Radiocarbon integrity of dissolved inorganic carbon (DIC) samples stored in plastic and glass bottles: implications for reliable groundwater age dating

Various approaches based on the natural variations of carbon isotopes (C-14 and C-13) in dissolved inorganic carbon (DIC) are routinely used to study groundwater dynamics and to estimate recharge rates by deriving groundwater ages. However, differences in C-14 activities in groundwater samples collected repeatedly from the same wells and discordantly young C-14 groundwater ages compared to noble gases led some authors to question the validity of radiocarbon dating. Poor sampling protocols and storage effects (C-14 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 C-13 analyses. The C-14 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 C-14 and C-13 contents (up to similar to 15 pmC and similar to 5 parts per thousand, 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 (CO2)-C-14 contamination and microbial activity.

Automated summary

This study evaluated the impact of storage protocols on carbon isotopes in groundwater and found that using plastic sampling bottles for storage led to contamination and underestimated age estimations. In contrast, using glass bottles for storage can avoid contamination and maintain higher analytical precision.