Aquifer classification and mapping at the national scale - phase 1 : identification of hydrogeological units

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Tschritter, C.; Westerhoff, R.S.; Rawlinson, Z.J.; White, P.A. 2017 Aquifer classification and mapping at the national scale - phase 1 : identification of hydrogeological units. Lower Hutt, N.Z.: GNS Science. GNS Science report 2016/51 52 p.; doi: 10.21420/G2101S

Abstract: Mapping and classification of aquifers in a consistent manner across New Zealand has been identified by GNS Science as an essential initiative to improve the sustainable management of the nation’s groundwater resources. Achieving this initiative is a recent objective of GNS Science’s SSIF (Strategic Science Investment Fund) funded “Groundwater Resources of New Zealand” (GWR) Programme. The objective of this study was to use the quarter-million scale geological map (QMAP) to identify a nationally consistent method to map hydrogeological units and potential aquifers. The resultant maps are the first phase of the aquifer delineation and characterisation that is planned within the GWR programme. The maps are intended to be used to define representative aquifer systems in New Zealand and to identify, characterise and assess groundwater resources with regard to their quality, quantity and flow pathways. The maps will enhance national monitoring and reporting of groundwater resources; and be a simple, relatable and informative dataset to enhance awareness and understanding of groundwater and associated environmental issues. The first national aquifer map for New Zealand was published in 2001. A partially-updated national aquifer map was re-issued in 2015. However, with increasing demand on groundwater in New Zealand, more consistent aquifer characterisation and mapping techniques are needed to improve understanding and appreciation of the resource. The QMAP, which is available as a seamless nationwide GIS dataset, includes comprehensive information about each surficially mapped geological unit as data attributes. As such, it provides a consistent, nationwide base map for aquifer mapping purposes. Two approaches (descriptive and numerical) were investigated using the QMAP attributes ‘MAIN_ROCK’, ‘SUB_ROCKS’ and age. The numerical solution provided a more suitable approach as it allowed the combination of different attributes, including a weighting component based on rock type predominance. Three maps were developed in this study. The first map showed hydro-lithological units that were based solely on the QMAP ‘MAIN_ROCK’ attribute and were grouped and ranked by assumed intrinsic permeabilities. However, the restriction to just the ‘MAIN_ROCK’ attribute yielded misleading results, e.g., areas with higher permeability classes were associated with basement rocks. Therefore, the second map, representing hydrogeological units, was based on the QMAP ‘MAIN_ROCK’, ‘SUB_ROCKS’ and age attributes. The hydrogeological units were ranked by assumed permeabilities of the rock types in each unit. This map showed that areas with known higher or lower permeabilities (e.g., Quaternary sediments and basement rocks, respectively) show expected higher or lower ranked hydrogeological units. The third map estimates aquifer potential as a result of the aggregation of the hydrogeological units of the second map. The resulting maps are currently available upon request, and it is intended to make them available through a web application in the future. The next phase of work will enhance the maps by incorporation of additional data sets, such as measured hydraulic properties and rainfall recharge maps. (auth)