Field degassing as a new sampling method for 14C analyses in old groundwater

Reika Yokochi, Ryan Bernier, Roland Purtschert, Jake C. Zappala, Yoseph Yechieli, Eilon Adar, Wei Jiang, Zheng Tian Lu, Peter Mueller, Gerard Olack, Roi Ram

Research output: Contribution to journalArticlepeer-review

9 Scopus citations

Abstract

Radiocarbon (14C) activity in groundwater can be used to determine subsurface residence time up to ∼40 kyr, providing crucial information on dynamic properties of groundwater and on paleoclimate. However, commonly applied sampling methods for dissolved inorganic carbon (DIC-14C) are prone to low level of modern atmospheric contamination, resulting in underestimation of groundwater ages that cluster around 30-40 kyr. We extract CO2 gas from groundwater using a device originally developed for studies of noble gas radionuclides. Carbon is collected in the gas phase, eliminating the possibility of fostering microbial activities and aqueous chemical reactions during sample storage. This method collects CO2-14C and radiokrypton (81Kr and85Kr) samples simultaneously. The presence of any shorter-lived85Kr is used to evaluate the degree of atmospheric contamination during sampling or mixing of young groundwater. Most groundwater samples showed lower CO2-14C activities than those of DIC-14C, presumably due to the absence of atmospheric contamination. Samples with81Kr age exceeding 150 kyr have no detectable CO2-14C except where mixing sources of young groundwater is suspected. These field data serve as confirmations for the reliability of the newly presented sample collection and CO2-14C method, and for the outstanding roles of radiokrypton isotopes in characterizing old groundwater.

Original languageEnglish
Pages (from-to)349-366
Number of pages18
JournalRadiocarbon
Volume60
Issue number1
DOIs
StatePublished - 1 Feb 2018

Keywords

  • Dissolved 14CO2
  • Field degassing
  • Groundwater residence time

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