Analysis of temporal trends in New Zealand's groundwater quality based on data from the National Groundwater Monitoring Programme

New Zealand's National Groundwater Monitoring Programme (NGMP) aims to provide a national perspective on groundwater quality, and to identify spatial and temporal trends in groundwater chemistry. In this investigation we assessed temporal trends in water quality at each NGMP site using the seasonal Mann-Kendall test and the seasonal Kendall Sen slope estimator. Hierarchical cluster analysis was conducted using the seasonal Kendall Sen slope estimator rates of change of 15 analytes from 108 NGMP sites, in order to define a set of categories representing distinct patterns of trends in groundwater quality that may be associated with specific causes. Cluster analysis revealed five trend-based clusters, with temporal patterns in groundwater quality that appear to relate to either natural water-rock interaction or human influence. The latter are possibly associated with changes in pumping regime and/or changes in land use in the groundwater recharge zone. Assignment of any monitoring site to one of the trendbased clusters defined in this investigation can provide a simple means of summarising temporal trends in water quality.