VULNERABILITY ASSESSMENT OF GROUNDWATER CONTAMINATION IN AN OVER-EXPLOITED AQUIFER SURROUNDING A LOW-LYING UNSANITARY LANDFILL SITE IN DHAKA USING COMBINED GEOLOGICAL MODELING, HYDROGRAPHS, WATER CHEMISTRY, AND ENVIRONMENTAL ISOTOPES (2H AND 18O) ANALYSIS

The studied landfill site in the designated flood flow zone is in Amin Bazar in Savar Upazilla, central Bangladesh. Overexploitation of groundwater developed a large cone of depression about 10km southwest of the project site, which perturbed the study area's groundwater dynamics. This phenomenon affected groundwater availability risked contamination from existing point and non-point sources. The hydrostratigraphic model and cross-sections revealed the study area to be characterized by one major aquifer belonging to the Plio-Pleistocene Dupitila Formation, which is composed of alternating fine to medium sand layers overlain by 5 – 17m thick floodplain deposits. Aquifer pump test analyses and lithologic characterization of subsurface materials imply that the aquifer is mainly semi-confined in nature and has moderate potential for groundwater development. Long-term water-level hydrographs of the Dhaleswari River from locations upstream and west of the landfill site show good agreement with the nearest groundwater stations, indicating the possibility of surface and groundwater interaction. Hydrogen and oxygen isotope analysis further confirms that groundwater is connected with surface water and recharged or mixed with isotopically light Dhaleswari River water. It is particularly true for the areas near the bends, where relatively coarser surface materials formed the meander bars, and the Madhupur clay unit is partly missing at the top. On the other hand, isotope measurements of the groundwater samples near the Turag River, located east of the landfill site, did not provide any evidence of recharge from river water sources. Overall, the groundwater of the study area varies from Ca-Mg-Na-Cl type to Ca-Mg-Na-Cl-HCO3 type water. Aqueous geochemistry data at and near the Karnatali River at the northern edge of the landfill disposal site signifies the existence of surface and groundwater interaction. Because of the high probability of mixing of surface water and groundwater through lateral and vertical movement, much care should be taken in waste disposal and landfill leachate treatment, especially during monsoons, to avoid further deterioration of groundwater and surface water quality in the study area.