Environment International, Vol. 9, pp. 401-407, 1983 0160-4120/83 $3.00 + .00 Printed in the USA. All rights reserved. Copyright © 1984 Pergamon Press Ltd. ENVIRONMENTAL IMPACT OF POLLUTED CANALS--A CASE STUDY FROM COLOMBO, SRI LANKA CITY S. V. R. Weerasooriya, A. Senaratne, and C. B. Dissanayakea Department of Geology, University of Peradeniya, Peradeniya,Sri Lanka (Received 24 May 1983; Accepted 31 October 1983) The city canal of Colombo, an effluent canal running through the city, was monitored for its nitrogeneous species and toxic trace metal contents. The canal is the dumping ground for municipal gar- bage, human and animal excreta, organic waste matter, etc. However, nitrate content of the canal water remains low, indicating an incomplete nitrification process. Due to the existing pH and redox conditions, the nitrogen species are present in a chemically reduced form. Since the Pb, Cd, and Zn contents of the canal water are high, industrial contamination is obvious. The environmental impact of such a polluted canal is enhanced by the presence of a large number of drinking water wells in the vicinity of the canal. Introduction This study deals with the environmental impact of the effluent canal of Colombo, a city with a population of 1.8 million and the capital city of Sri Lanka. The canal itself is highly polluted but is still used by a large number of slum dwellers for various domestic purposes. The dangers associated with polluted canals running through highly populated cities in Sri Lanka has been highlighted by Weerasooriya et al. (1982); theirs was a study on the environmental impact of nitrate distribu- tion in the lake-effluent-canal system in Kandy, the sec- ond largest city in Sri Lanka. Figure 1 is a schematic representation of the potential dangers of the Colombo canal to the environment. The present study was under- taken in view of this detrimental environmental impact of the canal on the population of Colombo. Figure 2 shows the location of the canal under study and its environs in Colombo. The canal runs through an extremely dense network of industrial and residential areas; on its route, it collects massive loads of municipal sewage and industrial effluents from the vicinity. The canal is approximately 18 m in width and is stone-paved only in some regions. In the other regions it is open; here the potentially polluted canal water percolates into the water table and moves laterally through the perme- aTo whom correspondence should be addressed. able alluvium and gravel to nearby drinking water wells. The water table is approximately 3-5 m below the ter- race and slopes outwards. The critical pollution prob- lems arise during the dry season, especially in February and March. During this season, the flow rate of the canal water is considerably decreased and lateral per- colation increased due to the lowering of the water table. The canal itself in most places emanates hydrogen sulfide; this is detectable by its odour, and the color of the water being black. Hardly any fish could be seen in the canal, and algal blooms were found thriving in most places. Despite the poor water quality of the canal, it is used by a large number of slum dwellers in the area for bathing and washing purposes. 401 Materials and Methods Figure 2 illustrates the location of sampling points along the canal. In order to check the water quality in the nearby drinking water walls, some well water samples were obtained from locations shown in Fig. 2. The samples were collected in precleaned polythene con- tainers and kept cool and dark until tested within a few hours after collection. The chemical analyses were car- ried out in the hydrogeochemical laboratories of the Geology Department of the University of Peradeniya. All samples were collected during July 1982.