Geochemical behavior of rare earth elements during the evolution of CO2-rich groundwater: A study from the Kangwon district, South Korea

A hydrochemical study of CO2-rich groundwaters in the Kangwon district, South Korea, was conducted to elucidate the behavior of rare earth elements (REE) as a result of mixing of CO2-rich waters. The REE fractionation during the formation of carbonate precipitates was also investigated. The CO2-rich waters can be classified into three chemical types (Na-HCO3, Ca-Na-HCO3, and Ca-HCO3) with characteristic isotope compositions. Chemical and isotope data indicate that the Na-HCO3 waters were influenced by a deep CO2 supply at depth, while other waters were formed by mixing between Na-HCO3 water and less-saline, shallow groundwater. Concentrations of total dissolved REE (ΣREE) in CO2-rich waters varied according to the water types and were highest in the Na-HCO3 waters (up to 3456 ng/L). Their ΣREE concentrations showed excellent correlations with parameters such as TDS, pH, alkalinity, δ18O, and tritium, indicating progressive mixing with shallow groundwater. The distribution of dissolved REE concentration showed different patterns dependent upon water type. The Na-HCO3 waters displayed an “S-shape” pattern with strong heavy REE (HREE) enrichment. In contrast, the Ca-Na-HCO3 water was characterized by a slight HREE enrichment, while the Ca-HCO3 water exhibited a relatively flat scattered, low REE (LREE) enrichment pattern. The REE patterns of the precipitates formed from the original waters and of the residual waters after removal of the precipitates were similar to those of the original CO2-rich waters, indicating that no REE fractionation occurred between precipitates and residual waters.