Seberang Perai rice Scheme irrigation Water Quality aSSeSSment

Rivers are the main sources of water for agricultural purposes in Malaysia. The Seberang Perai Rice Irrigation Scheme in the state of Penang is one of the eight main rice growing areas in Malaysia. The scheme consists of five (5) sub-schemes and irrigation water for each sub-scheme are supplied from either the Muda river, Jarak river, Kulim river or the Kerian river with pumping or gravity intakes. Agricultural fertilisation entails significant economic and environmental costs. Nitrogen (N) is the most commonly applied nutrient in crop production. However, excess N inputs, pesticides, and herbicides for rice crop not only adds costs to crop production but can be environmentally damaging to the plant and the environment. This study was carried out to improve irrigation water quality in order to ensure that environmental concerns are built into the operation and monitoring activities of the rice irrigation scheme. Water quality data were used to determine the water quality status whether in clean waters (80-100), slightly polluted waters (60-<80) or very polluted waters (<60) category and to classify the rivers in Class I, II, III, IV or V based on existing Water Quality Index (WQI). From 18 years-data for the period from 1990 – 2007, Water Quality Index (WQI) was computed based on 6 parameters: Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), Ammoniacal Nitrogen (NH 3 N), pH, Dissolved Oxygen (DO) and Suspended Solids (SS) as a means for water quality assessment. Increasing DO concentration and concurrent low concentrations of BOD and COD is the trend in the scheme. WQI calculated in this study ranged from 69.9% to 82.2%. Two rivers (Kerian and Jarak River) were considered slightly polluted in comparison with the general rating scale of the water quality index. Agricultural activities may be contributing to this trend, but water quality nonetheless is also affected seriously by through industrial and domestic use. The data also showed that the Muda and Kulim River is relatively clean. However, there is not much difference in the water quality index of the four rivers, between upstream and downstream sections. Based on the results of this study and it must be qualified here that the results are only referring to the period of the data sourced, the water quality of the rivers can be classified into Class III and IV by on overall river classification based on the DOE-WQI. The results of this study can be a basis for future studies on irrigation water quality in the other projects.