Characterization of the water quality in the Rekawa lagoon, Sri Lanka

Abstract

Rekawa lagoon is located in the Hambantota district in the Southern province of Sri Lanka. The lagoon is of high ecological and socio economic significance and is heavily utilized for aquaculture and fisheries purposes. The present study was conducted to assess the pollution status of selected sites in Rekawa lagoon in terms of water quality indices. The data was collected from five sampling sites in the lagoon; Site A: canal mouth, Site B: adjacent to residential areas, Site C: adjacent to agricultural lands, Site D: adjacent to hotels and shrimp farms, Site E: adjacent to mangrove forests. The water samples from each sampling site were analysed monthly for a period of six months from August 2023 to January 2024 to measure the water pH, salinity, turbidity, conductivity, DO (Dissolved Oxygen), TDS (Total Dissolved Oxygen), BOD5(Biological Oxygen Demand), temperature and Cr, Cd, Pb concentrations taking 5 replicates for each sample. In-situ parameters such as pH, salinity, turbidity, conductivity, DO, TDS, temperature were measured using multiparameter. The BOD5 values were measured according to APHA 2017. Heavy metal concentrations were measured using GBC SavantAA. The spatial and temporal variation of water quality parameters were assessed using one – way ANOVA followed by Tukey’s pairwise comparison by Minitab 17 software. Water quality index (WQI) was calculated from the water quality data. The WQI reflects moderate confidence due to the absence of microbiological parameters. The results of the present study indicated significant spatial variations of temperature, salinity, EC (Electrical Conductivity), pH, DO, BOD5, Cr, Cd, Pb and Zn concentrations. The highest mean surface water temperature was recorded at site C (32.5±0.11oC), highest mean surface pH at site A (8.35±0.06), highest mean conductivity at site A (28.48±0.28 mS/cm), highest mean water salinity at site A (10.31±0.21‰), highest mean DO at site A (8.09±0.01 mg/L), highest mean BOD5 at site C (4.77±0.02 mg/L), highest mean chromium concentration at site D (1899.00±0.00 ppb), highest mean cadmium concentration at site D (10.01±0.00 ppb), highest mean lead concentration at site D (157.02±0.00 ppb) and highest mean zinc concentration at site D (281.00±0.00 ppb). Salinity, EC, pH and temperature showed significant temporal variations among the sites. The highest mean surface water temperature was recorded in August, October and November (32.5 oC), highest mean salinity in August (12.00 ‰), highest mean conductivity in November (28.80 mS/cm) and highest mean pH in August (8.11). The highest WQI was recorded at site D (51.8) while the lowest was recorded at site E (26.6). According to the WQI the site D was categorized as poor while the other sites were categorized as good. The high load of organic pollutants released by the hotels and restaurants in the vicinity was identified as a possible cause for the degradation of water quality at site D as many point source wastewater outlets were identified at this site. Therefore, it is recommended to conduct adequate water quality monitoring and management activities in the Rekawa lagoon to ensure the continuous ecological and economic benefits acquired from the lagoon and to prevent the possible degradation.