LONG-TERM EFFECT OF TILLAGE AND RAINFALL ON HERBICIDE LEACHING TO SHALLOW GROUNDWATER

The interaction of conventional tillage (CT) and no-tillage (NT) crop production practices with rainfall on the movement of three herbicides into shallow groundwater was evaluated over 4 yr. Groundwater was sam pled from unconfined (<1.5m deep) and confined (<3 m and 4.5 to 11 m d eep) monitoring wells in 1989-1992 and analyzed for atrazine, alachlor , and cyanazine. Pesticide concentrations were cyclical: residues were highest soon after application, declined during the growing season, t hen increased during winter recharge. Alachlor and cyanazine were at n ondetectable levels within 3 mo after application. Atrazine residues, present in confined groundwater all year, ranged in concentration betw een 0.03 to 1.9 and 0.16 to 3.7 ug L(-1) for the CT and NT plots, resp ectively. Herbicide residues were higher in unconfined (<1.5 m deep) t han confined (<3 m deep) groundwater. Atrazine was sporadically detect ed in groundwater to 4.6 m, but not deeper. Lateral transport in confi ned groundwater to untreated areas was evident. The rapid movement of herbicides to groundwater with the first major rain after application suggest that preferential transport may be common. Results of this stu dy also indicate that timing, amount and intensity of rainfall relativ e to pesticide application may be the primary factors governing pestic ide leaching.