HYDROCHEMICAL AND STABLE-ISOTOPE SIGNATURES OF GROUNDWATER IN THE VIETNAMESE MEKONG DELTA AND THEIR MANAGEMENT IMPLICATIONS

Abstract

Groundwater is a strategic water source in the Vietnamese Mekong Delta (VMD), particularly during the dry season and in coastal areas affected by drought and salinity intrusion. However, the delta aquifer system is not a single homogeneous reservoir; it is a multi-layered sedimentary system separated by clay, silt and sand units, with distinct recharge sources, residence times, hydraulic connectivity and salinization risks. This study identifies aquifer-scale groundwater origin and evolution using an integrated stable-isotope and hydrochemical framework, thereby establishing source fingerprints for the main aquifers of the VMD. The dataset includes precipitation, surface-water and groundwater samples analyzed for δ¹⁸O, δ²H, ³H, ¹⁴C and hydrochemical parameters; the groundwater dataset comprises 132 stable-isotope samples, 132 hydrochemical samples, 65 ³H samples and 44 ¹⁴C/¹³C samples. The local meteoric water line is δ²H = 6.80δ¹⁸O + 2.23, whereas the regional groundwater line is δ²H = 4.92δ¹⁸O - 13.46 (R² = 0.85). The markedly lower groundwater slope indicates pre-recharge evaporation, mixing with paleo-brackish/saline water and water-sediment interaction. Vertically, the Holocene aquifer (qh) records modern recharge and high surface sensitivity; the Pleistocene aquifers (qp₃, qp₂₋₃ and qp₁) represent transitional, mixed paleo-meteoric water; the Pliocene aquifers (n₂² and n₂¹) retain strong paleo-salinity signatures; and the Miocene aquifer (n₁³) plots closer to the meteoric line and behaves as a deep, old groundwater reserve. These findings provide new evidence for recharge-zone delineation, aquifer-specific salinity monitoring, controlled deep pumping and the design of an integrated isotope-hydrochemistry-water-level monitoring network under climate change.