Impact of repeated pesticide applications on the binding and release of 14C-methamidophos to soil matrices under field conditions

The dissipation of 14C-methamidophos was monitored in the absence or presence of other pesticides using in situ soil columns in cotton fields. Samples were taken randomly in duplicate at 0, 6, 9, 12, 18, 24 and 30 months. The soils were analyzed for total, extractable and bound residues. The dissipation of 14C-methamidophos was rapid in the field environment; 3 hours after application, 12% was of the radioactivity lost due to volatilization and 88% was found in the 0-15 cm soil layer. With the passage of time bound residues in treated soil were less (11.52%) compared to those in untreated soil (13.47%). In general bound residues gradually increased with time and binding was higher in untreated soil at every sampling stage. The estimated time required for loss of 50% of radiocarbon was 73 days. In untreated samples the parent compound and three unknown spots were seen on TLC plates whereas four unknown spots with the parent chemical were found in the treated samples. (author)