Pesticide use by growers has been increasingly scrutinized for contributions to contamination of surface and ground water. Both surface and subsurface drip irrigation can reduce overland flow and thereby reduce surface transport of pesticides. Little is known, however, about leaching of pesticides when applied via drip systems. A 1-hectare subsurface drip irrigated (SDI) hop yard installed in 1992 at a Washington State University (WSU) experiment station in eastern WA was specifically designed for investigating the efficacy of chemigation of systemic aphicides and fertilizers. Irrigations were carefully controlled by real time input from buried soil water sensors that provided precise control and greatly limited any deep percolation losses below the root zone. Neutron scattering soil water measurements were taken weekly to verify soil water levels. For comparison to the experimental SDI hop yard, studies were also initiated in four commercial hop yards under either surface drip (DI) or furrow irrigation (FI) systems. The systemic aphicide imidacloprid (water solubility of 500 ppm) was applied a single time through the irrigation system of DI and SDI hop yards or directly to the furrows of FI yards. During the next three months after imidacloprid application, soil profiles near the drip emitters or within the treated furrows were periodically sampled 15 cm at a time to a depth of 150 cm. At the experimental drip yard, plant leaves were also collected. Soil samples were tested for parent imidacloprid residues, and leaf samples were analyzed for total imidacloprid residues (sum of parent and metabolites). Leaching of imidacloprid below the application zone was observed in the soil profiles of commercial yards. Although residues were found 90 cm deep in the profile, most of the pesticide remained in the top 60 cm. At the experimental drip yard, much of the imidacloprid residues were recovered near the emitters during 1997 (45-60 cm), but some leaching to the 60-75 cm depth was observed during 1998, Residues consistently found at the deepest layers of the profile in the experimental hop yard may represent translocation of imidacloprid from past heavy use of the product prior to 1997 and daily scheduled irrigation management. Leaf residues in 1998 rapidly increased after imidacloprid application and remained at several ppm through hat-vast. In general, movement of the comparatively water soluble imidacloprid was limited to the top 90 cm of the soil profile despite daily irrigation, but movement was better restricted by the use of soil moisture sensors for irrigation control than by daily scheduling.
