Legal Agricultural Use Determination for Imidacloprid Detections in California
Abstract
Imidacloprid is an insecticide used on a wide variety of agricultural and non-agricultural sites to control a variety of insect pests. Imidacloprid was initially registered in 1994, and as of September 10, 2021, 253 products containing imidacloprid were registered for agricultural and non-agricultural use in California. Imidacloprid’s physical and chemical properties indicate that it is persistent and mobile in the soil, and as a result, in 1999 it was placed on the California Department of Pesticide Regulation’s (DPR) Groundwater Protection List (GWPL) for future groundwater monitoring.
DPR conducted focused groundwater studies in 2003 and 2009 in different areas of California, targeting sampling locations based on pounds of imidacloprid applied for agricultural use, depth to groundwater, and well availability. Sixty-seven wells were sampled from seven counties for these studies. Imidacloprid, or its degradates, were not detected in any of the collected samples. In addition to the focused studies, DPR samples approximately 60 wells annually in areas known to be vulnerable to groundwater contamination by pesticides. Recently, DPR expanded its analysis of these wells to include more pesticides, including imidacloprid; subsequently some of these wells tested positive for imidacloprid. These detections prompted additional well sampling in high-use areas throughout California to reassess current imidacloprid concentrations in groundwater. Sixty-nine wells were sampled for imidacloprid in 2017 and 2019 as part of the latest imidacloprid study. In all, DPR has analyzed more than 600 samples from 307 wells for imidacloprid: some wells were sampled multiple times, other wells were sampled once but analyzed with two different methods, and some wells were only sampled once. Imidacloprid was detected above the reporting limit of 0.05 parts per billion (ppb) in 15 wells in Fresno, Santa Barbara, and Tulare counties, with concentrations ranging from 0.051 to 5.97 ppb. The highest concentration was detected in 2017 in a well that serves a vacant home and may not be representative of concentrations in active domestic wells. The concentration in this well dropped to 0.053 ppb when follow-up sampling was conducted in 2020. Imidacloprid degradates were also analyzed for in 144 well samples, and since there were no detections, subsequent samples were not analyzed for the degradates.
Pesticide use reporting data show that all 15 wells with imidacloprid detections were located in the same or contiguous sections where it was applied for agricultural purposes. Nine of these wells were also located within the same or contiguous section of another well with imidacloprid concentrations above the reporting limit. Given the historical use of imidacloprid in California and the locations of imidacloprid detections in groundwater relative to those use patterns, this report concludes these detections are the result of the legal agricultural use of imidacloprid.
DISCLAIMER: The mention of commercial products, their source, or use in connection with material reported herein is not to be constructed as an actual or implied endorsement of such product.