Transport and Persistence of Pesticides in Alluvial Soils: I. Simazine

Pesticide leaching is a concern in the alluvial valleys of western Washington, where high‐value crops are grown in soils with water tables s‐triazine] applied to red raspberry (Rubus idaeus L.) and strawberry (Fragaria × ananassa 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 1986 to 1989, and sampled soil (to 180 cm) and shallow groundwater monthly until April 1991, and at one site again in 1994. Most of the simazine remained in the surface 15 cm of the soil or degraded, but small amounts moved downward. Preferential flow soon after application was not the main cause of downward movement. Simazine was persistent, with disappearance half lives of 128 and 175 d at the two sites the first year. Some simazine remained in the soil 4 yr after the final application. Simazine was most mobile beneath raspberry at one site. This site had finer texture and slightly more organic matter, but a lower Kd for simazine than the other site. Small amounts of simazine reached shallow groundwater beneath both crops. The observed simazine leaching peak was more asymmetric than the peak predicted by the PRZM‐2 model, an indication of nonequilibrium adsorption‐desorption effects on simazine movement.