Temporal pattern in levels of the neonicotinoid insecticide, imidacloprid, in an urban stream.

Imidacloprid is a widely used insecticide with high runoff potential posing a significant threat to aquatic ecosystems. In order to determine the spatial and temporal concentrations of imidacloprid in Forester Creek, a tributary to the San Diego River, surface water samples were collected from two sites under wet-weather and dry-weather conditions. Solid phase extraction and liquid chromatography tandem mass spectrometry were utilized to quantify imidacloprid levels in all samples. Imidacloprid was detected with 100% frequency in surface water samples from Forester Creek with a median concentration of 16.9 ng/L (range: 3.8-96.8 ng/L). Over 60% of samples exceeded U.S. EPA's chronic exposure benchmark (10 ng/L). Temporal analysis displayed significantly higher levels in wet-weather than dry-weather (median 45.6 ng/L vs. 8.2 ng/L (p < 0.05)), demonstrating the influence of wet-weather runoff on stream quality. Imidacloprid generally followed a first flush pattern with the highest levels observed on the rising portion of the hydrograph as compared to the remainder of the storm, further indicating that the build-up and wash off from land surfaces during storms is a major source of imidacloprid into urban surface waters. To our knowledge, the present study is the first to document this first flush pattern for imidacloprid in an urban stream in southern California. Such data on the occurrence and levels of imidacloprid in this urban stream contribute to the limited knowledge on imidacloprid in urban environments and will promote a better understanding of sources and effects of the neonicotinoid pesticide within the southern California region.