Integrated assessment and classification of inland water chemistry in New Zealand

<h2>Abstract</h2> Understanding freshwater characteristics is essential for the ecological and environmental management of water resources. New Zealand was selected as the research area, and a water chemistry survey was conducted on freshwater resource management. Addressing the growing water demand for irrigation is essential for adapting to climate change in the primary sectors of New Zealand. The intensification of livestock farming in recent years has led to water pollution issues, particularly from animal waste. In view of these concerns, it is crucial to study the status of inland water characteristics in New Zealand. In November 2023, water samples were collected from 33 sites, and their chemical characteristics were investigated. According to the Piper-trilinear diagram, most of the sites (65.6 %) were classified as alkaline earth carbonate type, seven sites (21.9 %) as alkali non-carbonate type, two sites (6.3 %) as alkaline carbonate type, and two sites (6.3 %) alkaline earth non-carbonate type. The Stiff patterns show that Ca-HCO₃ and Na-HCO₃ types exist in most of the sampling sites, and the Na-Cl type was found in a specific location of the coastal area. According to the Gibbs diagram, this water chemistry is formed by precipitation, mineral dilution, and ion exchange during water flow. The applicability of irrigation was assessed using the United States Salinity Laboratory (USSL) diagram. Self-organizing maps (SOMs) demonstrated diverse water quality characteristics across sample sites. Water samples were classified into four groups: (1) South Islands surface water group, (2) North Islands downstream water group, (3) nitrate polluted groundwater group, and (4) sodium affected large ionic strength group. This study demonstrated the effectiveness of a comprehensive and integrated approach to assess inland water chemistry by combining various conventionally used diagrams (Piper-trilinear diagram, Stiff patterns, Gibbs diagram, and USSL diagram) and neural networks (SOM). Although this integrated approach was applied to New Zealand in this study, it is generally applicable to all regions and countries that depend on inland water for their water resources.