Neonicotinoids in Groundwater: Persistent Contaminants and Unresolved Risks

Abstract

Many people remember the ban on DDT in the 1970s, but what happened to insecticides after that? The agriculture industry quickly shifted to alternatives, with organophosphates becoming the dominant replacement. By the early 1990s, neonicotinoids emerged as a new class of insecticides, praised for their lower toxicity to mammals, effectiveness at low doses, and systemic action, which allows plants to absorb them for long-term pest protection. In theory, these qualities made neonicotinoids a safer and more efficient alternative. However, nearly 40 years after their introduction, they have emerged as new contaminants in groundwater, raising concerns about their environmental and human health impacts, which remain poorly understood.

The rapid increase in neonicotinoid use since the early 2000s has made them the most widely used insecticides in the United States today. These chemicals are applied to major crops such as corn, soybeans, and specialty fruits, as well as in residential pest control products and flea treatments for pets. While their use extends beyond agriculture, the majority is tied to agricultural applications, where they are primarily applied as seed treatments, but also through in-furrow, soil applications, and foliar sprays. Seed treatments gained favor for their ability to provide targeted, systemic pest protection from germination and minimize pesticide drift into non-target areas. However, this widespread adoption has led to unintended consequences, particularly their persistence in soil and water.

Neonicotinoids are highly mobile in water and can persist in the environment, with degradation times ranging from days to years depending on the compound and environmental conditions (Pietrzak et al. 2020). Imidacloprid, thiamethoxam, and clothianidin are among the most widely used neonicotinoids, and these compounds have been detected in groundwater across the US. Groundwater quality data from the EPA Water Quality Portal, collected from 1999 to 2024, reveals that at least one of these compounds was detected in wells across 30 of the 50 states (EPA Water Quality Portal 2024). In Wisconsin, detections have been particularly prevalent in areas with sandy soils and shallow groundwater table, such as the Central Sands Region (Senger et al. 2019; Romano et al. 2023). Recent monitoring efforts suggest that these chemicals are now present in groundwater throughout much of the state (Romano et al. 2024).

The detection of neonicotinoids in groundwater and elsewhere in the environment has raised concerns about their ecological and human health impacts. Since the late 2000s, research has documented lethal and/or sublethal effects on a range of organisms, including bees and butterflies, as well as aquatic vertebrates and invertebrates (Schneider et al. 2012; Morrissey et al. 2015; Rundlöf et al. 2015; Eng et al. 2019; Yang et al. 2023). Toxicology and biomonitoring studies have highlighted potential human toxicity, with concerns about impacts on thyroid, neurological, reproductive, and glucose health, along with evidence of exposure among the US population (Han et al. 2018; Buckley et al. 2022). However, the full extent of their health impacts remains unclear, highlighting the need for large-scale epidemiological studies.

Despite these concerns, there are currently no federal regulations addressing neonicotinoid contamination in drinking water. The EPA has not established maximum contaminant levels (MCLs), leaving individual states to set groundwater standards. However, federal regulatory action may be on the horizon. In 2022, the EPA found that clothianidin, imidacloprid, and thiamethoxam threaten endangered species and indicated potential restrictions under the Endangered Species Act. Meanwhile, states like New York, New Jersey, Nevada, Maine, and California have enacted measures such as banning neonicotinoids for lawn use or tightening controls on treated seeds.

These efforts reflect growing recognition of the need to address the widespread impacts of neonicotinoids, particularly their persistence in groundwater. Yet, critical questions remain. If neonicotinoids are phased out, what alternatives will replace them? Can non-chemical pest management strategies provide a viable solution? Or are we simply waiting for the next generation of insecticides to enter the market, and perhaps repeat this cycle decades later? How do we break the pattern of approving chemicals that, over time, prove harmful to our water resources, ecosystems, and human health?