Geochemical evolution and flow of groundwater impacted by long-term abstraction in the Mekong Delta, Vietnam

• Hydrogeochemical processes in the Mekong Delta aquifers were verified by carbon isotopes and modeling. • Radiocarbon ages reveal unexpected patterns in the multi-layered groundwater system. • Groundwater abstraction induces vertical flow components between the aquifers. • Cross-aquifer flow might intensify salinization by exchange with saline marine aquitards. In many large river deltas, growth in agricultural production and population has led to a continuous drop in groundwater levels of the often multi-layered delta aquifers. To investigate the impact on groundwater quality, a total of 119 groundwater samples from (semi-)confined aquifers were collected for hydrochemical and isotopic (δ 13 C DIC , 14 C DIC ) analysis from the Mekong Delta in Vietnam, one of the world’s largest river deltas. These aquifers are dominated by groundwater of the Na-Cl to Na-HCO 3 type showing a large variation in dissolved concentration (TDS of 0.4 to 30.8 g/L). Geochemical modeling confirmed that dissolution of reactive minerals (e.g., carbonates, pyrite), mixing with seawater, and cation exchange are the relevant hydrogeochemical processes. The highly saline groundwater component is probably due to an exchange with connate seawater in the marine sediments of the Delta. δ 13 C DIC values indicate that high DIC concentrations (up to 14 mmol/L) result from different sources (soil CO 2 , dissolution of carbonates, reduction of sulfate by organic matter, and the admixture of seawater DIC). Adjusted 14 C DIC groundwater ages range from approx. 3 to more than 30 thousand years, which suggests that the groundwater was recharged during low seawater levels in the Upper Pleistocene. Contrary to expectations, 14 C DIC ages do not show clear age trends along the presumed flow direction or depth of the groundwater. This provides evidence for vertical flow or leakage processes through natural or artificial hydraulic windows within the aquitards, presumably caused or intensified by the increasing groundwater abstraction over recent decades.