Abstract:
Kalu Oya basin, which is located near to sea outfall of Kelani river comprises with 18 sub catchments
having a total area of 78km2. As a consequence of numerous Colombo urban infrastructure development
projects, the urban wetlands in the area are being severely utilized and devastatingly affected the water
quality of the Kalu Oya basin with an enormous threat to the community. The present study assess the
spatial water quality risk in Kalu Oya basin in relation to urban wetland features and land use pattern
changes by using GIS maps and satellite images, Surface water quality variation of Kalu Oya basin at 45
locations and ground water at 38 locations were examined for five consecutive months duration from July
to October 2016. Water quality parameters such as pH, Electrical Conductivity (ECl, Dissolved Oxygen
[DOJ, N0s , PO+3-, SO+2-, Cl , HCO:-, F-, Na*, Kt, Ca2*, Mg2* and Fe were analyzed. Suitability of ground water for drinking purposes and the surface water for irrigation purposes were examined using water qualty
indices developed in the present study. Results revealed that EC, DO, Ca2*, F-, and Fe in ground water
exceeded the threshold limits declared by WHO standards. Sodium Absorption Ratio (SARJ, EC, PO43, K+ Cl- and Na* in surface water exceeded the threshold limits at Wattala, Hunupitiya, Hekitta
Thimbirigasyaya and Wanawasala areas. High level of anthropogenic activities were found to be the most
significant reason for the poor water quality in the sub catchments numbers of 15, 16 and 18 from the
total study area. These findings emphasized the need of introducing better catchment management
approaches on urban wetland water quality.
