An Assessment of Sediment and Nitrogen Input into the Upper Merbok Estuary, Kedah, Malaysia

Abstract

The increased loss of nutrients via agricultural runoff has serious potential ecological and public health implications. This paper discusses water quality and nutrient inputs into Merbok river in Northern Malaysia. Merbok river stretches 35 km long with depth ranging from 3 to 15 m. Merbok catchment areas of $$440~\mathrm{{km}}^{2}$$ are made up of alluvium deposits overlying an extensive span of ferruginous shale and mudstone with a few scattered outcrops of granite and ferruginous sandstone/quartzite. In 2006, the Merbok catchment consisted of 25.2 % urban areas, 48.6 % agricultural areas, 20.8 % forests, 0.6 % livestock farming areas, and 4.8 % was covered by other types of land uses. As early as 1990s, Merbok river has been classified as a very polluted river in terms of the ammonium nitrogen (NH4-N) and suspended sediment concentrations (SS). The results of our monthly survey of water quality and nutrient contents, which was carried out every month from October 2011 to September 2012 at 12 sampling stations, suggested and confirmed the previous findings. Such findings affirmed that nearly 3 decades ago, SS and NH4-N were high in the river catchment. The average SS was high (35 mg/l) (Class II of Landerarbeitsgemeinschaft Wasser (LAWA), (1998) water quality standard), at the lower end of Merbok river (M2), while the highest SS of 106 mg/l was recorded at Puntar river, which is upper tributary of Bongkok river. This is due to land clearing in the oil palm plantation and for rubber cultivation observed during sampling period. The maximum recorded NH4-N concentrations observed in the upper Merbok river ranged from 1.97 to 3.98 mg/l along Bongkok river. Moreover, relatively high concentrations were recorded from other tributaries such as the Lalang river (3.10 mg/l); Getah river (3.13 mg/l) and Tok Pawang river (3.53 mg/l), which were all above the limit of 2.4 mg/l or Class IV of the LAWA classification. The trend of data has referred to excessively contaminated rivers of heavy pollution burden. In addition, it was found that the maximum nitrate concentrations were increasing from 8.33 mg/l at the upper Bongkok river (B1) peaking at Bongkok B4 (9.24 mg/l) and decreasing to 6.01 mg/l near the tidal limit of Merbok river (M1) and 5.65 mg/l at M2 in the Merbok estuary, all of which were LAWA class III of heavily-polluted water. Land use and human activities in the upper catchment contributed to such significant pollution burden in the river.