Impact of sand mining on groundwater-surface water interactions in the Vietnamese Mekong Delta

Sand mining is a pressing socio-environmental concern in the Vietnamese Mekong Delta (VMD), profoundly reshaping river morphology, sediment transport, and hydrological processes. Although previous research has identified the adverse effects of sand extraction on surface water and channel morphology, its impact on groundwater has never been examined. This study addresses this unexplored relationship by providing the first investigation of how sand mining influences groundwater storage and surface water–groundwater (SW–GW) interactions, using an integrated approach of long-term field observations, remote sensing data, and hydrological records. Spatiotemporal analyses reveal a declining trend in groundwater storage anomalies (GWSA) and groundwater levels (GWLs), particularly in areas of intense mining activity. GWL decline rates in the unconfined aquifers within 1995–2017 vary spatially with a mean decline of 0.03 m/year. The maximum rate of GWL decline is 0.09 m/year and observed at the northeast and the minimum rate is 0.01 m/year and observed at coastal delta. GWSA has declined over 20 years in three phases: a sharp decrease (2002–2006, 3.2 × 10⁷ m³/year), partial recovery (2006–2015, 8 × 10⁶ m³/year), and a slower decline (since 2015, 4 × 10⁶ m³/year). The findings indicate that extensive riverbed incision caused by sand mining deepens channels, disrupts sediment redistribution, and accelerates groundwater depletion while reducing aquifer recharge. As a result, riverbed hydraulic connectivity is weakened, leading to a diminished SW-GW exchange. A conceptual model is proposed to illustrate the mechanisms driving these changes. These insights suggest that regulating sand extraction intensity, monitoring incision hotspots, and integrating sediment management with groundwater governance may help mitigate long-term hydrological degradation in the VMD and similar large deltas.