Moreau, M.; Saeed, H.; Narvaiza, M. 2026 Manawatū-Whanganui groundwater quality monitoring optimisation. Lower Hutt, NZ: Earth Sciences New Zealand. GNS Science report 2026/22. 62 p.; doi: 10.21420/Y146-FP35
Abstract
Regional State of the Environment (SoE) monitoring undertaken since the 1990s has evolved in an ad hoc fashion, although it has provided very useful information to inform management of groundwater resources to date. In contrast, national SoE monitoring, also initiated in 1990, has largely remained static. National groundwater quality monitoring is a collaboration between Earth Sciences New Zealand and all regional authorities. Horizons Regional Council (Horizons) commissioned Earth Sciences New Zealand to jointly review both regional and national groundwater quality monitoring programmes in the Manawatū-Whanganui region to ensure it achieves objectives and addresses critiques in respect to national consistency and transparency.This review is the second case study applying the Surveillance and Evaluative monitoring framework, where Surveillance involves passive characterisation of groundwater quality state and trends. In contrast, Evaluative monitoring is a more targeted approach, notably to assess the efficacy of policies and management initiatives. It is expected that within any given regional network there may be a number of monitoring sites that serve both purposes. In the first case study, undertaken by Earth Sciences New Zealand and Waikato Regional Council, a review methodology was developed with the potential to be applied consistently between regions. The development included extensive consultation with regional councils, relevant Crown Research Institutes (CRIs) and government departments to ensure fitness for purpose. Therefore, the aim of Horizons’ joint review is to realign regional and national monitoring, assess the feasibility of applying the methodology in hydrogeological settings different from Waikato’s, and clarify differences and limitations between national and regional groundwater quality reporting. This report proposes to replace the current SoE monitoring programme so that there are separate Surveillance and Evaluative networks. Development of an Evaluative network was out of scope for this review. The existing Manawatū-Whanganui region’s SoE monitoring programme consists of 34 wells, three of which are shared with the National Groundwater Monitoring Programme (NGMP) network. The SoE network was reviewed in respect to spatial location, hydrogeological characteristics, availability of information, site access, sampling logistics and, notably, the ability of sampling locations to record changes in groundwater quality that can be statistically assessed. To assess priority areas for both Surveillance and Evaluative monitoring, a geospatial index-overlay method was used. Spatial units for this method consisted of a 10 km x 10 km grid overlaid by hydrogeologically defined polygons. The combination of weights and ratings to mappable hydrogeologic units provided the basis for informed decisions on the design of the Surveillance regional monitoring network. This uses a variable monitoring site density approach, wherebyhigher priority areas were assigned greater densities. Priority areas for Evaluative monitoring with nitrate management and seawater intrusion management were also developed to support future network design. The index factor weightings were varied for Surveillance and Evaluative purposes and may be changed in a transparent fashion. The review of Surveillance monitoring involved comparison of the existing monitoring well distribution and characteristics with the index-mapped priority areas. The proposed Surveillance networks involve expansions of both the regional and the national networks with limited changes to the analytical suite and sampling frequency to compensate for the network expansion. Various adjustments will be required involving rationalising some existing sites, whilealso adding some new monitoring wells, which will require field confirmation of suitability. It is recommended that additional sites involve collaboration with a third party, typically with district councils monitoring drinking-water supplies, which will also generate operational efficiencies (auth)
