Present environment and historic changes from the record of lake sediments, Dhaka City, Bangladesh

摘要

Changes in the urban environment of Dhaka City have been evaluated from the geochemical compositions of bottom sediments from two lakes, Gulshan-1 and -2. Abundances of Pb, Zn, Fe_2O_3 and total sulfur in a Gulshan-1 core gradually increase toward the sediment-water interface. Three stages of condition change can be recognized in Gulshan-1, based on trace metal concentration patterns. The basal Stage Ⅰ corresponds to background Dhaka sedimentation, whereas gradual increase in Stage Ⅱ represents the beginning of pollution. Marked increases in Pb and Zn in Stage Ⅲ reflect rapid urban development and increased emissions to the atmosphere. The condition of the Dhaka environment is compared to that of Japan based on Zn-Fe_2O_3 and Pb-Fe_2O_3 relations. Zn-Fe_2O_3 ratios in Stage Ⅰ Dhaka sediments are similar to normal Japanese sediments, whereas Stage Ⅱ data lie on the same trend as Japan urban sediments. Dhaka Stage Ⅲ samples have greater Zn:Fe_2O_3 ratios than Japanese urban sediments, suggesting Zn pollution from poorly-controlled industrial sources. In contrast, the Dhaka Pb-Fe_2O_3 trend lies between normal and urban Japanese sediments. Although vehicle emissions in Dhaka are now significant, the data suggest that Pb pollution from this source remains in its early stages.