Occurrence and distribution of neonicotinoids and fiproles within groundwater in Minnesota: Effects of lithology, land use and geography

Due to the widespread use of insecticides in agriculture and for urban pest control, there is the potential for contamination of groundwater systems. Five neonicotinoids, fipronil, and nine transformation products (desnitro-imidacloprid, imidacloprid olefin, imidacloprid urea, acetamiprid-n-desmethyl, thiacloprid amide, 6-chloronicotinic acid, fipronil desulfinyl, fipronil sulfide, and fipronil sulfone) were studied in samples from 15 springs and 75 unique wells from 13 counties over four years (2019-2022) in Minnesota. Up to 13 neonicotinoids and fiproles were identified in groundwater samples from springs and 10 from wells. Springs were particularly susceptible to contamination, with clothianidin found in 41 % of springs (maximum concentration: 200 ng/L) followed by thiamethoxam (31 %), imidacloprid (22 %), thiacloprid (19 %), and acetamiprid (12 %). Clothianidin was the most common analyte in well samples (13 %), followed by thiamethoxam (12 %), acetamiprid (14 %), imidacloprid (10 %), and thiacloprid (2 %). Hydrostratigraphy was an important factor in neonicotinoid detection with frequencies of detection highest in sandy or karst aquifers. Regional land use, especially the fraction of agricultural land, and imperviousness influenced observed concentrations within springsheds and well catchment areas. There were several significant correlations between neonicotinoid detections with geochemistry indicators including tritium/groundwater age, dissolved oxygen (DO), and total nitrate plus nitrite (total oxidized nitrogen).