Radiocarbon integrity of dissolved inorganic carbon (DIC) samples stored in plastic and glass bottles: implications...

Various approaches based on the natural variations of carbon isotopes (¹⁴C and ¹³C) in dissolved inorganic carbon (DIC) are routinely used to study groundwater dynamics and to estimate recharge rates by deriving groundwater ages. However, differences in ¹⁴C activities in groundwater samples collected repeatedly from the same wells and discordantly young ¹⁴C groundwater ages compared to noble gases led some authors to question the validity of radiocarbon dating. Poor sampling protocols and storage effects (¹⁴C contamination) for radiocarbon analysis are a critical factor in explaining age determination discrepancies. We evaluated the impact of storage protocols on carbon isotope exchange with atmospheric carbon dioxide by comparing glass versus standard plastic field sampling bottles for various storage times before radiocarbon and ¹³C analyses. The ¹⁴C bias after 12 months in pre-evacuated glass vials was minimal and within analytical precision. However, storage of DIC samples in plastic sampling bottles led to marked changes in ¹⁴C and ¹³C contents (up to ∼15 pmC and ∼ 5 ‰, respectively, after 12 months), meaning contamination led to younger groundwater age estimations than it should have been. Protocols for sampling and storing DIC samples for radiocarbon using pre-evacuated glass bottles help avoid atmospheric ¹⁴CO₂ contamination and microbial activity.