Modelling the photochemical generation kinetics of 2-methyl-4-chlorophenol, an intermediate of the herbicide MCPA (2-methyl-4-chlorophenoxyacetic acid) in surface waters

2-Methyl-4-chlorophenol (MCP) is the main transformation intermediate of the herbicide MCPA in surface waters and it is more toxic than its parent compound. MCP is produced from MCPA by both direct photolysis and •OH reaction. The latter process has higher yield of MCP from MCPA: 0.5 vs. 0.3 for the direct photolysis. Our model results show that the formation rate of MCP would be higher in waters that contain low organic matter and high nitrate and nitrite. Such conditions are favourable to MCPA direct photolysis and •OH-induced transformation, which are both inhibited by organic matter, while •OH formation is enhanced by nitrate and nitrite. Good agreement is obtained between model predictions and field data of MCPA transformation in the Rhˆone river delta (Southern France). The field data also suggest that MCP undergoes slightly faster transformation than MCPA in that environment.