Definition of hydrochemical facies for New Zealand's groundwaters using data from the National Groundwater Monitoring Programme

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Daughney, C.J.; Reeves, R.R. 2003 Definition of hydrochemical facies for New Zealand's groundwaters using data from the National Groundwater Monitoring Programme . Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences science report 2003/18 68 p.

Abstract: Groundwater is used extensively in New Zealand for domestic, municipal, industrial and agricultural water supply, and so it is essential that its quality be monitored and maintained. The National Groundwater Monitoring Programme (NGMP), established in 1990, is presently run by the Institute of Geological & Nuclear Sciences in close collaboration with 15 regional authorities in order to gain a national perspective on groundwater quality, and to identify spatial and temporal trends. This study aimed to define hydrochemical facies for New Zealand’s groundwaters, based on medians of 15 chemical parameters from 110 sites sampled through the NGMP. Hierarchical Cluster Analysis (HCA), coupled with complimentary statistical and graphical methods, revealed six distinct hydrochemical facies (clusters) in the NGMP data. Analyte variations between the facies were related to degree of water-rock interaction, aquifer lithology, aquifer confinement, and degree of human impact. Four of the six facies were representative of oxidised, surface-dominated systems, and had compositions similar to the global average estimate for river water. The remaining two facies were representative of reduced, subsurface-dominated systems, and had compositions similar to the global average estimate for groundwater. The facies defined by HCA provided a useful framework for comparison of NGMP sites in different regions of the country, and in many cases, provided information about aquifer properties where such details were missing. (auth)