Definition of hydrochemical facies in the New Zealand National Groundwater Monitoring Programme

• Published in: Journal of hydrology, New Zealand New Zealand, 2005-01, Vol.44 (2), p.105-130
• Main Authors: Daughney, Christopher J., Reeves, Robert R.
• Format: Article
• Language: English
• Subjects: Aquifers; Centroids; Cluster analysis; Earth sciences; Earth, ocean, space; Error rates; Exact sciences and technology; Facies (Geology); Graywackes; Groundwater; Hydrology. Hydrogeology; Land use; Lithology; National Groundwater Monitoring Programme (NGMP); Orchards; Plantations; Quality; Water chemistry
• ISSN: 0022-1708; 2463-3933

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 study, hierarchical cluster analysis was used to define hydrochemical facies using the medians of 15 chemical parameters from 110 NGMP sites. The cluster analysis revealed six distinct hydrochemical facies in the NGMP data. Four of the facies had compositions similar to the global average estimate for river water, and the remaining two facies had compositions similar to the global average estimate for groundwater. Analyte variations between the facies were related to degree of water-rock interaction, redox potential, aquifer lithology, aquifer confinement and degree of human impact, but bore no obvious relationship to use of the surrounding land. The hierarchical cluster analysis approach provided a framework for comparison of NGMP sites in different regions of the country. It permitted estimation of baseline water quality with as little as one monitoring site per aquifer, and in several cases, it provided information about aquifer properties where such details were missing.