Analysis of heavy metals in bed sediments of Dhanmondi lake

Abstract

The Dhanmondi Lake, the largest urban lake of the Dhaka City, has been receivIng domestic as well as industrial sewage for a long time. The primary objective of the study was to assess the heavy metal contamination of the sediment column of Dhanmondi Lake at different locations. Secondary objectives include the determination of any possible relationship between metal concentration with particle size, vertical distribution of metal, assessment of association of metal with the organic and inorganic fractions and estimation of partition coefficient of a specific heavy metal. Sediment samples from ten locations of Dhanmondi Lake were collected from various depths. Locations were selected considering the probable sources of contamination from sewer lines connected to the Lake. In general, the soil samples were found to be grayish to black in color indicating high organic content. This can be attributed to disposal of domestic and industrial sewage. However, at greater depths reddish silty clay soils were distinctly visible which represent the original bed soil of the Lake. Samples were oven dried, ground and sieved. Then the samples were digested and the concentrations of different heavy metals (copper, lead, cadmium, chromium and mercury) at different depths were determined for each particle size using an Atomic Adsorption Spectrometer. The general trend of copper distribution associated with particle sizes indicates that the copper concentration increases with the decrease of particle size upto 0.30 mm, then it decreases with the decrease in particle size ranging between 0.30 mm to 0.15 mm. This trend changes again and the copper concentration increases with decrease in particle size. In case of lead, the concentration increases with the decrease in particle size upto 0.15 mm. Then it decreases with the decrease in particle size of 0.15 - 0.075 mm. Finally, it increases slightly with the decrease in particle size. No general pattern of cadmium distribution with particle size has been observed. However, test results indicate that higher cadmium concentration is associated with particle size between 0.6 - 0.3 mm at all locations irrespective of depth. Concentration distribution of chromium with particle sizes generally followed two distinct patterns: (a) chromium concentration increases upto 0.15 mm particle size, then decreases for particle sizes O.I 5 - 0.075 mm and then increases again; (b) chromium concentration decreases upto 0.075 mm particle size and then increases. However, in both of the patterns the lowest cadmium concentration was found to be associated with particle sizes of O.075mm. Mercury is probably the most toxic metal available in the environment even at very low concentrations. But due to lack of resources very limited number of samples were tested for the presence of mercury. Vertical distribution of mercury concentration as well as those of other metals indicated a higher level near the bed surface, which subsequently decreased as the depth increased. Samples collected from near the Pilkhana BDR Gate exhibited highest concentration for all metals among all the locations. However, the average concentration level throughout the lake sediment is alarming. The laboratory results indicate that the major fractions of the heavy metals are associated with the organic fraction of the soil. However, it should be noted that during the washing of the samples to separate organic fraction using NaOH, a significant portion of iron (oxy) hydroxides (and probably some other oxide particles as well), which provides the surface for adsorption of heavy metals on sediments, might have been washed out from the inorganic fraction. In general, the heavy metal concentration was found to increase with the increase in organic content. In order to estimate the partition coefficient of a specific heavy metal, a synthetic sample was prepared in the laboratory and was dosed with different metal solutions. The analysis of the synthetic sample does not provide any conclusive evidence. This may be attributed to the fact that the synthetic samples were analyzed only 72 hours after dosing which might not have given enough time to allow proper adsorption of all metals. Also, the chemical composition of the synthetic soil sample might have been different than that of bed sediment of Dhanmondi Lake. At present the restoration work of the Dhanmondi Lake is in progress. A maSSIve excavation campai!,'Il was conducted to remove the contaminated bottom sediment. At some locations the excavation was carried out to reveal the original formation. However, at most of the places this was conducted to partially remove the contaminated soil leaving the highly contaminated soil layer exposed for the fresh water to be introduced. This will definitely pollute the overlying water layer. In addition, these excavated contaminated soils were used for landfilling purposes within the city areas leaving the population and aquatic biota susceptible to heavy metal pollution.