14C Ages of Groundwater in Iceland

Abstract

During the last few years an effort has been made to date Icelandic groundwater by the radiocarbon dating technique. In Iceland at least three different sources of carbon in the water are expected: atmospheric CO2, soil CO2 of organic origin and CO2 leached from the rock. Moreover, there may be a supply of CO2 from magmatic sources in some geothermal systems. Addition of carbon from sources other than the atmosphere and organic soil dilutes the 14C concentration of the water and in this way yields higher apparent 14C age. Earlier studies have suggested that boron concentrations of the water samples can be taken as a measure of the amount of 14C-dead CO2 in groundwater leached from the rock. After applying boron-based corrections, the values for most of the cold and warm groundwaters lie in the range 30 to 110 pMC, which is in accordance with indications from other chemical tracers and the estimated residence time for the groundwater. The calculated boron based 14C dilution is considerably less in the water samples from the NW-Peninsula and the Southern Lowlands compared to the samples from the Skagafjordur region. This is in accordance with the expected real ages of the groundwater. The ɬ13C values in cold surface waters range from -1 to -7 ÅÒ whereas in soil waters the ɬ13C is much lighter and ranges from -16 to -22 ÅÒ. In the geothermal waters the ɬ13C values usually fall in the range -6 to -13 ÅÒ. A linear relationship is observed for the ɬ13C of groundwater and the calculated 14C boron-based dilution, except where the waters emerge in thick peat soil resulting in low ɬ13C values.