WATER-MOVEMENT AND ISOPROTURON BEHAVIOR IN A DRAINED HEAVY CLAY SOIL .2. PERSISTENCE AND TRANSPORT

In a study of isoproturon applied to winter wheat in a heavy clay soil, high concentrations of herbicide were detected in overland flow (surface runoff) water, mole drain water and field drain water. The amount of isoproturon detected in the field drain, over two major rainfall events in 1993, was estimated to be 2.7% of that potentially available at the soil surface. The peak drain water concentration of isoproturon in the first significant drain flow event was in excess of 500 ppb. For these two rainfall events, 1% of the compound originally applied was lost to the drainage system. Comparison of isoproturon with chloride and sulphate concentrations suggests different origins for the drain water, with the majority of isoproturon being carried down to the drainage system by preferential flow from the soil surface. A residue of 5-10% of the herbicide persisted in the top soil and did not appear to be degraded in the period of May to June.