Groundwater Residence Time in Iceland Depicted by Stable Water- and Carbon Isotopes

Iceland is young (the oldest rocks ca 16M years) and characterized by active and widespread volcanism, defined along a Neovolcanic rift zone.  More than 90% of the Icelandic bedrock is of basaltic origin. The permeability of the Tertiary lava pile is 1-4*10^-14 m² but for younger geological formations especially within geothermal areas the permeability is several orders of magnitude higher due to active fissures and faults. The deeper crust has however a very low permeability as pores and fissures are filled with secondary minerals.

 

Due to the high permeability in the upper crust groundwater is often mixed with water components originating from different conditions, of different age and in some cases also affected by water-rock interaction.   Thus groundwater dating is complex and to succeed in estimating groundwater residence time it is of vital importance that interdisciplinary methods are applied to understand the geochemistry, geology and hydrology of a specific groundwater system.

 

In this presentation an overview of using stable water- and carbon isotopes to estimate groundwater residence time is given. It is demonstrated how stable water isotopes including the second order parameter; deuterium excess, can be used to estimate relative ages. Also how radiocarbon age estimations have successfully been used when comprehensive corrections for “dead carbon” from the bedrock and CO₂ gas from the deep crust or mantle are applied together with δ¹³C to correct for modern carbon of organic origin.