The Manawatu aquifers,North Island,New Zealand: Clarification of hydrogeology using a multidisciplinary environmental tracer approach

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 (¹⁸O, ³H, ¹³C and ¹⁴C) 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 ¹⁴C. Copyright © 2001 John Wiley & Sons, Ltd.