Contamination of rice field water with sulfonylurea and phenoxy herbicides in the Muda Irrigation Scheme, Kedah, Malaysia

The purpose of the present study was to investigate the potential risk of herbicide contamination (2,4-dichlorophenoxy (2,4-D), 2-methyl-4chlorophenoxyacetic acid (MCPA), metsulfuron, bensulfuron, and pyrazosulfuron) in the rice fields of the Muda Irrigation Scheme, Kedah, Malaysia. The study included two areas with different irrigation water sources namely non-recycled (N-RCL) and recycled (RCL) water. Periodic water sampling was carried out from the drainage canals during the planting period of the wet season 2006/2007 and dry season 2007. The HPLC-UV was used to detect herbicide residues in the water samples collected from the rice fields. The results showed that the concentration of sulfonylurea herbicides such as bensulfuron and metsulfuron in the rice field was 0.55 and 0.51 mu g/L, respectively. The potential risk of contamination depended on the actual dosage of each herbicide applied by farmers to their rice fields. The potential risk of water pollution by the five herbicides studied in the area with RCL water tended to be more widespread compared to the area with N-RCL water due to surface water runoff with higher levels of weedicides to the surrounding areas. During the two seasons, 50-73 % of the water samples collected from the area receiving RCL water contained the five herbicides studied at concentrations of more than 0.05 mu g/L, and this percentage was higher than that from the areas receiving N-RCL water (45-69 %). During the wet season, the overall total mean concentration of the eight herbicides found in the samples collected from the area with RCL water (6.27 mu g/L) was significantly higher (P< 0.01) than that from the area receiving N-RCL water (2.39 mu g/L). Meanwhile, during the dry season, there was no significant difference (P>0.05) in the herbicide concentrations between the areas receiving RCL (6.16 mu g/L) and N-RCL water (7.43 mu g/L) water.