Journal
HYDROLOGICAL PROCESSES
Volume 15, Issue 17, Pages 3269-3286Publisher
JOHN WILEY & SONS LTD
DOI: 10.1002/hyp.272
Keywords
New Zealand; hydrogeology; groundwater recharge; groundwater flow; artesian aquifers; environmental isotopes; groundwater residence times; hydrochemistry
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A recently developed approach to carbon isotope methodology (process recognition via isotope diagrams) is applied in a multidisciplinary study of precipitation-recharged aquifers of the lowlands-plains area of the Manawatu (south-west North Island). Urban and rural areas rely on groundwater from the upper levels of a deep basin sequence comprising marine and terrestrial sediments of Pleistocene age. Hydrochemical and isotopic (O-18, H-3, C-13 and C-14) data are merged with known details of geology and hydrogeology to reveal two separate confined aquifers within the depth range to 200 m. The shallower of these, below unconfined, locally recharged groundwater, is recharged on the foothills of the Ruahine Range to the north-east of the study area; flow direction is NE-SW. The deeper confined aquifer is recharged on the Tararua Range to the immediate east; flow direction essentially is transverse (SE-NW) to that in the shallower aquifer. Two processes are identified as dominant contributors to concentration and isotopic composition of dissolved inorganic carbon (DIC), namely addition of CO2 from decay of organic materials and carbonate dissolution. Limitations of carbon isotope methods in determining residence times are illustrated by the data. Although the confined groundwater is essentially tritium-free, only a few samples showed conclusive evidence of significant ageing on the time-scale of C-14. Copyright (C) 2001 John Wiley & Sons, Ltd.
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