Water quality modelling and improvement of water safety of rambakan oya reservoir catchment in Maha Oya irrigation division

Abstract

The Rambakan Oya reservoir is the primary water source for the Mahaoya water supply scheme and provides irrigation for paddy cultivation. In early 2020, this reservoir was highly affected by algal blooming throughout its catchment area. Therefore, this comprehensive study was conducted to identi-fy the sources of pollutants and propose water safety plans for the Rambakan Oya reservoir catch-ment. Water samples were collected from twenty different locations on the surface of the reservoir, by taking into account the outlets of streams to the reservoir. The results indicated that Nitrite, Chemical Oxygen Demand (COD) and total coliform are the most potential pollutants beyond the safe level for drinking water. To analyze the potential pollutant loading based on various land use patterns, the Storm Water Management Model (SWMM) was employed. Moreover, using a hazard matrix analysis, all potential hazards, both visible and hidden that could contaminate the water in the reservoir were thoroughly investigated in accordance with module number three of Water Safety Plans (WSP). The findings show that the potential pollutant loading at critical outlets increased with deforestation, while the impact of development activities on pollutant load was relatively insignificant. On the other hand, forest extent of 30% only affected a 5% increase in pollutant loading. These findings highlight that the primary cause of the undesirable impact on the Rambakan Oya reservoir is the runoff from cattle farms, carrying water contaminated with faecal matter and urine, rather than land development activi-ties. Moreover, surface runoff generated from agricultural lands and faecal pollution due to domestic effluents is in the next higher-order level of risk. Therefore, risk mitigation measures alone cannot en-sure the safety of the reservoir, the contribution and support of stakeholders are also crucial to en-hancing the safety of the Rambakan Oya reservoir.