Surface water quality assessment for the management of hydrological regimes: Kalu Oya and Mudun Ela catchment in Sri Lanka

Abstract

This study aims to assess the relationship between surface water quality and the land-use of an urban catchment named ‘Kalu Oya and Mudun Ela catchment’ located in the western province, Sri Lanka. Ten judgmental sampling locations of the streamflow were selected to measure water temperature, pH, salinity, conductivity, Total Dissolved Solids (TDS), Biological Oxygen Demand (BOD₅), Chemical Oxygen Demand (COD), oil & grease, nitrate and dissolved phosphorous levels, flow rates over five months of the period from June to October 2018. All these parameters varied independently with a significant difference from other sites (ANOVA, p < 0.05). Water temperature and nitrate levels showed a strong clustering pattern with positive Moran’s index values supported by higher Z-scores. (Moran’s index 0.557 and 0.5106, Z-score 2.773 and 2.419 respectively). Higher oil & grease levels exceeding the accepted levels were observed at all the sampling sites. Oil & grease values showed a strong positive correlation with the flow rate (Coefficient-0.807, P value-0.005) revealing that higher oil & grease levels in the stream may be resulted by subsurface flushing. Site 02 is affected by the effluent discharged by the Colombo fish market having higher phosphate and TDS levels. Clustering pattern of nitrate and dispersed pattern of oil & grease of the Kalu Oya and Mudun Ela catchment show evidence of the poor drainage system in the catchment. Improving the drainage system may enhance water infiltration by reducing the volume of runoff and improve the water quality by that it can be used as an effective urban storm water drainage system in the hydrologic water regime.