Hydrochemical and stable-isotope signatures of groundwater in the Vietnamese Mekong Delta: aquifer-scale differentiation, uncertainty and management orientations

Tóm tắt

Groundwater is a strategic water source in the Vietnamese Mekong Delta (VMD), especially during dry-season drought and salinity intrusion. The delta, however, is a multilayered aquifer system rather than a homogeneous groundwater reservoir. The working dataset was compiled from the national project DTDL.CN-54/22 and related hydrogeological records; it includes precipitation, surface-water and groundwater samples collected during 2023-2025 and earlier monitoring records where explicitly stated. After quality screening, the groundwater dataset used for isotope-hydrochemical interpretation comprises 132 stable-isotope records, 132 hydrochemical records, 65 ³H records and 44 paired ¹⁴C/¹³C records, classified by aquifer and checked against well metadata. Stable isotopes were reported relative to VSMOW; repeated laser analyses were accepted only when the standard deviation of the final injections was below 0.05‰ for δ¹⁸O and 0.5‰ for δ²H. The local meteoric water line is δ²H = 6.80δ¹⁸O + 2.23, whereas the optimal regional groundwater line is δ²H = 4.92δ¹⁸O - 13.46 (R² = 0.85; RMSE = 0.34; AICc = -299.69). The lower groundwater slope is interpreted cautiously as the combined result of pre-recharge evaporation, mixed recharge histories, paleo-brackish/saline components, diffusion and water-sediment interaction, rather than as a single diagnostic mechanism. Hydrochemical facies, Cl⁻, EC/TDS, Na⁺/Cl⁻, Cl⁻/Br⁻ and HCO₃⁻ were used as independent checks on the isotope interpretation. The results demonstrate aquifer-scale differentiation: the Holocene aquifer (qh) is surface-sensitive and affected by modern recharge; the Upper Pleistocene (qp₃), Middle-Upper Pleistocene (qp₂₋₃) and Lower Pleistocene (qp₁) aquifers form a transitional domain; the Upper/Middle Pliocene (n₂²) and Lower Pliocene (n₂¹) aquifers preserve stronger paleo-salinity and older-water signals; and the Upper Miocene aquifer (n₁³) behaves as a deeper strategic reserve. These findings support aquifer-specific monitoring, controlled deep pumping and adaptive groundwater management under climate-change stress.