Water Quality Monitoring for Management of Diffuse Nitrate Pollution

Aotearoa New Zealand is making significant investment in national policy and standards, regional plan rules and on-farm actions to reduce the impact of intensive land use on groundwater quality and stream health. Existing freshwater monitoring networks in New Zealand and overseas have predominantly been developed to yield information on the state and trend of freshwater but are not designed, and have often proven ill-suited, to robustly establish cause-effect relationships between improvement actions and water quality outcomes. This document provides background information on approaches to water quality monitoring network design (with a focus on groundwater), statistical power analysis, and the requirements for effective change-detection monitoring network design. Three change detection case studies are provided to demonstrate application of the monitoring design framework and tools. The case studies include an analysis of the capability of the existing national SOE groundwater quality network to detect a reduction from current measured nitrate concentrations to a hypothetical 2.4 mg/L NO3-N target implemented over a 30-year period, based on 30 years of quarterly sampling. The analysis suggests that the improvement would only be detected with a suitable degree of confidence in 40% of wells after 30 years. This means that the effectiveness of policies intended to reduce nitrate concentrations are very unlikely to be identifiable in the groundwater monitoring network within the timeframes required for effective natural resource management. This study concludes that we need to rethink our approach to change detection water quality monitoring. Integrated design of surface water and groundwater quality change detection networks to select optimal change detection monitoring sites across the interconnected hydrological system will be a key part of this.