Geochemical options for water security in a coastal urban agglomerate of Lower Bengal Delta, Bangladesh

Khulna- situated in the Lower Bengal Delta of Bangladesh- is one of the most stressed coastal agglomerates globally under the realm of rapid changes in the climate pattern. This research assesses the geochemical options for water security of Khulna City Corporation (KCC) by analysing a total of 47 surface- and 62 groundwater samples collected during both monsoon and winter from stations uniformly distributed over the city following standard methods. The dissolved ions from three possible water sources such as the ponds and lakes, the river Mayur and groundwater generally show a similar relative dissolved ions concentration pattern where the alkali metals (Na+ + K+ ) exceed the alkaline earths (Ca2+ + Mg2+ ), and the strong acidic anions (Cl- + SO42-) and the weak acidic anions (HCO3- ) are of equal importance, indicating natural softening and mixing of water from both plagioclase and carbonate weathering, and seawater sources. Although the spatial variation with reference to mean concentrations of dissolved ions among different sampling stations during both monsoon and winter seasons in all the water sources such as the ponds and lakes, the river Mayur and the groundwater is not statistically significant at 5% level, the variation in dissolved ion concentrations among the water sources is statistically significant at 5% level. The temporal variation in concentrations between monsoon and winter seasons for all water sources is statistically significant for most dissolved ions, suggesting active material exchange in the system. The molar ratios among the dissolved ions suggest that both continental/meteoric and seawater (saline water)/oceanic contributions are important in KCC water. The ions in ponds and lakes and in river Mayur might have derived from similar sources; however, the sources are varied in the case of groundwater. The geochemical mechanism controlling the ponds and lakes, and the Mayur water chemistry is commanded by the process of rock weathering, while the winter chemistry of most groundwater is controlled by the process of evaporation and crystallization. Water from the three potential sources of KCC indicates mixed hydrochemical facies where the process of natural softening is active, while certain KCC groundwater indicates reverse softening and up-coning of saline front. An assessment of potable water quality among the three sources suggests that the ponds and lakes, and the river Mayur have the geochemical potential to provide water security to the KCC, particularly the river Mayur during monsoon. Thus, a plan for maintaining especially the Mayur as a freshwater reservoir is necessary for meeting the freshwater challenges in KCC.