Dissipation of 4‐chloro‐2‐methylphenoxyacetic acid (MCPA) in a rice field

The dissipation of the herbicide 4‐chloro‐2‐methylphenoxyacetic acid (MCPA) in a California rice field was investigated. Samples of water, rice plants and mud were collected at intervals for 20 days and analysed for MCPA by established methods for plants and mud, and by a novel method utilising XAD‐2 macro‐reticular resin for water. Water residues declined to less than 0.01 parts/106 of MCPA within 14 days after application; plant residues declined from 20 parts/106 to 1 part/106 within 20 days; mud residues remained constant at about 0.1 parts/106. Photolysis of dilute aqueous MCPA solutions with either sunlight or an indoor photoreactor yielded 4‐chloro‐2‐methylphenol as the major product; o‐cresol and 4‐chloro‐2‐formylphenol also were identified. While photosensitisation was observed with water taken from the rice field, microbial degradation proceeded at an even faster rate. All environmental compartments except air contained measurable amounts (>0.01 parts/106) at some time, but water contained the bulk of the applied MCPA whose eventual disappearance was shown to be due to biological and chemical degradation and not dilution.