ASSESSMENT OF SEASONAL HEAVY METAL POLLUTION AT THE ESTUARIES OF SOME SELECTED CANALS FEEDING THE RIVER KARNAPHULI

Abstract

The main goal of this study was to evaluate the sessional variation of heavy metals concentration in surface water and sediments at the estuaries of major cannals (locally known as khal) feeding to Karnaphuli river and their associated health risks. Additionally, some important physico-chemical parameters of surface water were also studied to assess the present environmental loads at the study points. Ten major khals, which serve as the natural drainage to the Karnaphuli river were chosen for collecting the surface water and sediments samples; among them six from the Chattogram city side and four from the opposite side of Karnaphuli river. Concentrations of eight heavy metals, like Cu, Cr, Cd, Pb, Ni, Mn, As and Zn in surface water and sediment samples during the wet and dry seasons were determined using flame atomic absorption spectroscopy (FAAS). Beside these, some important physico-chemical parameters, such as temperature, pH, DO, BOD, COD, TDS, EC, salinity and hardness of surface water were measured using portable multi-analyzer and standard methods, where applicable. Some of the studied parameters at some locations showed higher values than the limits recommended by WHO, especially in the dry season. The values of all parameters, except temperature were cosideably higher in the wet season than in dry season. The average concentrations of Cu, Cr, Cd, Pb, Ni, Mn, As and Zn in surface water samples during wet were found 57.48 ± 7.07, 52.79 ± 5.63, 14.94 ± 3.15, 52.15 ± 5.41, 34.34 ± 3.89, 64.90 ± 10.31, 9.40 ± 1.55 and 60.37 ± 6.77 µg/L and in dry season the concentrations were 65.24 ± 9.40, 62.50 ± 6.52, 22.20 ± 4.41, 72.24 ± 9.14, 38.28 ± 5.34, 69.90 ± 11.13, 15.03 ± 3.53 and 75.03 ± 9.78 µg/L respectively. Again, the average concentrations of those heavy metals in surface sediments samples were 38.84 ± 3.50, 46.92 ± 3.84, 2.18 ± 0.42, 42.98 ± 3.97, 43.39 ± 3.55, 1.34 ± 0.19 and 91.93 ± 7.72 mg/kg in wet season and 51.13 ± 2.88, 59.23 ± 1.65, 2.52 ± 0.14, 61.05 ± 4.09, vi 49.66 ± 5.70, 58.55 ± 8.112, 21 ± 0.74 and 104.44 ± 5.02 mg/kg respectively in dry season. The average concentration of Cu, Cr, Cd, Pb and Ni in sediments were found higher than the limit recommended by USEPA, 2019. However, the study also found that the concentration of heavy metals in both water and sediment samples were found higher in the dry season than that of wet season at most of the locations. The metal pollution index (MPI) shows that water of all the studied feeding khals are highly polluted in both wet and dry seasons. Potential carcinogenic and non carcinogenic health risks through dermal contact were found within the acceptable limit recommended by USEPA for lifetime exposure to the surface water. Most of the environmental indices calculated, like enrichment factor (EF), degree of contamination (Cd), pollution load index (PLI), geoaccumulation index (Igeo), potential ecological index (PERI) and nemerow pollution index (PN) for heavy metals in surface sediments showed no potential risk; although the contamination factor (CF) and modified contamination degree (mCd) indicate that the sediments of the studied khals became moderately and highly contaminated respectively with heavy metals. Pearson’s correlation coefficient was applied to find out the association between pollutants in the investigated samples. The results showed that physiochemical parameters of surface water, and concentration of heavy metals in surface water and sediments are influenced by seasonal water flows. Moreover, heavy metals showed variation from one location to others, and it was considerably higher in the industrial sites than that of municipal waste discharge sites. The obtained comprehensive data for physico-chemical parameters of surface water, and toxic heavy metals in the surface water and sediments at the estuaries of studied khals feeding to Karnaphuli river may help the policy maker for making effective legislation to minimize the ecological risks of the river through proper monitoring, controlling and protection.