Wash off of imidacloprid and fipronil from turf and concrete surfaces using simulated rainfall

The surface runoff of imidacloprid granular product (GR) from turf surfaces, and imidacloprid emulsifiable concentrate (EC), fipronil suspension concentrate (SC) products and fipronil byproducts from concrete surfaces was investigated during 1h rainfall simulations at 50 mm/h or 25 mm/h with product incubation times of 1.5 h, 1 d, 7 d, and 14 d. About 57.3% of the applied mass of imidacloprid, corresponding to an event mean concentration of 392.0 μg/L, was washed off from the concrete surfaces after 1.5h of incubation. After 1 d, 7 d, and 14 d of incubation on either turf or concrete surfaces, up to 5.9% of the applied mass of pesticide was removed in each of the run-off events. The maximum concentrations of pesticides were observed in the initial fraction of the runoff collected in the first rainfall event. They were 157.8, 3267.8 and 143.3 μg/L for imidacloprid GR, imidacloprid EC and fipronil SC, respectively. Imidacloprid was not persistent on concrete surfaces, with run-off concentrations below detection limits in 7d incubation experiments. The cumulative mass losses of imidacloprid from turf and fipronil from concrete had a linear relation with cumulative surface run-off depth, while cumulative mass losses of imidacloprid from concrete surfaces were better fit by a power function of the cumulative surface run-off depth. The concentrations of fipronil in the runoff from the third rainfall event at 14 d incubation time were still relatively high and ranged from 12.0 to 31.0 μg/L. A toxicity unit approach was also employed to evaluate the potential acute toxicity of fipronil and its byproducts to aquatic organisms.