Measurement of pesticides leaching by using pan lysimeter method on a sandy upland field

Extent of pesticides leaching was measured by using the pan lysimeter method on a sandy upland field in Chiba Prefecture, Japan. A collecting pan (30 X 30 cm) in the lysimeter was installed into the soil profile horizontally at 50 cm below the soil surface. Total volume of leachate and its ratio to the total rainfall during the monitoring period (125 days) was 608.2 mm and 65%, respectively. Daily average volume of leachate was increased by two heavy rains. Test chemicals were dimethoate, simazine, metolachlor, diazinon, chlorotharonil, and Cl- as a tracer. Among five pesticides, only dimethoate and simazine were detected in the leachate. Maximum concentrations of dimethoate and simazine were 18.4 ppb and 1.0 ppb, respectively. The dimethoate concentration in leachate reached maximum when Cl- concentration in leachate began to rise. The simazine concentrations in leachate were lower with delayed detection periods and a broader elution profile when compared with those of dimethoate. During the monitoring period, the total mass of pesticide leaching and its ratio to the application rate for dimethoate were 2.99 mg/m(2) and 3.5%, respectively. The corresponding values for simazine were 0.33 mg/m(2) and 0.4%. respectively. Daily average amount of dimethoate leaching was significantly increased by the first heavy rain.