Paleogroundwater in the Moutere Gravel Aquifers Near Nelson, New Zealand

Abstract

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 (50-100m) tap "pre-industrial" Holocene water (termed the "modern" component) with 14C concentrations of 90 +/10 percent modern carbon (pMC) and delta-18O values of -6.8 +/0.4 per mil, 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 per mil). 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.