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This study aims to holistically describe the flow sources, pathways and lag times of water through the rivers and aquifers of the West Coast catchments. This information is required to ground truth and improve groundwater flow models and management tools. Such improvements will help to prevent degradation of rivers and aquifers from land-use activities that impact on cultural, recreational and economical values, as well as on drinking water supplies and quality. This study is mainly funded through the MBIE Endeavour programme ‘Te Whakaheke o Te Wai’ and complemented by the MBIE Envirolink fund (C05X2005). Groundwater chemistry and age tracer data were assembled from a range of sources: (i) GNS’s national datasets (National Groundwater Monitoring Programme, National Tracer Survey), (ii) regional datasets (State Of the Environment monitoring and specific site investigations), (iii) existing data from the Water Dating laboratory and (iv) water samples collected in March 2020 as part of this study. The aggregated dataset consists of 100 sites (including 97 bores, two springs and one river) and spans the period November 1998 to March 2020. The environmental tracer data (age, isotopes, temperature, gas concentration and chemistry) was combined and interpreted using graphical analysis (e.g. Piper diagram) spatial analysis and multivariate statistics. These analyses inform characterisation of groundwater flow and hydrochemical processes in the West Coast aquifers. Short (less than 10 years) residence times are typical for groundwater resources in the region. Three geographically distinct hydrochemical signatures are consistent with topography and geological distinct areas: pristine, alpine foothills (cluster A); impacted coastal or fluvial (clusters B1 and B2) areas; and a dilute valley with indication of land-use impact (clusters C1, C2 and C3). West Coast rivers and streams are hydraulically well connected to adjacent Holocene gravel aquifers. These aquifers are mostly oxic in nature. There is does not appear to be a relationship between well depth and groundwater age, indicating absence of confined or disconnected groundwater condition throughout large parts of the region’s aquifers. In the Grey Valley, groundwaters have tracer signatures indicative of recharge from local rain and rivers. This area has the largest groundwater demand and abstraction for the region (Mourot and White 2020). Time series groundwater age data for the valley was not sensitive enough to identify impact of abstraction on groundwater circulation from increased abstraction between 2012 and 2017 (WCRC 2014). It is expected that significant change in water allocation may be rapidly reflected in future water age monitoring data due to the short residence time of water in this system (auth)

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)

Groundwater age, chemistry, gas, and isotope tracers were applied in the Ohau and Waikawa catchments of the Horowhenua Groundwater Management Zone to understand groundwater dynamics, source, and hydrochemical processes. The measurement of river and stream water age provided a detailed understanding of the ability of geologic formations to let rainwater enter into large groundwater systems with large groundwater stores. Groundwater age tracer results allowed for the identification of: groundwater recharge rates; areas of recharge and discharge; and waters that were recharged during a previous climatic period and therefore not relevant for studies of anthropogenic impacts on groundwater resources. The delineation of recharge sources using this tracer data set, including radon, provided insight on groundwater and surface water interactions and associated nutrient contamination pathways. Hydrochemistry (particularly nitrate), stable isotope, and gas data so far indicate groundwater recharge is dominated by local rain. Indication of river-recharged groundwater was not prevalent but is indicated to occur at sites near the Ohau River and the Waikawa Stream. The origin of two groundwater pollutants that contribute to lake water eutrophication, phosphorous (P) and nitrogen (N), can be identified using groundwater age: P from geologic sources is indicated by elevated P only in old groundwater (>100 years), and N from land-use intensification is indicated by elevated N concentrations only in waters younger than 60 years. Radon data provided detailed understanding of groundwater discharge areas in the Ohau River and Waikawa Stream. Significant stretches of groundwater discharge into the river/stream are indicated by high radon concentrations in the lower reaches of the Ohau River and Waikawa Stream just upstream from where they intercept and flow across the Quaternary sands. It is inferred that the permeability of these sands is too low to allow groundwater discharge to the sea. Instead, at the end of the transmissive quaternary gravels, their groundwater flow discharges into the surface waterways. High groundwater nitrate concentrations in the Ohau and Waikawa catchments were identified as the result of groundwater recharge by local rain which carries land surface derived nitrate into the groundwater systems. Nitrate concentrations in the groundwater of the Ohau catchment are lower than in the southern Waikawa catchment. The tracer signatures indicate partial groundwater recharge from the river in the Ohau catchment, implying that the lower nitrate concentrations in the Ohau catchment are likely to be a result of dilution of the nitrateladen rainfall recharge derived water by near-pristine river water. In the Manakau catchment, absence of stream or river recharge signature implies that nitrate laden groundwater is not diluted by low-nitrate stream water, leading to the observed higher groundwater nitrate concentrations. The gravels in the Ohau catchment are highly transmissive and as a result the water flowing through these gravels is relatively young as it nears the discharge area of the gravel system and returns into the river. Groundwater is older in the less-transmissive gravels of the Manakau catchment where it discharges into the Manakau Stream at the interception between the Quaternary gravel and sand near the coast. Groundwaters in the Ohau and Waikawa catchments are typically not highly reduced and therefore not expected to be associated with complete denitrification. The estimated oxygen reduction rates for these gravel aquifers suggest that the groundwater requires decades in the groundwater system to reach anoxic conditions favourable for denitrification reactions. The longer groundwater transit time through the Manakau catchment may allow for greater denitrification potential in the gravels, before the water returns into the stream, than in the Ohau catchment. In the lower Waikawa Stream, downstream from the main groundwater discharge area, dual nitrate isotope signature indicates occurrence of denitrification in the flow system. Decreasing nitrate concentrations downstream indicate that at least part of this denitrification occurs in the stream. However, higher denitrification potential in the groundwater system of the Manakau Stream catchment may also provide this environmental service of denitrification in the groundwater system, so part of the immediate denitrification signature in the groundwater discharge area of the stream may originate from the groundwater flow system. The Ohau groundwater system, with faster transit, only shows normal N retention isotopic signature (typical nitrate isotope signature from pastoral soils) in the lower reaches of the river. (auth)

Groundwater is an important source of water for the Wairau Plain. Concerns about its sustainable yield and contamination from nutrients have prompted several hydrogeological studies over time to better understand the groundwater system. The main aim of this study is the review and consistent interpretation of age-tracer, isotope and chemistry data, collected between 1968 and 2019, to obtain the hydraulic parameters necessary to understand the declining water levels and spring flows and to assess the groundwater storage that feeds the river and springs. (auth)

This report interprets the results from groundwater age tracer and isotope measurements in the Christchurch aquifer system in 2017. The study included carbon-14, tritium and trace gases measured at GNS, stable isotopes (oxygen-18 and deuterium), and water chemistry measured at the same time (results given in Appendices 1 and 2). The interpreted mean ages and water sources are compared with results from measurements from each decade between 1976 and 2006 to see how the system is responding to exploitation. The goal is to improve understanding of the hydrogeological processes in the Christchurch aquifer system and allow for better management of the resource. It is hoped that the work will lead to improved conceptual models and support better digital modelling of the system. The 2017 survey showed that the 14C ages of deep water in the Christchurch aquifer system are continuing to become older as groundwater abstraction continues (in particular, the 2017 ages are older than the 2006 ages). In addition, the steep west-east gradient in ages continues to be observed (e.g. from 400 years to 1450 years in the central transect). The system is clearly accessing deep storage, which is much older on the eastern than on the western side. However, in addition to the increasing ages of deep water, there are indications of inflow of increasingly younger water on the western and northern sides of Christchurch. This has implications for long-term risks to the quality of the system, such as risk of microbial contamination, because such contamination would be carried by very young water penetrating from the surface. Shallow layers are more vulnerable than deeper levels, so drinking water should be drawn from deeper levels. Monitoring ages and groundwater sources with tritium and gas age tracers and chemical measurements is recommended. Risk of saline intrusion on the coastal side of the system to deeper aquifers is considered small because artesian pressures recover each winter after summer drawdown, despite continued groundwater abstraction. However, shallow levels of the system are more vulnerable due to possible drawdown by abstraction and sea-level rise due to climate change. Breakthrough of conservative contaminants (such as nitrate) brought in by young water inflow to the deep system may be a possibility in time and will be worth monitoring with nitrate and 14C measurements. The two deep wells near the Waimakariri River gave very young ages, but the inferences to be drawn from this are unclear at present. It is recommended that more emphasis should be given to 14C measurements in the future to monitor the functioning of the deep system. This is because 14C can give the real mean ages of the water, and therefore provide fundamental information on the deep system flow paths and storages. (auth)

The National Institute for Water and Atmospheric Research established, and is leading, the 'National Hydrological Project (NHP), with GNS Science (GNS) and Landcare Research as sub-contracted partners. This report details the GNS activities in the NHP task "design and conceptualisation of catchment-wide isotopic data layers, including data-acquisition of isotope field samples, in selected case studies" and aims to refine isotopic data layers, for stable isotopes and tritium, to make them applicable for studies that estimate groundwater - surface water interaction at the catchment-scale. We describe the main activities carried out between May 2017 and May 2018: (1) the sampling design in three selected case study areas (Southland, Horizons and Gisborne regions) and associated collected water samples for NHP are described (with the first analyses results expected in the course of the next financial year 2018-19); (2) outline of the theory and rationale behind the sampling design; and (3) a review of existing tritium and stable isotope measurements, obtained outside the NHP, from both national and regional case study scale sampling programmes. The theory and review strengthen the rationale of the sampling design, and combined they lead to recommendations for further work, the most important of which is to align other streams of existing research to overcome the huge data sparsity resulting from using only data collected though the NHP, and to make the important step towards the national scale. (auth)