Assessment of heavy metal contamination of sediments of some polluted rivers

Abstract

Buriganga, Sitalakhya and Turag are some of the 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 these rivers. Sediment samples have been collected from five locations of Sitalakhya river and available data from previous studies on ten locations of Buriganga and Turag river have been used for sediment analysis. Samples were collected in April 2011 from Sitalakhya river and analyzed for the concentrations of Cr, Pb, Zn, Cu and Cd using atomic absorption spectrophotometer (AAS). Aqua regia digestion has been performed for the dissolution of the sediment samples prior to the determination of heavy metals. The metal ion concentrations in the sediment samples have been compared with USEPA sediment quality guidelines. Based on this comparison, the sediment samples have been characterized as “heavily polluted”, “moderately polluted” and “not polluted”. The sediments of Buriganga river assessed in this study have been found to be highly polluted with respect to Cu, Pb and Zn; not polluted to moderately polluted with respect to Cd; moderately to highly polluted with respect to Cr. The sediments of Sitalakhya river assessed in this study have been found to be unpolluted to moderately polluted with respect to Cr; moderately to heavily polluted with respect to Cu; not polluted to moderately polluted with respect to Zn; not polluted to heavily polluted with respect to Pb; not polluted with respect to Cd. The sediments of Turag river assessed in this study have been found to be moderately to highly polluted with respect to Cr, Cu and Zn; not polluted with respect to Pb and Cd. Toxicity characteristics leaching procedure (TCLP) test for sediment samples have been performed for the heavy metals Pb, Cd, Cr, Cu and Zn. The metal concentrations in the TCLP samples have been found to be well below the regulated level as per USEPA. Therefore, the sediments are not likely to readily leach these metals in the water. A major objective of this study was to assess suitability of different methods for assessment of sediment quality. The methods assessed included metal pollution index, marine sediment pollution index, toxic unit, geo-accumulation index, PIN index, potential ecological risk index, contamination factor, degree of contamination, pollution load index, mean sediment quality guideline quotient, Pearson’s correlation, principal component analysis and cluster analysis. The methods differ in a number of ways, especially with respect to data requirement. The suitability of different methods in the context of Bangladesh has been assessed utilizing the sediment quality data used in this study.