Radiocarbon, 18O, and chemical concentrations have been used to identify groundwater recharged during the last ice age near Nelson, New Zealand. Moutere Gravel underlies most of the Moutere Depression, a 30-km-wide system of valleys filled with Plio-Pleistocene gravel. The depression extends northwards into Tasman Bay, which was above sea level when the North and South Islands of New Zealand were connected during the last glaciation. The aquifers are tapped by bores up to 500 m deep. Shallow bores (50–100 m) tap “pre-industrial” Holocene water (termed the “modern” component) with 14C concentrations of 90 ± 10 percent modern carbon (pMC) and δ18O values of −6.8 ± 0.4, as expected for present-day precipitation. Deeper bores discharge water with lower 14C concentrations and more negative 18O values resulting from input of much older water from depth. The deep end-member of the mixing trend is identified as paleowater (termed the “glacial” component) with 14C concentration close to 0 pMC and more negative 18O values (-7.6). Mixing of the modern and glacial components gives rise to the variations observed in the 14C, 18O, and chemical concentrations of the waters. Identification of the deep groundwater as glacial water suggests that there may be a large body of such water onshore and offshore at deep levels. More generally, the influence of changing sea levels in the recent past (geologically speaking) on the disposition of groundwaters in coastal areas of New Zealand may have been far greater than we have previously realized.

Abstract: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 ( 18 O, 3 H, 13 C and 14 C) 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 CO 2 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 14 C.