Tannery industry impacts on hydrogeochemistry and heavy metal contents of two major rivers in Dhaka

Dhaka city is highly vulnerable to environmental pollution as a result of overpopulation, rapid industrialization, and urbanization in recent decades. A survey was carried out during winter season of 2019 to investigate the effects of tannery industry effluents on hydrogeochemistry, chromium, nickel and lead concentrations in river water and sediments of Buriganga and Dhaleshwari rivers passing by the side of Dhaka City. Water and sediment samples were collected from the downstream of tannery industrial effluents discharge sites. The pH of the water samples collected from Buriganga river ranged between 7.04 and 7.60, whereas it was between 7.23 and 8.45 from Dhaleshwari river. The water samples of Dhaleshwari river had more organic carbon (258±102 mg C L-1), NH4+-N (134±72.4 mg NL-1) and NO3--N (24±24 mg NL-1) compared to the water samples of Buriganga river (167±87 mg C L-1, 114±26 mg NL-1, and 11±6 mg NL-1, respectively). The sediments from Buriganga river had very high chromium (Cr) (15.23±11.68 mg kg-1), and lead (Pb) contents (36±23mg kg-1) compared to the Cr (1085±2453 mg kg-1), and Pb (17±9 mg kg-1) contents in sediment samples from Dhaleshwari river. By contrast, the sediment samples from Dhaleshwari river had more nickel (Ni) content (232±143 mg kg-1) than in sediments from Buriganga river (128±50mg kg-1). The water samples from Buriganga river had Cr, Pb, and Ni contents below the detection limit, except ones ample for Pb (0.06 ppm) and Cr (0.01 ppm) and another for Ni (0.07 ppm). Only one water sample from Dhaleshwari river had detectable Pb (0.01 ppm) contents. Thirteen water samples from Dhaleshwari river had Cr ranged from0.01 to0.16 ppm while the remaining had Cr below the detection limit. The results indicated that occurrence of large quantity of heavy metals and organic pollutants in the sediments and river water can be a threat to the ecological status of two major rivers in Dhaka city and of the ecology of agricultural ecosystems near Dhaka city