TRANSPORT AND PERSISTENCE OF PESTICIDES IN ALLUVIAL SOILS - I - SIMAZINE

Pesticide leaching is a concern in the alluvial valleys of western Was hington, where high-value crops are grown in soils with water tables < 3 m deep. We conducted this project to determine the longterm leaching pattern of simazine [2-chloro-4,6-bis-(ethylamino)-s-triazine] applie d to red raspberry (Rubus idaeus L.) and strawberry (Fragaria x ananas sa Duch.). Simazine was applied to strawberry and raspberry in paired plots at two sites. Strawberry received a split application (total 2.2 kg ha(-1)) in August and November, and raspberry received a single 4. 5 kg ha(-1) application in November. We applied simazine yearly from 1 986 to 1989, and sampled soil (to 180 cm) and shallow groundwater mont hly until April 1991, and at one site again in 1994. Most of the simaz ine remained in the surface 15 cm of the soil or degraded, but small a mounts moved downward. Preferential flow soon after application was no t the main cause of downward movement. Simazine was persistent, with d isappearance half lives of 128 and 175 d at the two sites the first ye ar. Some simazine remained in the soil 4 yr after the final applicatio n. Simazine was most mobile beneath raspberry at one site. This site h ad finer texture and slightly more organic matter, but a lower K-d for simazine than the other site. Small amounts of simazine reached shall ow groundwater beneath both crops. The observed simazine leaching peak was more asymmetric than the peak predicted by the PRZM-2 model, an i ndication of nonequilibrium adsorption-desorption effects on simazine movement.