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14 C‐Dating Model for Groundwater Affected by CO 2 Inputs From Deep Underground Formations
Author(s) -
Wang Tao,
Chen Jiansheng,
Zhang Chenming,
Zhan Lucheng,
Li Ling
Publication year - 2020
Publication title -
water resources research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.863
H-Index - 217
eISSN - 1944-7973
pISSN - 0043-1397
DOI - 10.1029/2019wr025155
Subject(s) - groundwater , groundwater recharge , geology , hydrogeology , aquifer , radiocarbon dating , isotopes of carbon , sedimentary rock , crust , mantle (geology) , carbonate , carbonate minerals , dilution , geochemistry , earth science , hydrology (agriculture) , soil science , total organic carbon , calcite , paleontology , environmental chemistry , geotechnical engineering , chemistry , physics , organic chemistry , thermodynamics
Groundwater age, defined as the time since recharge, is useful for estimating groundwater renewal rates. Radiocarbon ( 14 C) activity has been widely applied for groundwater age dating since the 1950s. However, this approach can be complicated by other 14 C‐free carbon sources, which dilute the 14 C content at the time of groundwater recharge. Several corrective models have been introduced to determine the dilution factor using carbon isotopes and chemical mass balances, but these models do not account for the influence of deep crust and/or mantle (endogenic) carbon sources and are hence not applicable for dating groundwater affected by endogenic CO 2 . Therefore, we developed a new 14 C groundwater dating model including three carbon end‐members (biogenically derived sedimentary carbon, carbonate carbon, and deep crust and/or mantle carbon) to quantify the dilution factor. This model was tested by age dating groundwater samples from the United States that were influenced by endogenic carbon (based on their δ 13 C value), showing that the conventional method overestimated the 14 C apparent age by ~19% on average. The average uncertainty for the conventional and new method is approximately 1.58% and 11.2%, respectively. The new method proposed here can be applied to groundwater age dating in other regions where aquifers are influenced by endogenic CO 2 . However, its application requires a priori identification of local geological, hydrogeological, and geochemical conditions.