Stormwater quality and pollutants loading from catchments at Sungai Johor

Abstract

Sungai Johor is an important water resource in terms of aquaculture, agriculture and source of drinking water for the entire Kota Tinggi, Skudai, and Iskandar Puteri districts as well as Singapore. However, a tropical downpour may increase pollutants loading in the river which in turn may affect its beneficial uses. The purpose of this study is to investigate the concentrations and loading of pollutants in stormflow. Comparisons were made against baseflow for two different landuses, agricultural and suburban catchment areas. Three sampling stations namely Sungai Sebol (A1), Sungai Penggeli (A2) and Rantau Panjang (A3) were chosen to represent agricultural landuse, while Sungai Kemang (U1), Sungai Kampung Kelantan (U2) and Sungai Pemandi (U3) as the suburban. Parameters studied include water quality (DO, BOD, COD, TSS, pH and NH3), nutrients (NO3-, NO2-, TN, PO43- and TP), metals (As, Cd, Cr, Cu, Mn, Ni, Hg, Zn, Al, Fe and Pb) and organophosphorus pesticides (malathion and chlorpyrifos). Comparison were made based on concentration, pollutant loading, pollutograph and first flush to evaluate the effect of storm event on the water quality of the studied tributaries. The study shows in general, during baseflow, most parameters were within the Class II and Class III of NWQS. However, stormflow increases the concentration and loading of pollutants, in particular for BOD, COD, TSS, NO3-, TP, Hg and Fe. The use of pollutants load, rather than concentration, is a better representative for river quality because the amount of rainfall, water discharge and catchment size do affect the measured water quality of river during a storm. Parameters in agricultural catchment were dominantly classified in Type 1 while most of the parameters in suburban followed the Type 2 classification of pollutograph. Most of the pollutants in A1 and A2 catchments did not exhibit any first flush phenomenon while the pollutants in suburban catchments only show moderate first flush. Therefore, based on the positive and strong correlation between pollutants and amount of rainfall, it can be concluded that rainfall does influence the mobilization of pollutants into surface water during storm events. The study suggests that stormflow does have a great effect in increasing pollutant concentration and loading, which then may affect the existing beneficial uses of the river.