Quantifying the timing and source of baseflow into the tidally fluctuating Meghna River, Bangladesh

Rivers carry enormous quantities of particulate and dissolved elements from the continents to the oceans and inland seas. Much of the dissolved material fluxes are derived from baseflow, which includes both groundwater and short-term riverbank storage. Understanding of the timing and volumetric fluxes of groundwater to the river is important to how these deposits develop. In lowland regions with prolonged dry seasons, baseflow supplies the majority of river flow. During the dry season, the Meghna River undergoes a dynamically fluctuating river stage, owing to ocean tides. The study area is a 10 km reach of the Meghna River in the middle of the Ganges-Brahmaputra-Meghna Delta, 200 km from the Bay of Bengal. In this study, three different methods were employed to estimate volumetric flux. The first river-based method was differential gaging with an Acoustic Doppler Current Profiler (ADCP). Continuous discharge measurements were made every 30-45 minutes from a moving boat during ebb tide, which lasts approximately 8 hours, over five consecutive days. The width of the upstream and downstream ends of the study reach were 1.5 and 2.0 km, respectively. Baseflow was calculated by subtracting water coming from the tidal prism from total produced water along the 10 km (8.5x10⁶ to 5.5x10⁷ m³/day). The second river-based method used 24-hour averaged radon (²²²Rn) activity measured continuously at both the upstream and downstream ends of the study reach on consecutive days. Radon activity in 3 shallow monitoring wells represented the groundwater end-member (208±23 dpm/L). Radon activity in the upstream and downstream were 3.0±1 and 1.5±0.3 dpm/L, respectively. The estimated aquifer discharge based on measured radon activities at the downstream end of the study reach was 1.0x10⁶ m³/day. The third, aquifer-based method was an analytical model based on Boussinesq equation calibrated with observed hydraulic heads at two well-characterized sites on opposing sides of the river within the connected, shallow aquifer. This model calculated the timing of discharge of both groundwater and bank-storage over the study reach, over a semi-diurnal tidal cycle. The estimated volumetric flux was used to estimate the time to develop observed solid-phase concentrations of arsenic along the riverbank.