Groundwater flow system in Bengal Delta, Bangladesh revealed by environmental isotopes

A total number of 328 groundwater samples are analysed to evaluate the groundwater flow systems in Bengal Delta aquifers, Bangladesh using environmental isotope (H-2, O-18, C-13, H-3, and C-14) techniques. A well-defined Local Meteoric Water Line (LMWL) delta H-2 = 7.7 delta O-18 + 10.7 aEuro degrees is constructed applying linear correlation analyses to the monthly weighted rainfall isotopic compositions (delta O-18 and delta H-2). The delta O-18 and delta H-2 concentrations of all groundwater samples in the study area are plotted more or less over the LMWL, which provides compelling evidence that all groundwaters are derived from rainfall and floodwater with a minor localized evaporation effects for the shallow groundwaters. Tritium concentration is observed in 40 samples out of 41 with values varying between 0.3 and 5.0 TU, which represents an evidence of young water recharge to the shallow and intermediate aquifers. A decreasing trend of C-14 activity is associated with the heavier delta C-13 values, which indicates the presence of geochemical reactions affecting the C-14 concentration along the groundwater flow system. Both vertical and lateral decrease of C-14 activity toward down gradient show the presence of regional groundwater flow commencing from the unconfined aquifers, which discharges along the coastal regions. Finally, shallow, intermediate, and deep groundwater flow dynamics has revealed in the Bengal Delta aquifers, Bangladesh.