Uncertainties lying on the groundwater age-dating in a highly-utilized and fractured-rock aquifer system

Groundwater age provides valuable information on groundwater vulnerability and sustainability and can shed light on groundwater flow patterns and the effectiveness of remediation strategies. Production wells near the Korea CO₂ Storage Environmental Management (K-COSEM) site have been actively used for the agricultural purposes. They are characterized by variable depths of screen zones (22-150 m bgs) penetrating into the fractured mediums, therefore, expected to capture a mixture of water from a wide range of pathways. 17 samples were collected aiming at determining groundwater age at production wells twelve of which from the private wells and the other five monitoring wells located inside K-COSEM site. The groundwater age distribution was evaluated based on the multiple age tracers including CFCs, 3H, tritiogenic 3He and radiogenic 4He. Specifically, a Bayesian Markov Chains Monte Carlo (MCMC) was performed to make an inference about the age distributions of the 17 wells. The deviance information criteria (DIC) was used to find the best model among several ones in reproducing the observed data. It was found that the age distribution of the study site was relatively young (avg.: 11.3-120.2 y), indicating water is predominantly extracted from shallow layers. A stepwise approach was used to identify the importance of various factors affecting the inference of groundwater age, including 1) groundwater mixing, 2) degassing loss during vadose zone travel, 3) local contaminant source for CFCs, and 4) non-uniform influx of radiogenic 4He along the fractured materials. Overall results provided an insight that the combination of Bayesian approach and DIC criterion can help to constrain the primary mechanisms associated with the age tracer distribution.