Moreau, M. 2025 West Coast State of the Environment report: groundwater quality 1998–2024. Lower Hutt, N.Z.: GNS Science. GNS Science report 2025/08. 35 p.; doi: 10.21420/R8EF-WC65
Abstract
The West Coast Regional Council (WCRC) commissioned GNS Science (GNS) to prepare a report of the State of the Environment (SoE) for groundwater quality in the region. This report includes: a summary of groundwater resources in the West Coast region and their monitoring, an update on the state of and trend in groundwater quality in the region from the previous report (Moreau 2019), a state and trend assessment script for WCRC use, and recommendations for future monitoring. This study is based on groundwater quality data regularly collected by WCRC that contained over 19 analytical variables, including microbial indicators. It also includes data collected as part of the National Groundwater Monitoring Programme (Moreau and Cameron 2023); results from the 2022 national Pesticides Survey led by ESR (Close and Banasiak 2023) and from a regional characterisation study (Moreau et al. 2021). Statistical trend analysis was performed using R software (version 3.6.2, NADA and LWP-Trends libraries). Time periods considered for the analysis were: 2019 to 2024 for State and Exceedances; and 2014–2024, 2004–2024 and 1999–2024 for Trends. Following the removal of sites due to minimum data point requirements. State was assessed at 48 groundwater quality sites. Exceedances were assessed at 21 sites, and Trends were assessed at 29 groundwater quality sites.Groundwater quality remains relatively dilute, with median conductivities ranging from 66 to 250 S/cm (5th and 95th percentiles). Nitrate concentrations (<0.01 to 6.9 mg/L as N) are higher than previously reported (0.18 to 5.8 mg/L as N) over the 2012–2017 period (Moreau 2019). However, unlike previously reported, there were no occurrences of exceedance of the drinking-water Maximum Acceptable Value (MAV) of 11.3 mg/L as N (Water Services 2022). The 2022 screen for trace inorganic variables identified occasional occurrence of aluminium, barium, chromium, cobalt, copper, strontium, vanadium and zinc, at or close to the detection limit, which can be explained by natural sources. Microbial contamination remains a groundwater quality issue for WCRC groundwaters, with E.coli detected above the MAV at 70% of the monitoring bores. The 2022 pesticides screen only yielded two isolated detections (clopyralid and picloram herbicides) at low concentrations. Concurrent trends were a common pattern at monitoring sites and highlight localised dynamics in each hydrogeological system. The long-term monitoring sites showed trending variables consistent with possible indication of increasing temperature and carbon uptake, which may be related to climate change and/or human activity. (auth)
