flow times (minimum groundwater age) and figures of age distributions, of groundwater from water supply wells sampled in April 2019 from 21 Hastings District Council (HDC) wells and 7 Napier City Council (NCC) wells. These results have been interpreted with respect to Section 4.4.2.1 of the Drinking-water Standards for New Zealand: 2005 (Ministry of Health 2018). Available data from previous age-tracer measurements of groundwater from the same wells have been incorporated in the analysis. Incorporation of such historical time-series data improves the robustness of the steady-state age interpretation (i.e. long-term average condition). The purpose of the study is to improve the understanding of the source water characteristics (travel time from land to the well) and the associated potential risk of exposure to pathogen contamination. The Drinking-water Standards for New Zealand: 2005 (Ministry of Health 2018) allow assessment of compliance with bore water security criterion 1 (Section 4.4.2.1) using residence time determination by the tritium, CFCs and SF6 methods. Using these methods, for a groundwater supply to comply with the criterion, the percentage of water that has been present in the aquifer for less than one year (the young fraction) must be less than 0.005%. Regarding the HDC wells, Omahu Nos.1 and 2, Wilson Road NEW, Lyndhurst Nos.5 and 7, Brookvale No.3, Whirinaki, and Esk are modelled to have minimum residence times of less than one year with resulting young fractions of more than 0.005%, and therefore these wells do not satisfy the residence time criterion (Section 4.4.2.1) of the Drinking-water Standards for New Zealand: 2005 (Ministry of Health 2018) at the present time. The Whirinaki well was previously modelled to marginally satisfy the residence time criterion, but variable and inconsistent age-tracer and hydrochemistry concentrations points toward unusual mixing processes and variable water sources. Consequently, the mixing parameters are not sufficiently constrained for these wells to exclude the presence of water younger than one year. Using our best estimates of modelled groundwater age, all the other HDC wells, including Portsmouth Road, Eastbourne Road Nos.1–5, Waipatu, Whakatu OLD and NEW, Tuckers Lane, Ferry Road, Haumoana Beach Road Nos. 2 and 3, Parkhill Road and Waipatiki, are modelled to have minimum residence times of greater than one year with resulting young fractions of less than 0.005%, and therefore are inferred to satisfy the residence time criterion (Section 4.4.2.1) of the Drinking-water Standards for New Zealand: 2005 (Ministry of Health 2018) at the present time. The NCC wells T2, T3, T5, T6, T7, A1, and C1 are also modelled to have minimum residence times of greater than one year with resulting young fractions of less than 0.005%, and therefore are inferred to satisfy the residence time criterion (Section 4.4.2.1) of the Drinking-water Standards for New Zealand: 2005 (Ministry of Health 2018) at the present time. The interpretations of water age distributions presented in this report should be used along with other information to inform a risk-based, multi-barrier approach for ensuring the safety and security of groundwater bores for drinking water supply. There is a clear groundwater age pattern throughout the Heretaunga Plains Aquifers. Wells within the unconfined Holocene gravel deposits, including Omahu and Brookvale, discharge very young groundwater. This is consistent with the extremely high hydraulic conductivities observed within these deposits. Further, due to the wells that are screened through multiple aquifers, young water can also be present in wells that discharge relatively old groundwater overall. Groundwater within the confined aquifer system becomes progressively older along its flow towards the coast. The drinking water wells southwest of Napier contain water with MRT between 20–40 years, with minimum groundwater ages between 2–6 years. Further toward the coast the groundwater becomes significantly older, with MRT 50–90 years, and minimum groundwater ages between 7–11 years. Close to the coast the water is even older, indicating sluggish flow in this part of the aquifer. Greater groundwater flow velocities in the confined aquifer toward the coast is indicated further south in the centre of the plains. A tongue of very young groundwater with MRT < 5 years extends nearly halfway towards the coast. It is likely that this band represents a buried paleo river channel that is still hydraulically connected to the Ngaruroro River and thus enables fast seaward flow of water lost from the river. Groundwaters in this band have water contributions as young as less than one year. At the southern margin of the confined aquifer near the coast, older water of MRT > 60 years and minimum groundwater ages of > 13 years indicates more sluggish flow in this part of the aquifer. (auth)