Hydrogeochemical processes and groundwater quality of over-exploited Dupi Tila aquifer in Dhaka city, Bangladesh.

Dhaka is one of the highly populated cities in the world. Increased urbanization and population growth in Dhaka are mostly dependent on groundwater, with 78% of the supply water coming from the Plio-Pleistocene Dupi Tila aquifer. This research was conducted with the objectives of identifying ion chemistry, hydrochemical processes and their relationship to groundwater quality and finding out the impact of over-exploitation on Dupi Tila aquifer. Three consecutive semi-confined aquifers have been delineated up to the explored depth: the Upper Dupi Tila aquifer (UDA), Middle Dupi Tila aquifer (MDA) and Lower Dupi Tila aquifer (LDA). Hydrogeochemical processes and water quality have been inferred from bivariate plots, correlation of major ions, piper plot and stable isotopes analysis. The total dissolved solids show an increasing downwards trend from an average of 267 mg/l in UDA to an average of 284 mg/l in LDA. Waters in all three aquifers as well as surface water (SW) are mainly of Ca/Mg-HCO3 type. The weathering of aluminosilicates controls the concentration of the major ions. Even though there is a significant decline in piezometric level both in UDA (> -85 m Public Works Department reference datum (PWD)) and MDA (> -65 m PWD), there is no evidence of groundwater recharge through direct infiltration from the river. The stable isotopes indicate most of the UDA, MDA and LDA waters are mainly from local precipitation. Increasingly enriched mean values for stable isotopes for the subsequent aquifers UDA, MDA and LDA, as well as increasing PCO2 for UDA < MDA < LDA, indicate increasingly warmer recharge conditions. The overall groundwater quality in the aquifers is good. At present, we have no indication that groundwater exploitation has altered groundwater quality of the Dupi Tila aquifer. Therefore, it is of vital importance to regularly monitor water quality for the purposes in order to timely detect any potential water quality alteration that could be aggravated by the vast decline in piezometric level, in view of the long-term sustainable development of the groundwater resources of this city.

Groundwater salinization and freshening processes in coastal aquifers from southwest Bangladesh.

The hydrogeochemical processes in the multilayer aquifers of southwest (SW) coastal area of Bangladesh provide important information for quantifying hydrochemical differences between different aquifers. Hydrogeochemical processes affecting groundwater chemistry in this coastal area have been evaluated by interpreting conventional plots, ionic delta, HFE-diagram, stable isotopes and geochemical modelling. The median TDS distribution of the aquifers has an increasing trend from below 1000 mg/l in the deep aquifer (DA) to 2622 mg/l in lower shallow aquifer (LSA) and 7012 mg/l in upper shallow aquifer (USA). Na+ is the dominant cation in all the aquifers. HCO3- is the dominant anion in DA with high median concentration (495 mg/l), which is more than double than that in LSA (214 mg/l) and USA (159 mg/l). The groundwater in the DA is freshened NaHCO3+ type due to cation exchange process that enhances second stage of calcite dissolution. Few NaCl+ type DA waters found in the SW corner of the study area indicate the presence of connate water confined in the inter-basin during the Holocene transgression. The stable isotopes indicate that the DA waters have been deposited during warmer periods. In contrast, both the LSA and USA are dominated by NaCl- type water; some CaCl- type waters have also been found in these aquifers, indicating salinization. The salinization processes consist of infiltration of redissolved evaporite salts, reverse cation exchange, and mixing with marine influenced flood water. Thus, the knowledge of different processes controlling freshening and salinization will help properly manage and preserve the environmental characteristics in the aquifer systems of SW Bangladesh as well as similar complex coastal geological settings.