QUANTIFYING GROUNDWATER DISCHARGE TO THE MEGHNA RIVER USING DIFFERENTIAL GAGING, RADON AND HYDRAULIC GRADIENTS

Much attention has been given to quantifying submarine groundwater discharge (SGD) to oceans through subaqueous deltas. Current studies indicate that in lowland river deltaic systems, SGD often occurs as baseflow through a system of paleochannels. Riverbank aquifers in deltas are, however, very dynamic with levels that fluctuate frequently with tides and seasonal flooding. These river fluctuations force the adjacent shallow groundwater to change flow direction and redox conditions, inducing oxic surface waters with fresh dissolved organic carbon (DOC) into otherwise reducing groundwater. In this study we attempted to quantify the contribution of groundwater discharge to the Meghna River using three independent approaches, namely: differential gaging, hydraulic gradients and a tracer mass-balance method using radon-222. The Meghna River is the third largest river on the Ganges-Brahmaputra-Meghna (GBM) delta. We focused on the early dry season when groundwater levels are still relatively high and river levels are low. Preliminary results show very good agreement between the radon approach and the differential gaging assessments. Comparisons between discharge in the northern and southern ends of the study reach during low tide on immediately adjacent days indicated approximately 300 m³/s (8%) of the peak river discharge (3,600 m³/s) measured at the southern end came from groundwater discharge across the 13 km study reach. This amount was comparable to that estimated during a 24-hour Rn time-series at the northern and southern border of the study reach. Hydraulic gradients in the river bank aquifers agreed with the magnitude and timing of the observed peak groundwater discharge occurring during low tide. A shortcoming of this study was that only one ADCP was available to measure discharge in the river, so synoptic measurements couldn’t be made. In spite of this, the differential gaging found a similar groundwater discharge to the Rn method, indicating the errors were reasonably small during the relatively steady low tide stage. Establishing a reliable protocol for assessing groundwater contribution to the lowland deltas in the area will have big implications for understanding the role of groundwater in the release of As accumulated in the river aquifer system.