Estimating Groundwater Recharge in a River-Dominated Deltaic Aquifer Using Water-Level and Chloride Tracers: a case study in the Mekong Delta

Abstract

Understanding groundwater recharge is crucial for sustainable groundwater development. However, estimating recharge rates remains challenging due to the heterogeneous nature of hydrogeological, land use, and climatic conditions. This study applies two complementary approaches Water Level Fluctuation (WLF) and Chloride Mass Balance (CMB) to estimate recharge rates in An Phu Ward, An Giang Province, Mekong Delta. The study used data collected from five monitoring wells, aligned perpendicular to the Hau River and ranging in depth from 20 to 24 meters. Groundwater levels were automatically recorded at 15-minute intervals from May 2023 to November 2024. Groundwater samples were collected monthly for chloride (Cl−) analysis, while rainwater was sampled during the rainy season. The results indicate that groundwater levels exhibit strong seasonal variability and are heavily influenced by fluctuations in river water level. Using the WLF method over a one-year time step, recharge rates were estimated to be 21%, 12%, 12%, 20%, and 21% of annual rainfall at wells W1 through W5 in 2023, respectively. Based on short-term water table responses to heavy rainfall events, recharge rates were further estimated at 18.1 ± 4.3%, 33.4 ± 6.5%, and 38.5 ± 10.3% at wells W3, W4, and W5, respectively. In contrast, the wells (W1, W2) closest to the river showed a strong river-groundwater connection (correlation coefficient r = 0.98), and no significant recharge signal was observed during heavy rain events. Using the CMB method, recharge rates were estimated to be approximately 16±3%, 17±2%, and 21±4% at wells W1, W3, and W5, respectively. However, substantially lower values were observed at W2 (5±1%) and W4 (6±3%) likely due to the increase of Cl− concentration due to irrigation. These findings highlight the influence of river aquifer interactions and irrigation practices on both groundwater levels and Cl− concentrations, introducing potential uncertainties in recharge estimation. Despite inherent limitations, the combined application of the WLF and CMB methods enhance the assessment of groundwater recharge in deltaic aquifers. The study highlights the importance of careful implementation and context-specific interpretation when using multiple approaches for groundwater recharge estimation.