Dating of Icelandic Groundwater by Stable Water Isotopes, Tritium and 14C

Abstract

From around 1960 stable water isotopes have been measured in natural Icelandic waters to trace origin and delineate groundwater flow. Comprehensive information on water chemistry is also accessible for many of these waters and Tritium is available for a limited number. Additionally, carbon isotopes have been measured in many of the samples to evaluate carbon sources and reactions as well as water age. Different isotopic methods have been used to estimate groundwater ages. Stable water isotopes have been used to estimate relative groundwater ages, especially to detect if a pre-Holocene component is present. Measurements have revealed groundwater that is more depleted than any modern precipitation, taken to indicate that the groundwater has a pre-Holocene component from a time when the climate was colder and precipitation thus more depleted. Changes in deuterium excess of low temperature groundwater has also been interpreted as age dependent, either reflecting prolonged water-rock interaction or a change in the precipitation source area reflecting different climate regime. Water age has also been estimated through carbon isotope measurements. To correct for dead carbon from the Icelandic bedrock (mainly basalt) the concentration of the incompatible element Boron (B) has been used to estimate carbon concentration leached from the rock (Sveinbjornsdottir et al., 2001). This B-based correction has been found to work well within well-defined watershed areas. However, this method fails in areas where marine sediments are part of the bedrock (Sveinbjornsdottir et al., 2004). In order to correct apparent 14C ages geochemical modeling approaches have also been applied by estimating the different sources of dissolved inorganic carbon in the groundwater, identified as leaching from primary rocks, deep degassing of CO2 (in case of geothermal water), as well as atmospheric and organic carbon (Stefánsson et al., 2016, 2017). In the presentation the use of water- and carbon isotopes to evaluate groundwater age will be given and methods to correct for the various sources of dissolved inorganic carbon in Icelandic groundwater will be discussed. References Stefánsson et al., 2016. Mantle CO2 degassing through the Icelandic crust: Evidence from carbon isotopes in groundwater. Geochim. Cosmochim Acta 191: 300-319. Stefánsson et al., 2017. Isotope systematics of Icelandic thermal fluids. J. Volcanol. Geotherm. Res. 337: 146-164. Sveinbjornsdotir et al., 2001. Isotopic and chemical charcteristics of old “ice age” groundwater, North Iceland. In Water-Rock Interaction, Rosa Cidu (ed). 205-208. Sveinbjornsdotir et al., 2004. Isotopic characteristics of natural waters in the Southern Lowlands of Iceland. In Water-Rock Interaction, Wanty & Seal II (eds). 1401-1405.