Pesticides Lurk in Every Sample from Two Western Mexican Rivers

Every single water sample collected from two river basins in western Mexico over two years contained pesticides, according to a new study that offers one of the most detailed portraits yet of agricultural chemical pollution in the region’s surface waters. The research, published in Environmental Monitoring and Assessment, analyzed 80 water samples from 10 sites across the Marabasco basin and the Armería sub-basin during 2023 and 2024, covering both dry and wet seasons. Of the 49 pesticides screened using gas chromatography coupled to mass spectrometry and high-performance liquid chromatography tandem mass spectrometry, 20 were detected. The ubiquity of these compounds, present at every sampling site throughout the entire study period, suggests that pesticide contamination is not an occasional event tied to specific applications but a persistent feature of these watersheds.

The most frequently detected compounds tell a story about how land use shapes water chemistry. The herbicides ametryn, atrazine, and 2,4-D appeared alongside the insecticides DEET and diazinon at the highest detection frequencies. DEET, an insect repellent more commonly associated with personal use than agriculture, signals the contribution of domestic activities to water pollution, while the herbicides reflect intensive cultivation in the surrounding landscape. Concentrations, though generally low in absolute terms, reached notable peaks: picloram at 8.22 nanograms per milliliter, pyraclostrobin at 4.25, fipronil at 2.41, and diazinon at 1.64 nanograms per milliliter. These are trace-level concentrations, but the study’s risk calculations show that trace amounts of certain insecticides can still pose significant threats to aquatic life.

Statistical analysis revealed a clear seasonal pattern. The wet season increased both the frequency of detection and the concentrations of several pesticides, a finding consistent with the idea that rainfall mobilizes chemicals from fields into waterways through runoff. When rains hit recently treated crops, water carries dissolved and suspended pesticides into streams and rivers, effectively flushing the agricultural landscape into the aquatic environment. Yet the dry season offered no reprieve: pesticides were detected at all sites throughout the study, indicating that these compounds persist in the environment or continue to enter waterways even when rainfall-driven transport is minimal. Irrigated and rainfed agricultural areas exhibited the highest pesticide diversity, confirming that farming intensity correlates with the chemical complexity of nearby waters.

To translate detection into ecological meaning, the researchers calculated risk quotients, comparing measured concentrations against toxicity thresholds for algae, aquatic invertebrates, and fish. The results were sobering. Insecticides such as fipronil, diazinon, and chlorpyrifos, along with the fungicide carbendazim, posed significant risks to these organisms in both average and worst-case scenarios. Risk quotients above one indicate that measured concentrations exceed levels expected to cause harm, and several compounds crossed that line for multiple trophic levels. Fipronil, a phenylpyrazole insecticide that disrupts insect nervous systems by blocking GABA-gated chloride channels, is particularly notorious for its toxicity to aquatic invertebrates at extremely low concentrations. Diazinon, an organophosphate that inhibits acetylcholinesterase, similarly threatens non-target aquatic species. Chlorpyrifos, another acetylcholinesterase inhibitor, has been widely restricted in many countries but remains in use in Mexican agriculture.

The prioritization analysis, which ranked pesticides by both detection frequency and magnitude of ecological risk, identified eight priority compounds requiring attention. Ametryn and diazinon emerged as the top candidates for immediate management action. This kind of ranking matters because regulators and watershed managers cannot address 49 different chemicals simultaneously; knowing which ones drive the greatest risk allows targeted interventions. The eight priority pesticides represent a manageable list that could inform monitoring programs, restrictions on use near waterways, or promotion of alternative pest control strategies. The study’s authors recommend strengthening integrated pesticide management measures and environmental regulations to reduce the risks they identified and preserve water quality in the region.

What makes these findings particularly striking is the geographic and temporal scope. The Marabasco and Armería basins drain a landscape where intensive agriculture, including sugarcane, lime orchards, and other crops, coexists with growing urban centers. The region’s tropical climate, with its pronounced wet and dry seasons, creates conditions where pesticide transport dynamics differ from temperate agricultural zones. Heavy seasonal rains can deliver pulses of chemicals to waterways, while the dry season concentrates pollutants in reduced flows. The study’s two-year monitoring window captured this variability, but the persistence of contamination across all sites and seasons suggests that the problem is structural rather than episodic, rooted in how agriculture and domestic pest control are practiced throughout the watershed.

The ecological implications extend beyond individual species. Algae form the base of aquatic food webs, and herbicide-driven disruptions to primary production can cascade upward. Aquatic invertebrates, which include insects, crustaceans, and mollusks, occupy a middle trophic position and serve as food for fish, birds, and other predators. Invertebrate communities are also sensitive bioindicators, with their composition reflecting water quality and habitat condition. Fish, as top predators in these systems, integrate exposures across their food web and can accumulate persistent compounds in their tissues. When insecticides threaten all three groups simultaneously, the structural integrity of the entire aquatic ecosystem is at stake, potentially reducing biodiversity, altering community composition, and impairing ecosystem functions such as nutrient cycling and organic matter decomposition.

The study also highlights the role of domestic pesticide use, particularly DEET, in water pollution. While agricultural runoff is often assumed to be the primary source of surface water contamination, the presence of DEET at high detection frequencies points to household insect repellents, vector control programs, and urban wastewater as additional pathways. In a region where dengue and other vector-borne diseases drive widespread repellent use, the environmental footprint of personal protection measures deserves attention. This finding complicates the conventional narrative that frames pesticide pollution as purely an agricultural problem and suggests that effective management will require addressing both rural and urban sources.

For Mexico, where pesticide regulation has been described by the OECD as facing governance challenges, studies like this one provide the empirical foundation for policy reform. The country has recently moved to phase out glyphosate and restrict genetically modified maize, signaling a shift in agricultural chemical policy. The identification of eight priority pesticides in western Mexican waters offers regulators a concrete starting point for monitoring, restriction, or substitution strategies. The authors’ call for integrated pesticide management aligns with global efforts to reduce reliance on synthetic chemicals through biological control, agroecological practices, and precision application technologies that minimize off-target movement. Whether these recommendations translate into meaningful changes in how pesticides are used across the Marabasco and Armería basins will depend on regulatory will, economic incentives, and the engagement of the farming communities whose practices ultimately determine what flows into the region’s rivers.

Subject of Research: Assessing pesticide pollution in surface water of two basins in western Mexico: spatiotemporal variability, potential ecological risk assessment, and pesticides prioritization

Article Title: Assessing pesticide pollution in surface water of two basins in western Mexico: spatiotemporal variability, potential ecological risk assessment, and pesticides prioritization

Article References: García, A. R., Rodríguez-Aguilar, B. A., Ceballos-Magaña, S. G., Peregrina-Lucano, A. A., Calderón-Araya, R., Palma-Rubilar, P., Muñiz-Valencia, R., & Helguera-Gómez, E. O. (2026). Assessing pesticide pollution in surface water of two basins in western Mexico: spatiotemporal variability, potential ecological risk assessment, and pesticides prioritization. Environmental Monitoring and Assessment, 198(9), Article 994. https://doi.org/10.1007/s10661-026-15849-2

Image Credits: AI Generated

DOI: 10.1007/s10661-026-15849-2

Keywords: Assessing, pesticide, pollution, surface, water, basins, western, Mexico, spatiotemporal, variability, potential, ecological