Comparative study of copper contamination in the water and sediment column of the Ramna lake and Dhanmondi lake

Abstract

The primary objective of this study was to assess the level of copper contamination in the lake water and sediment of Dhanmondi and Ramna lakes. The other major objectives were determination of equilibrium speciation of copper in lake water in order to assess its toxic effects, determination of spatial distribution of major copper species, evaluation of copper adsorption characteristics on different sizes of bed sediment particles of both the lakes and assessment of gross bioaccumulation of copper in fish. Water and sediment samples of Dhanmondi and Ramna lake were collected from different sampling locations for laboratory analysis during three different periods of the year namely, dry period (April to May), wet period (June-July) and intermediate period (August-January). Sediment samples were collected to assess the nature of copper adsorption on the bed sediment particles. In addition, bioaccumulation of copper in fish of the lake was assessed by determining copper concentrations in various parts including gill, stomach etc. of a fish (Nilotica) of Dhanmondi lake. Both the Dhanmondi and Ramna lake water and sediment layer were found to be heavily contaminated with copper. Maximum and minimum concentration of copper in the water ofDhanmondi lake was 0.2913 mgll and 0.062 mgll and in Ramna lake 0.2047 mgll and 0.0723 mg/l respectively. Major aqueous copper species found in both the Dhanmondi and Ramna lakes were Cu(OH)" Cu'+, CuCO,. concentration ofCu(OH), was maximum. Cu'+, the free and most toxic form of copper species was also present in considerable amounts. pH was found to be the major controlling factor for copper speciation. With a decrease in pH, ionic copper (Cu'+) concentration increases thus increasing the toxicity of water. Conversely, with an increase in pH, ionic copper concentration decreases with increased adsorption reducing toxicity. Analysis of sediment samples suggest that maximum amount of copper was adsorbed on particles retained on sieve #30, #40, #50 (i.e. size < 1.19mm. to> 0.297mm.). In most cases copper absorption increased with the decrease in particle size upto a certain level (0.42 mm). After that particular particle size, copper absorption decreased with the decrease in particle size. From the analysis of bed sediment it was found that maximum copper concentration in top and bottom layer of Dhanmondi lake was 59.45 mg/kg and 52.28 mglkg respectively. In Ramna lake copper concentration in top and bottom layer was 47.77 mg/kg and 46.21 mglkg respectively. Study of bioaccumulation of copper revealed that copper is present in excessive amount in the different parts of fish in Dhanmondi lake. Maximum copper concentration was found in the stomach (7.442 mglkg) of fish. Thus, human consumption of the fishes from Dhanmondi lake may be harmful. At present renovation work is underway at the Dhanmondi lake. It seems that the lake is being dredged indiscriminately and the dredged spoils are being piled along the shore. If not done upto a proper depth this dredging process is likely to expose the heavily contaminated deeper sediment layer to lake water. The freshly filled lake water will get contaminated with copper through diffusion and desorption from the newly exposed bed. In addition, the dredged spoils piled along the shore may release copper following rainfall into the lake and surrounding areas.