Assessment of Surface Water Quality of an Urban Wetland: Thalangama Wetland, Colombo, Sri Lanka

Abstract

Thalangama wetland is situated within the Kelani river basin, Colombo District, Sri Lanka. It is one
of the most attractive landscapes in Colombo that provides a wide range of ecological, social and
economic functions. Wetland water supports aquatic life and is being prominently used for agriculture
and bathing and contact recreational activities without knowing if the water quality is appropriate for
such uses. This study was therefore conducted to assess the surface water quality of Thalangama
wetland using key-physicochemical parameters and to examine the suitability of wetland waters for
bathing and contact recreational water, aquatic life, irrigation and other agricultural activities. A total
of 10 water quality parameters covering temperature, pH, salinity, TDS, EC, DO, BOD, COD, nitrate,
and total phosphate were measured at eight sampling sites of the wetland. The study was conducted
in August 2020. The spatial variation of water quality parameters was assessed using one-way
ANOVA followed by Tukey’s pairwise comparison. Results were compared with ambient water
quality standards of Sri Lanka. The study revealed that the surface water quality parameters including
temperature, EC, TDS, DO, BOD, COD, nitrate and total phosphate show a significant spatial
variation among the sampling sites. Surface water pH and BOD at all the sampling sites were within
the ambient water quality standards for water suitable for bathing and contact recreational water and
water suitable for aquatic life, irrigation and agricultural activities. EC at all the sampling sites were
within the limits of water suitable for irrigation and agricultural activities. Nitrate and total phosphate
concentrations at all the sampling sites were within the limits of water suitable as bathing and contact
recreational water and water suitable for aquatic life. DO at six out of eight sampling sites were within
the limits of water suitable as bathing and contact recreational waters and water suitable for aquatic
life, irrigation and agricultural activities. COD at all the sampling sites were not within the limits of
water suitable as bathing and contact recreational water and water suitable for aquatic life. The study
identifies agricultural practices, livestock rearing, construction activities and urban discharges as
major anthropogenic influences that degrade the surface water quality of the wetland. Hence, the
study determines the necessity for effective management of the surface water of Thalangama wetland
to prevent pollution and to ensure the water quality of the wetland is within the recommended
standards.

Keywords: Urban wetlands, Surface water quality, Spatial variation, Physicochemical parameters, Ambient water quality standards