EFFECT OF AQUIFER HETEROGENEITY ON THE SUSTAINABILITY OF DEEP GROUNDWATER RESOURCES IN THE BENGAL DELTA NEAR A MEGA-CITY PUMPING CENTER

Groundwater pumping beneath Dhaka, a mega-city in Bangladesh, has created an extensive drawdown cone. Outside the city, shallow groundwater is often contaminated with dangerous concentrations of arsenic (As) and deep groundwater is considered a reliable source of As-safe drinking water. Unfortunately, vertical head gradients induced by Dhaka pumping may threaten the deep aquifer with downward migration of shallow groundwater elevated in As and/or dissolved organic carbon capable of causing in-situ As release. This study explores the role of geologic heterogeneity in this downward flow. Five realizations of heterogeneous hydraulic conductivity were simulated with sequential indicator simulation conditioned to 400 borehole logs within the study area. A regional MODFLOW model with high resolution in the Dhaka area was developed for flow simulation through both heterogeneous and equivalent homogeneous systems. Advective particle tracking was used to determine the transport time and flowpaths of shallow groundwater. Results indicate that deep Dhaka pumping has induced vertical groundwater flow from the rivers and shallow aquifers in the surrounding regions. Although the overall shape and extent of the Dhaka pumping cone of depression is the same in both homogeneous and heterogeneous systems, there is substantial local variability in the vertical hydraulic gradient and travel times in heterogeneous systems. Both the minimum and maximum travel times in heterogeneous systems differ by an order of magnitude compared to that of equivalent homogeneous systems. Moreover, in the heterogeneous systems, although high gradients exist across clays, vertical flow is largely impeded; travel times to deep aquifer zones are thousands of years. In contrast, areas with thick sandy sediments showing near-zero vertical gradients are vulnerable to vertical flow of shallow groundwater in the deeper part of the aquifer system within a few decades. The results have implications for managing the deep groundwater resource in the Bengal Delta and in similar areas with contaminated shallow groundwater.