Insecticide and fungicide concentrations in irrigation runoff and soils from a lettuce field in the Salinas Valley, California, 2019 and 2020

Irrigation runoff and soil samples were collected from a lettuce field located at the USDA-ARS Spence Research farm in the Salinas Valley, California to measure neonicotinoid insecticides (clothianidin and imidacloprid) and a fungicide (azoxystrobin) applied via coated seed and drench treatments. The field trial was designed to evaluate four treatments with replication: 1) control, untreated seed, 2) imidacloprid treated seed, 3) clothianidin treated seed, and 4) azoxystrobin treated seed with an imidacloprid drench. Samples were collected from each treatment over two lettuce growing seasons: August to October 2019 and September to October 2020. Runoff samples were collected over both growing seasons during 6 irrigation events each year. Surface soils were collected each year before planting and pesticide application and then again at the end of each trial. Soil cores (1 m) were collected in 2019 at the end of the that year's trial to assess potential subsurface transport. Water samples (n=131) were filtered (0.7 um) and extracted via solid phase extraction. Soil samples (n=106) were extracted using an accelerated solvent extraction system followed by carbon solid phase extraction. Both water and soil extracts were analyzed via liquid chromatography tandem mass spectrometry. In both years of the study, clothianidin and imidacloprid were detected in runoff and soil samples collected from each treatment. Clothianidin concentrations ranged from non-detect (ND) to 576 ng/L in runoff and ND to 29 ng/g in soil samples. Imidacloprid concentrations ranged from ND to 4877 ng/L in runoff and ND to 20 ng/g in soil samples. Imidacloprid variability between replicates was especially high in Year 2 in Treatment 4; concentrations measured in Treatment 4, Rep D were one to two orders of magnitudes higher than concentrations measured in Treatment 4 Reps A, B, and C. Azoxystrobin was also detected in runoff samples in both years and from all treatments, with concentrations ranging from ND to 237 ng/L. However, in contrast to the clothianidin and imidacloprid soil results, azoxystrobin was only detected in two soil samples during the entire two-year study (0.24 and 0.57 ng/g).