Heavy metal contamination of benthic sediment of turag river near industrial effluent discharge locations

Abstract

Metal pollution has harmful effect on biological systems and does not undergo biodegradation, therefore get accumulated in river bed. The present study intends to assess the level of contamination of sediment by heavy metals and radioactive substances of the Turag River flowing through the Northern and Western sides of Dhaka City. To estimate the geochemical environment of the river, eleven trace elements, namely Cr, Mn, As, Ni, Cu, Zn, Cd, Pb, Th, V and U, were examined from the freshly deposited river bed sediments of Turag River using Aqua-regia and analyzed at Environmental Engineering Laboratory of the Department of Civil Engineering, BUET following Standard Method 3111 using Atomic Absorption Spectrophotometer (Shimadzu AA 7000). Vanadium and Radioactive metals like Thorium and Uranium were measured at the Institute of Nuclear Science & Technology, Bangladesh Atomic Energy Commission (BAEC)using TRIGA Mark II research reactor and High Purity Germanium detector (HPGe) based instrumental neutron activation analysis (INAA).All the sediment samples were collected from eleven different locations and from each location three to five samples were collected at varying depth (0-5cm, 5-10cm, 10-15cm, 15-20cm and 20-25cm) and after preparation samples were analyzed using standard procedure. Trace element contamination assessment reveals that, the benthic sediment is contaminated to low to moderate level with Zn, As, Ni and Cu, while contamination withMn, V and Th is at moderate to severe level in this areas, with a common trend of concentration of V>Mn>Th>Zn>U>As >Cu >Ni>Cr>Pb>>Cd. As expected, the highest concentrations of metals in the sediment samples were most frequently found in the upper layers while the lowest concentrations were mostly observed in the bottom layers. This indicates that the untreated and/or partially treated industrial and municipal wastewater discharges along the river bank are major sources of pollution.Results showed that metal concentration ranged between 161 mg/kg and 34 mg/kg for V, 58.35 mg/kg and 4.96 mg/kg for Mn, 35.3 mg/kg and 6.2 mg/kg for Th, 11.66 mg/kg and 1.11 mg/kg for Zn, 5.48 mg/kg and 1.37 mg/kg for U, 11.06mg/kg and 0.16 mg/kg for As, 7.12mg/kg and <MDL for Cu, 2.59mg/kg and 0.28 mg/kg for Ni, 2.7mg/kg and <MDL for Cr and 3.56 mg/kg and 0.04 mg/kg for Pb at different depths. The highest contamination degree of the sediment was noticed at Location 1 (Dying industry discharge point) and lowest at Location 9 (near Dhaka Ashulia Highway) and the contamination level varies systematically with the sampling depth and the distance from the contamination source. The presence of elements in Turag River is somehow lower than UCC data except for As, V and U. However, it is anticipated that, the sediment quality may degrade in the near future due to increasing anthropogenic inputs in the river basin, hence proper management strategies are required to control the direct dumping of wastewater in the river.