Groundwater delivery of neonicotinoids to streams.

Neonicotinoids are mobile and persistent insecticides whose presence in streams poses a threat to stream invertebrates. This study sought to understand how stream baseflow concentrations of three common neonicotinoids relate to their delivery in groundwater from agricultural land. In a study area with large areas of irrigated potato (Solanum tuberosum L.) and vegetable crops, some grain and dairy agriculture, and small areas of suburban development, neonicotinoid concentrations in streams exceeded the aquatic life benchmarks for imidacloprid and clothianidin at the sites with higher percentages of potato and vegetable crops in their groundwater contributing area. The stream concentrations are similar to field-edge groundwater concentrations after adjusting for the density of agricultural land cover within a groundwater travel-time corresponding to their usage period and assuming the neonicotinoids persist or degrade slowly in the aquifer. If neonicotinoids do not degrade in the aquifer, the stream concentrations can be explained by a mean loss to groundwater from the land in potato and vegetable agriculture of 0.16 (range 0.002-0.3) g ha-1 year-1 for imidacloprid, 1.6 (range 0.02-4.1) g ha-1 year-1 for thiamethoxam, and 0.5 (range 0.05-1.0) g ha-1 year-1 for clothianidin. Given that only approximately one-third to two-thirds of the water entering the streams was young enough to contain these neonicotinoids, if neonicotinoid loss to groundwater stays the same and they persist in the aquifer, their concentrations will continue to increase as those streams come to steady-state with applied neonicotinoids.