Assessment of heavy metal contamination in sediment of the Buriganga-Turag river system

Abstract

The Buriganga and the Turag rivers are situated on the Southern and Western sides of Dhaka City. This is a good example of site where human pressures and ecological values collide with each other. Buriganga and Turag are some polluted rivers around Dhaka city. Encroachment, disposal of untreated domestic and industrial wastewater and dumping of solid wastes have degraded the overall quality of the rivers. The present study investigated the extent of pollution of sediments of those rivers. One of the aims of this research was to assess the level of heavy metal contamination in the sediment using advanced statistical techniques and different pollution indices and finally to analyze the ecological risk due to sediment contamination in the Buriganga –Turag river system. Under this study, sediment samples were collected from 15 (fifteen) locations of the Turag river and available data from previous studies on 05 (Five) locations of the Buriganga were used for sediment analysis. Samples were collected in April, 2011 in case of Turag river and in May, 2010 in case of Buriganga river and analyzed for the regional variability for the concentrations of Cr, Pb, Zn, Cu and Cd- all of concern because of their potential toxicity, using Atomic Absorption Spectrophotometer. Aqua regia digestion (USEPA method 3050) has been performed for the dissolution of the sediment samples prior to the determination of heavy metals. Metal concentrations found to be higher for the Buriganga river than the Turag river. The sediments of the Buriganga river assessed in this study have been found to be highly polluted with respect to Cu, Pb and Zn; unpolluted to moderately polluted with respect to Cd and moderately polluted to highly polluted with respect to Cr on the basis of USEPA sediment quality guideline. The sediments of the Turag river assessed in this study have been found to be moderately to highly polluted with respect to Cr, Cu, Zn; unpolluted with respect to Pb and Cd on the basis of USEPA sediment quality guideline. In order to determine the similarities and differences among sampling sites, concentration data of the heavy metals analyzed statistically by using Principal Component Analysis (PCA) methods. Cd-Cu-Zn; Pb-Cr may have same or similar source input in the sediments of Buriganga river and Cr-Zn; Pb-Cu in the sediments of Turag river on the basis of Principal Component Analysis. Significant correlation between the contaminants of Cd and Zn (r=0.99), Cd and Cu (r=0.98), Zn and Cu (r=0.97), Pb and Zn (r=0.89), Pb and Cr (r=0.85), Cd and Pb (r=0.82), Pb and Cu (r=0.80), Cr and Cu (r=0.75) in Pearson’s correlation for the heavy metals of Buriganga river indicates that those contaminants have same or similar source input. Significant correlations between the contaminants of Cr and Zn (r=0.45), Pb and Cd (r=0.45), Pb and Cr (r=0.49) in Pearson’s correlation for the heavy metals of Turag river indicates that those contaminants have same or similar source input. Different types of indices have been used to assess the current pollution status in river sediments of the Buriganga and the Turag rivers. The Buriganga and the Turag river have a low to appreciable potential ecological risk due to heavy metal contamination according to Ecological Risk Index. As per Sediment Quality Guideline Quotient (SQG-Q) the Buriganga river sediments are moderately to highly impacted and the Turag river sediments are moderately impacted to adverse biological effects.