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Jul
Pesticide Contamination of Nonagricultural Streams Underscores Further Threats to Biodiversity
(Beyond Pesticides, July 1, 2025) Published in Water Research, a study highlights the various routes for pesticide contamination, with the results identifying the presence of over 80 substances in streams without adjacent agricultural land use. “Our findings underscore the necessity of further investigating the non-agricultural entry pathways of pesticides and biocides to effectively mitigate their impacts on streams in non-agricultural catchments,†the authors state. They continue, “These streams often serve as critical refuge habitats and sources of recolonization, making their protection essential for biodiversity conservation.â€
In analyzing nonagricultural streams, the researchers find pesticide contamination that, while lower than levels found in streams directly next to agricultural land, can occur through various routes and threatens biodiversity in essential ecosystems. As the authors describe: “Although pesticide concentrations were lower than in agricultural streams, the potential toxicity of pesticides was associated with a significant reduction in sensitive insect populations, as indicated by the SPEARpesticides index. Notably, 40% of the studied streams did not achieve a good status according to the pesticide specific SPEARpesticides indicator.â€
The SPEARpesticides indicator is used “to identify pesticide effects on the aquatic invertebrate community. It measures the abundance of pesticide-sensitive species (“species at riskâ€) in relation to the abundance of all taxa,†the study explains. The calculated value then correlates with “five quality classes (high, good, moderate, poor and bad) following the ecological status classes laid out under the WFD [European Union (EU) Water Framework Directive].â€
The study focuses on examining “13 streams located predominantly in protected areas with no agricultural land use in their catchments,†which leads to the hypothesis that, “Given the absence of direct pesticide input via runoff, any detected pesticide contamination is likely attributable to atmospheric deposition.†These areas were chosen due to the role of protected areas in contributing to biodiversity conservation.
Researchers continue: “Streams within protected areas may enhance freshwater biodiversity by providing suitable habitats and serving as sources for recolonizing species. Undisturbed stream stretches play a pivotal role in the recovery of pesticide-impacted downstream areas.†(See studies here and here.)
Previous research finds that pesticide contamination of natural areas is ubiquitous, even in areas considered remote and pristine, such as the Alpine glaciers, U.S. national parks, high-altitude tropical cloud forests, and the Maya Mountain Protected Areas. (See studies here, here, here, and here.) “Remarkably, not only legacy persistent organic pollutants (POPs) were found, such as organochlorine pesticides, which are known to be prone to atmospheric transport, but also current-use pesticides like organophosphates, chlorothalonil, terbuthylazine, pendimethalin, and glyphosate,†the authors state.
They continue: “In recent years, several studies monitoring pesticides in ambient air have shown the presence of a wide range of pesticides in ambient air, in total >100 different substances with detection frequencies reaching up to 100% of samples. Sampling sites included remote sites, such as national parks, forests, and even polar sites, indicating medium- and long-range transboundary transport.â€
A recent study in Nature Reviews Earth & Environment, covered in the Daily News here, highlights how the pesticides used in global crop production pose risks to ecosystem and human health through multiple pathways. As the authors note, “Once applied to crops, pesticides are transported through surface and groundwater flows, atmospheric dispersion and wildlife migration.†Residues in food products, such as fruits and vegetables, as well as bioaccumulation in animals that are consumed as food, also threaten consumers around the world, particularly those relying on international trade.
The researchers reference an additional study, which identifies a potential transboundary transport of pesticides involving the Tijuana River watershed, as two pesticides (methidathion and mevinphos), both banned in the U.S. but not in Mexico, are detected in rivers within U.S. boundaries. “The presence of pesticides in the atmosphere and their long-distance travel can lead to exposure of local and adjacent biodiversity and communities, as well as damage to non-target crops,†the authors state. (See more on pesticide drift and water contamination here and here.)
In the current Water Research study, the researchers aim to “(i) characterize the toxicity profiles of these streams, (ii) identify whether the source of detected pesticides could be linked to atmospheric transport from surrounding agricultural activities, (iii) investigate the role of substance properties in influencing pesticide concentrations through their propensity for atmospheric transport, and (iv) assess the ecological relevance of pesticide levels by analyzing their impact on the macro-invertebrate community composition in the streams.â€
The 13 streams included, located in Germany and in the national pesticide monitoring program of small lowland streams (known as “Kleingewässer-Monitoringâ€) as well as the FLOW project, were selected because they do not have any expected pesticide input by runoff or leaching. “Of the 13 streams, eight sites were monitored in only one of the three years, four in two years, and one in all three years,†the researchers note. They continue: “The sites span five federal states in Germany. Except for two, all monitoring sites are located within protected areas, including biosphere reserves, landscape conservation areas, nature parks, and NATURA 2000 sites.â€
In discussing the methodology, the authors say, “Water sampling was conducted from April to July/August in 2018, 2019, and 2021 during the main pesticide application period, while benthic macroinvertebrates were sampled after the main pesticide application period in June.†Following sampling, analyses of pesticide toxicity to invertebrates is assessed and recorded using toxic units (TU), which are “calculated from the individual measured pesticide concentrations divided by the respective LC50 [concentration that is lethal to 50% of a group] in acute standard laboratory test systems.â€
The results show that potential pesticide toxicity, represented as maximum toxic units (TUmax), is “associated with a significant reduction in sensitive insect populations, as indicated by the SPEARpesticides index,†the researchers report. They continue: “The correlation of SPEARpesticides with TUmax shows a significant decrease in SPEARpesticides with increasing potential toxicity. This indicates that macroinvertebrate communities may be affected by pesticide toxicity in the investigated streams.†(See recent Daily News entitled Study Adds to Wide Body of Science Highlighting Benefits of Organic for Insect Biodiversity that highlights research from Germany on the insect apocalypse.)
In further explaining the results, the authors note that these values are: “driven by insecticides and biocides not approved for agricultural use or phased out during the study period, such as fipronil and neonicotinoid insecticides. They likely originated from veterinary products, residues from past agricultural use, illegal use, or impurities in approved pesticides.†Both fipronil and neonicotinoids are widely used in land management in the U.S., with a wide body of science linking these chemicals to health and environmental impacts.
Overall, 118 substances were analyzed, 81 of which are detected at least once in the streams. 69 samples reveal concentrations above their respective limit of quantification (LOQ) and the regulatory acceptable concentration (RAC) is “exceeded 14 times in 10 samples in 9 different streams by fipronil, imidacloprid, clothianidin, and cypermethrin.†As a note, all threshold exceedances are caused by insecticides. The researchers state that, “[O]bserved threshold exceedances can be mainly attributed to the structural problem of the current regulatory approach where authorizations are granted for several years without reevaluation in the case of new scientific evidence.†(See more on regulatory deficiencies here.)
The above insecticides “exhibited by far the highest potential toxicity towards invertebrates, followed by fungicides,†the authors say. Many of these chemicals, however, are not approved for use in the EU. As an explanation, the researchers relay: “Imidacloprid, clothianidin, and fipronil have high persistence in soil (half-lives ranging from 142 to 545 d), and they have been detected in soil several years after the last application, therefore an input into streams from past use can be expected. Other possible explanations include illegal application practices and impurities in approved pesticides. However, input from uses other than agriculture is also possible because pyrethroids, imidacloprid and fipronil were all approved as biocides, and imidacloprid and fipronil were also approved as veterinary products during the study period.†Fipronil residues are also linked to rinsate from bathing dogs, which can lead to contamination in nearby waterways. (See previous coverage on pesticides and pets here and here.)
As the authors summarize, this study “indicates that pathways other than atmospheric transport from agricultural application may be more relevant for pesticide exposure in non-agricultural streams.†Additional studies support this, such as a French study that finds acute toxicity in crustaceans from pesticides in mountain lakes most likely originated from livestock treated with veterinary drugs.
The study results lead the researchers to summarize that their work “highlights the need to scrutinize pesticide entry pathways other than those related to crop spraying to mitigate pesticide risks in streams in non-agricultural catchments.†The indirect contamination that is not addressed through regulatory processes threatens crucial habitats and overall biodiversity, further emphasizing the need to reevaluate the role of pesticides in agriculture.
Prior research supports the current study findings and documents the role of pesticide transport through the atmosphere. One study finds that “pesticide deposition in snow at U.S. National Parks correlated best with cropland area within 75 to 300 km, depending on pesticide half-life.†This showcases how there is no universal radius of influence that can be defined, as different pesticides and their properties can determine the degree of their impacts to nontarget areas.
Additional studies provide “evidence that pesticides contribute significantly to the ongoing biodiversity crisis,†the researchers say. “For example, a German national monitoring study on pesticide exposure and effects in agricultural streams recently identified pesticides as the dominant stressor for vulnerable insects. Comparable results were found in Australia, Europe, North America, and South America.†In California, research finds that “the decline of several amphibian species in the Sierra Mountains within the last decades of the 20th century was strongly associated with upwind pesticide use in the intensely agricultural Central Valley, with cholinesterase-inhibiting pesticides showing the strongest association.†This class of pesticides can include organophosphates and carbamates.
The direct and indirect effects of pesticides, from both agricultural and nonagricultural methods, threaten all life. These findings support the need for alternative practices that protect biodiversity, public health, and the environment and mitigate the harmful effects of chemical-intensive land management. Organic agriculture offers a holistic solution that removes these threats, as well as goes a step further to not only protect but enhance biodiversity, while also providing increased nutritional quality and other benefits. (See more on health and environmental benefits here and here.)
Lend your voice to the organic movement by taking action. >> Tell your Congressional Representative and Senators to cosponsor bills supporting organic agriculture. Additionally, become a Parks Advocate to encourage your community to transition to organic. The Parks for a Sustainable Future program provides in-depth training to assist community land managers in transitioning two public green spaces to organic landscape management, while aiming to provide the knowledge and skills necessary to eventually transition all public areas in a locality to these safer practices. For more information, please email our team at [email protected].
All unattributed positions and opinions in this piece are those of Beyond Pesticides.
Source:
Schweiger, L. et al. (2025) Pesticide contamination is associated with invertebrate community change in non-agricultural streams, Water Research. Available at: https://www.sciencedirect.com/science/article/pii/S0043135425008115.
