Passive and Discrete Sampling of Neonicotinoid Pesticides in Saginaw, Michigan (United States) and Implications for the Protection of Aquatic Life

Neonicotinoid pesticides are widely applied in urban and agricultural settings despite their toxicity to aquatic organisms at low concentrations. Monitoring for six neonicotinoids (acetamiprid, clothianidin, dinotefuran, imidacloprid, thiacloprid, thiamethoxam) in the Saginaw River watershed of Michigan shows detections of imidacloprid exceeding final chronic values (FCVs) developed to protect aquatic life. The study design implemented both discrete and passive surface water sampling to capture the episodic nature of pesticide release. Fourteen sites were sampled monthly from August-October 2021 and April-July 2022. One or more neonicotinoids were detected in 86% of discrete and 100% of passive samples. Imidacloprid was detected at the highest maximum concentration (220 ng L ^-1 ), followed by clothianidin (98 ng L ^-1 ), and thiamethoxam (32 ng L ^-1 ). Development of aquatic life values for imidacloprid, clothianidin, and thiamethoxam, pursuant to Michigan statute and Rule 57 (Water Quality Standards), resulted in FCVs of 29 ng L ^-1 , 81 ng L ^-1 , and 280 ng L ^-1 , respectively. Seven out of 14 sample locations exceeded the FCV for imidacloprid. The most sensitive species included in derivation of neonicotinoid aquatic life values included mayflies ( Neocloeon triangulifer, Cloeon sp., and McCaffertium sp. ) and a midge ( Chironomus dilutus ). This study provides new insight on monitoring for neonicotinoid pesticides and weighs the costs and benefits of passive and discrete sampling methodologies.