title: 2023 Groundwater residence time assessment of  Heretaunga Plains municipal water supply wells in the context of contamination risk via young water flow paths and impact by Cyclone Gabrielle
Creators: name: Morgenstern, Uwe
Publisher: name: GNS Science
Year: 2023

description: The Heretaunga Plains aquifers provide drinking water to much of Hastings District and Napier City. Monitoring age distributions of water in drinking-water supply wells provides insights that help councils to understand the groundwater resource and identify potential pathways for pathogens via fast flow paths. An absence of young water in drinking-water supply wells significantly reduces the likelihood of pathogen contamination through an aquifer. The purpose of this study is to improve understanding of source-water travel time from land to the sampled wells and associated potential risks of exposure to pathogen contamination. This report provides new results of age-tracer analyses and modelled mean residence times (MRT), as well as piston flow times (minimum groundwater age) and figures of age distributions of groundwater from Hastings District Council (HDC) and Napier City Council (NCC) water-supply wells sampled from 2020 to 2023. Monitoring of age tracers enables detailed understanding of aquifer function, particularly regarding potential contamination risks and changes in aquifers that have the potential to cause problems in the future. The new age-tracer data enabled more refined age-distribution parameters than was previously possible to derive (Morgenstern and van der Raaij 2019). With the addition of the new data, it was possible to better constrain the mixing parameter, with higher piston flow components ruling out young water, resulting in higher minimum residence times. Except for the Waipatu and Tucker Lane wells, the age-distribution parameters for all HDC wells sampled changed since they were last interpreted. Except for the Portsmouth Road well, these changes are related to refinements in interpretation due to additional data rather than changes in the aquifer system. At Portsmouth Road well, the groundwater flow has changed after a switch to low pumping in 2020. At low pump mode, the well discharges slightly older water. For the Lyndhurst and Eastbourne Road No. 1 and 5 wells, the various age tracers consistently indicate water of variable age over time. The Wilson Road; Eastbourne No. 2, 3 and 4; Waipatu; Whakatu; and Tuckers Lane wells were found to draw water with more consistent ages over time. Among the NCC wells, the age-distribution parameters had not changed for well T2 but had changed for wells T5 and T6. None of the investigated wells contained water younger than one year, indicating low pathogen contamination risk. There is a clear groundwater-age pattern throughout the Heretaunga Plains aquifers. Wells within the unconfined Holocene gravel deposits, including Omahu and Brookvale Road, discharge very young groundwater. This is consistent with the extremely high hydraulic conductivities observed within these deposits (e.g. Morgenstern et al. 2018). Further, because some wells are screened through multiple layers of the aquifers, young water can also be present in wells that discharge relatively old groundwater overall; for example, Brookvale Road well No. 3. Groundwater within the confined aquifer becomes progressively older as it flows towards the coast. Drinking-water wells southwest of Napier contain water with MRT between 20 and 50 years, and minimum groundwater ages in this area are between 4 and 12 years. Further towards the coast at A2 and A3, the groundwater becomes significantly older, with MRT 45–50 years and minimum groundwater ages between 13 and 15 years. Greater groundwater flow velocities in the confined aquifer towards the coast are indicated further south in the centre of the plains. A tongue of very young groundwater with MRT 3–8 years extends from the river recharge area nearly halfway towards the coast (Portsmouth Road, Wilson Road, Hospital, Lyndhurst). 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. This is also supported by the fact that the young water in this band coincides with river-recharge signatures (Morgenstern et al. 2018). The new Wilson Road well has water contributions as young as 1.4 years. The other wells in the confined aquifer do not contain groundwater with MRT close to one year. Older water of MRT >70 years indicates more sluggish flow at the southern margin of the confined aquifer near the coast. SF6 concentrations in the Portsmouth Road and Wilson Road wells rose steadily between May 2016 and November 2017, indicating an SF6 plume moving through this part of the aquifer system. The highest SF6 concentration was observed at Portsmouth Road. Further downstream at Wilson Road, the excess SF6 was more dilute. After further dilution, the SF6 plume also reached the Hospital well in 2017. Further downstream, the Lyndhurst Road wells did not show statistically significant SF6 contamination until 2018, but the 2019 data showed that the plume had also arrived there. The SF6 plume is also present at the Flaxmere Hawke’s Bay Regional Council test bore. After peaking around 2018/19, the SF6 concentrations in the groundwater at Wilson Road and Lyndhurst No. 7 have now, in 2023, returned to below equilibrium concentrations with the atmosphere, and the SF6 concentrations at Lyndhurst No. 5 have nearly returned to pre-plume levels. The brief nature of the SF6 spikes in this part of the aquifer, with decreasing magnitude downstream, indicates a local SF6 contamination source east of Roys Hill. In addition to the SF6 plume in the western part of the Heretaunga Plains aquifers, there is also a CFC-12 plume moving through the central part of this aquifer system near the coast around the Ngarururo River mouth and the lower Karamu Stream. These SF6 and CFC-12 plumes moving through the aquifer system are not concerning in and of themselves from the point of view of drinking-water standards. Their concentrations are only slightly above ambient concentrations and well below maximum permittable levels. However, they are important because they indicate flow connections in the aquifers, and, as these plumes of man-made substances are carried through the aquifer, other unmeasured contaminants originating from the same land-use or industrial activities as the SF6 and CFC-12 plumes may also be leaching into this groundwater. In February 2023, Cyclone Gabrielle provided an opportunity to test whether extreme flooding events can change groundwater flow dynamics in confined parts of aquifers due to increased hydraulic loading, causing young water to be pushed to parts of aquifers that usually contain older water, which may increase the risk of contaminants moving into drinking-water sources. The data indicate that groundwater ages in the investigated wells did not change significantly because of Cyclone Gabrielle. The wells that showed slight changes in age-tracer concentrations (T2; A2; Wilson; Lyndhurst No. 5 and 7; and Eastbourne No. 1, 2 and 5) consistently showed older water after Cyclone Gabrielle. Other wells, despite showing no detectable changes in age-tracer concentrations, contained water that was more evolved after the cyclone, indicated by decreased dissolved oxygen and elevated methane, ammonia and phosphorous concentrations (15002, 981, 1032, 5900, 17234). These observations all point toward older (probably deeper) groundwater having been pushed by increased hydraulic loading into the active groundwater flow paths. With no younger water detected in the investigated wells following Cyclone Gabrielle, there is no indication of increased risk of pathogen contamination in the confined aquifer system following extreme flooding events.