Assessment of flux of organic carbon and heavy metals from benthic sediments of a polluted canal

Abstract

Continued disposal of untreated industrial and domestic waste has significantly degraded the ecosystem of Tongi khal (canal), one of the peripheral river systems around Dhaka city. Sediment of polluted water bodies often serve as a major source of contaminants for the overlying water. However, there is limited data on the sediment quality of Tongi khal. In this study sediment and water quality of Tongi khal during the critical dry period have been assessed, and fluxes of selected nutrients, organic carbon and heavy metals from sediment to water column have also been assessed through laboratory experiments. Result of this study has identified the anoxic condition in the river during the dry season, with very low dissolved oxygen ( 0.2 mg/L), negative ORP (redox potential) and high sulfide concentrations (as much as 177.5 µg/L) in the river water. Dry period water quality of Tongi khal is characterized by high concentrations of organic matter (highest BOD5 91.9 mg/L, COD 212 mg/L), ammonia (highest concentration 25.2 mg/L), phosphate (up to 8.31 mg/L), and sulfate (as high as 295 mg/L). Water quality of Tongi khal becomes worse at the end of the dry season (i.e., March-April). Furthermore, water quality of Tongi khal deteriorates as one moves from upstream of the industrial locations to downstream (towards Balu River), due to the discharge of industrial (as well as domestic) effluents from industrial establishments. Benthic sediments of Tongi khal contains high concentration of organic matter varying from 5.8 to 11.3 percent (wet weight basis). Sediment samples also contain relatively high concentrations of Pb (up to 71.4 mg/kg) and Cr (up to 74.8 mg/kg); Cd and Hg concentrations were found to be relatively low. All sediment samples have been found to be “moderately polluted” with respect to chromium, and one sediment sample has been found to be “heavily polluted” with respect to lead, according to USEPA guideline. Although concentration of Pb and Cr were high in the sediment samples, TCLP test results suggest that these metals (Pb, Cr, Cd) are strongly bound to the sediment samples, and would not be released to water overlying water column easily. However, high concentration of Hg (around 6 times more than the USEPA limit) in the leachate of TCLP test indicates that sediment-bound mercury (Hg) could be released into water easily. Very high concentration of potential sediment oxygen demand (pSOD, as high as 37,191 g O2/m3 Sediment/day) of the Tongi khal sediments indicates the presence of high concentration biodegradable organic matter in the sediment. Results of this research suggest that accumulated pollutants in the bed sediment is an important contributor to the deteriorated water quality of Tongi khal. Significant flux of ammonia (as high as 1,152 mg/m2.day) from sediment to water column was observed due to ammonification process. However, ammonia concentration in water has been found to decrease with time together with an appreciable rise in pH, indicating significant algal uptake of ammonia; nitrification was not an important mechanism (for reduction of ammonia concentration) in the anoxic condition of the experimental systems. Flux of other nutrient constituents (e.g. phosphate) was not significant, but enough to support algal growth in the water column. Appreciable and sustained flux of organic carbon from sediment to water column was observed during the course of the experiments. Flux of lead and chromium from sediment to water was not very significant, in agreement with the TCLP test results. However, physical disturbance of sediment could immediately make the overlying water layer anoxic, and promote enhanced release of nutrient (e.g., ammonia), organic matter (also observed during determination of pSOD), SS, and heavy metals. Sustained disturbance, as could be expected during dredging, would severely deteriorate the water quality of Tongi khal.