Assessment of Pollution in Lagoons Using Water Quality Indices: A Case Study in Sainthamaruthu Coastal Lagoon

An understanding about pollution status is vital for remediating water resources, which is lacking in context of Sri Lanka’s lagoon system. The aim of this study was to assess the pollution in a lagoon in terms of degree, dynamics and sources of pollution using water quality indices. The water samples collected from up, mid and down streams of Sainthamaruthu lagoon across different rainfall seasons were analyzed for pH, temperature, Electrical Conductivity (EC), Total Dissolved Solids (TDS), Dissolved Oxygen (DO), ammonium nitrogen (NH₄⁺), nitrate nitrogen (NO₃⁻) and Total Phosphorus (TP). The measured variables were subjected to computation of Water Quality Index (WQI), Comprehensive Pollution Index (CPI) and statistical analyses. Subsequently, it was found that upstream region of the lagoon was severely polluted during North East Monsoon (NEM) whereas significantly higher concentrations of nutrient pollutants (NH₄⁺= 4.5 mg/L, TP=3 mg/L) and physiochemical attributes (EC=3007μS/cm, TDS=801.7 mg/L) respectively were associated with the higher pollution in NEM. Meanwhile, the other regions were moderately polluted throughout the year. Besides, the greater WQI (greater than 100) observed at all the regions across the different rainfall seasons implied that Sainthamaruthu lagoon was not suitable for drinking and fish culture. Furthermore, the results of Principal Component Analysis (PCA) revealed that the pollution in Sainthamaruthu lagoon is caused by mixed sources of natural and anthropogenic factors including mineralization, sea water intrusion, effluent discharge and wastewater release. Ultimately, this study scientifically proved the occurrence of pollution in Sainthamaruthu lagoon and the facts identified could be used during remediation of the lagoon in near future.