Organophosphate pesticides are among the most heavily used chemical pest controls on Earth, accounting for roughly 40 percent of global pesticide consumption. They were widely adopted as replacements for the notoriously persistent organochlorine pesticides of the mid-twentieth century because they break down more quickly in the environment. Yet that relative impermanence has not kept them out of aquatic ecosystems. Spray drift, agricultural runoff, groundwater leaching, careless disposal of empty containers, and the washing of application equipment all provide routes by which these compounds reach lakes and rivers, where they can persist long enough to be taken up by organisms. In Ethiopia, chlorpyrifos-methyl is commonly applied in crop production, while diazinon finds its way into the environment partly through flea collars used on sheep, dogs, and cats in rural areas.

The research team, led by Tarekegn Fentie Yimer of the University of Gondar, targeted Lake Tana and Lake Hayqe because neither had ever been assessed for the ecological and human health risks of these compounds, despite intensive pesticide use by smallholder farmers and horticulture companies in their catchments. Lake Tana covers 3,150 square kilometers at an altitude of 1,786 meters, is recognized by UNESCO as a Biosphere Reserve, and supports endemic fish, bird, and plant species alongside a fishery with an estimated potential of 10,000 tons per year. Lake Hayqe, by contrast, is a closed-basin lake with a watershed of only about 77 square kilometers but an average depth of 37 meters and a maximum depth of 81 meters. Both lakes supply Nile tilapia, a staple food for surrounding communities, making them ideal natural laboratories for tracing pesticides from farm fields to dinner plates.

Between February and May 2023, the researchers collected water from 20 sites on Lake Tana and 5 sites on Lake Hayqe, along with 30 Nile tilapia from three Lake Tana fishing districts and 10 fish, five tilapia and five common carp, from Lake Hayqe. Fish muscle tissue was freeze-dried, homogenized, and extracted using the QuEChERS method, a technique whose acronym promises a process that is quick, easy, cheap, effective, rugged, and safe. All samples were then analyzed by gas chromatography coupled with triple quadrupole mass spectrometry, one of the most sensitive analytical approaches available for trace organic contaminants. The method performed well: recoveries ranged from 96.7 to 106.0 percent, well within the accepted 80 to 120 percent window, and detection limits were low enough to quantify every residue found.

Of the eighteen organophosphate pesticides screened, only a handful appeared. In water, chlorpyrifos-methyl, ethion, and malathion were detected at every site sampled, with chlorpyrifos-methyl ranging from 1.41 to 1.91 micrograms per liter. That figure is dramatic when set against the United Kingdom Environment Agency’s non-statutory maximum acceptable concentration of just 0.01 micrograms per liter for the protection of freshwater life, meaning the lakes carry concentrations more than a hundred times that benchmark. Malathion, by contrast, remained below the World Health Organization’s drinking-water guideline of 2 micrograms per liter. In fish muscle, four compounds were detected: chlorpyrifos-methyl, diazinon, ethion, and malathion, at concentrations generally between about 11 and 25 micrograms per kilogram. Notably, malathion levels in fish exceeded the WHO maximum residue limit of 2 micrograms per kilogram, while diazinon stayed comfortably within its 200 to 500 microgram limit.

The spatial patterns told a story about sources. Statistical analysis revealed significant differences among sampling sites for every detected compound, with the highest chlorpyrifos-methyl and malathion values clustering near the mouths of tributary rivers such as the Gumara, Rib, and Megech, which drain intensive agricultural areas like Fogera and Gorgora, and near Bahir Dar, the region’s largest city. Principal component analysis, a multivariate technique that condenses many correlated measurements into a few underlying axes of variation, confirmed that ethion and chlorpyrifos-methyl dominated the first component explaining nearly 60 percent of the variance in water data, while malathion distinguished the Megech site. Tilapia consistently accumulated more pesticide than common carp in Lake Hayqe, suggesting species-specific differences in uptake and metabolism.

The ecological risk assessment produced the study’s most alarming numbers. The team calculated risk quotients by dividing measured environmental concentrations by predicted no-effect concentrations derived from acute toxicity data for three representative organisms: fish, Daphnia, and algae, chosen to span primary producers, primary consumers, and higher trophic levels. For Daphnia, the risk quotients for chlorpyrifos-methyl reached 275 in Lake Tana and 268 in Lake Hayqe, values far above the threshold of 1 that signals high risk. Ethion also posed high risk to Daphnia, with quotients between roughly 19 and 22, while malathion registered a medium risk. Fish and algae fared considerably better, with low to medium risk quotients across the board. The pattern identifies Daphnia as by far the most sensitive receptor, a finding consistent with studies in Ethiopia’s Lake Hawassa and in surface waters of northwestern Bangladesh and Greece.

The human health picture was far more reassuring. Using standard exposure assumptions, a fish consumption rate of 30 grams per day and an adult body weight of 60 kilograms, the researchers estimated daily intakes and hazard quotients for each pesticide. Every hazard quotient came in well below 1, and the combined hazard index was approximately 0.06 for both lakes, indicating a negligible non-carcinogenic risk from eating fish under the conditions assessed. Diazinon contributed the largest share of the hazard index, yet even at maximum detected concentrations the total remained an order of magnitude below the level of concern. The authors caution, however, that these estimates rest on generalized consumption and body weight assumptions that may not reflect every local community, and that the assessment addressed only non-cancer endpoints.

The study’s authors are candid about its limitations. Composite sampling, while analytically practical, blurred spatial resolution, particularly in Lake Hayqe where all water samples were merged into a single composite. Sampling occurred during a single season, so rainfall-driven pulses of pesticide runoff may have gone unmeasured, and the smaller sample size from Lake Hayqe limits direct comparison between the lakes. Still, the baseline established here carries a clear message for policymakers: pesticide governance in Ethiopia remains underdeveloped, many generic pesticides circulate without full registration, and farmers often handle chemicals without precautions. The researchers call for multi-season monitoring, larger and non-composited samples, and site-specific exposure data, alongside improved management practices to stem the flow of organophosphates into lakes that sustain fisheries, tourism, and hydropower. Protecting the plankton at the base of these ecosystems, the data suggest, may be the most urgent task of all.

Subject of Research: Ecological and human health risk assessment of organophosphate pesticide residues in water and fish from two Ethiopian highland lakes

Article Title: Ecological and human health risk assessments of organophosphate pesticide residues in water and fish from two highland lakes in Ethiopia

Article References: Yimer, T. F., Kassa, Y., Mulugeta, M., Akele, M. L., Alemu, A. K., Yenealem, D., Amare, A., Moges, Z., Birhan, T. A., & Tibebe, D. (2026). Ecological and human health risk assessments of organophosphate pesticide residues in water and fish from two highland lakes in Ethiopia. Heliyon, 12(15), Article e45551. https://doi.org/10.1016/j.heliyon.2026.e45551

Image Credits: AI Generated

DOI: 10.1016/j.heliyon.2026.e45551

Keywords: organophosphate pesticides, Lake Tana, Lake Hayqe, Ethiopia, chlorpyrifos-methyl, Daphnia, ecological risk assessment, bioconcentration, Nile tilapia, water contamination, fish consumption, risk quotient

The analytical backbone of the study was a modified QuEChERS extraction procedure coupled with ultra-high performance liquid chromatography tandem mass spectrometry, a combination that allows trace-level quantification of multiple pesticide residues across chemically distinct matrices. This approach matters because herbicides behave very differently depending on where they end up. Some dissolve readily into water and travel with runoff; others bind to soil particles and hitch a ride on eroded sediment. By sampling soil, sediment, and water simultaneously and repeatedly across wet, dry, and transitional seasons, the team could trace those partitioning behaviors in a real watershed rather than inferring them from laboratory models alone.

One compound dominated the picture: diuron. This phenylurea herbicide, widely used to control weeds in orchards and cane fields, was detected frequently, with soil concentrations ranging from 0.309 to 175.406 micrograms per kilogram and sediment values reaching 91.292 micrograms per kilogram. Diuron’s persistence is well documented in other systems; it degrades slowly, adsorbs to organic matter, and can linger long after application. Its prominence in both soil and sediment of the Nala watershed suggests a reservoir of legacy and ongoing inputs that continues to feed the aquatic system, even between spraying seasons.

In the water column itself, the headline compounds were different. Ametryn, a triazine herbicide used heavily in sugarcane cultivation, reached a maximum concentration of 30.294 micrograms per liter in surface water, while mesotrione, a triketone herbicide, peaked at 20.264 micrograms per liter. These are striking figures for ambient surface waters, and they reflect the intensity of herbicide use in a watershed where sugarcane and other subtropical crops receive multiple applications per growing cycle. The fact that the dominant compounds shift between matrices underscores a central theme of the study: there is no single herbicide problem, but rather a suite of compound-specific contamination patterns.

The seasonal dynamics proved equally revealing, and somewhat counterintuitive. In soil, the predominant herbicides, particularly diuron and ametryn, showed significantly elevated concentrations in spring, which aligns neatly with pre-planting and early-season weed control. Yet in sediment, the predominant herbicides displayed no significant seasonal differences at all, hinting at a buffered, slowly exchanging pool that accumulates residues over time. Surface water told a third story: several compounds varied significantly across seasons, but the patterns diverged by compound. Butachlor, a chloroacetanilide used in paddy and wetland systems, reached its highest median concentration in summer, while ametryn and mesotrione showed their numerical maxima in spring.

Those contrasting temporal signatures allowed the researchers to disentangle two forces that shape pesticide fate in agricultural catchments. Application timing clearly controls the initial pulse of herbicides entering the environment, which explains the spring peaks in soil and in compounds applied early in the season. But once residues are mobilized, rainfall and hydrological processes take over, governing transport, redistribution, and dilution across the watershed. The summer peak of butachlor, a compound associated with flooded rice systems, likely reflects both its use pattern and the intense monsoon-season runoff that flushes it from fields into streams. In effect, the watershed integrates a chemical calendar written by farmers with a hydrological calendar written by the climate.

Space mattered as much as time. Higher residue levels clustered primarily in the southwestern agricultural areas of the watershed, where cultivation intensity is greatest, while downstream redistribution became evident during the rainy season as runoff carried dissolved and particle-bound herbicides toward lower reaches. This spatial structure has practical implications: it identifies hotspot zones where mitigation efforts, such as vegetated buffer strips, controlled drainage, or adjusted application schedules, would deliver the greatest reduction in downstream loading. It also demonstrates that a watershed cannot be managed as a uniform unit; pollution pathways are dictated by the geography of farming and the connectivity of the drainage network.

The ecological risk assessment is where the study’s findings become most alarming. Using the risk quotient approach, which compares measured environmental concentrations to toxicity thresholds for aquatic organisms, the team identified diuron, ametryn, and imazapic as the principal risk drivers in surface water. Maximum risk quotients reached 1251.23 for diuron, 841.50 for ametryn, and 789.20 for imazapic, values that are orders of magnitude above the threshold of 1 typically used to flag unacceptable risk. More concerning still, the median risk quotients for these three compounds remained above 1, meaning that at typical, not just extreme, concentrations, the water poses sustained ecological concern throughout much of the year.

The risk landscape beyond the top three drivers was more nuanced. Several additional herbicides also exhibited median risk quotients greater than 1, though at lower magnitudes, indicating chronic pressure from a broader suite of compounds. Others showed high maximum risk quotients but lower medians, a pattern the authors interpret as episodic risk, where brief pulses after rainfall or application events push concentrations past toxic thresholds even if average levels appear tolerable. Episodic exposure is ecologically significant because aquatic communities often respond to peak concentrations, and short pulses during sensitive life stages, such as larval development, can cause disproportionate harm. A monitoring program that samples infrequently would miss exactly these events, which is why the monthly resolution of this study is methodologically important.

What emerges from the Nala watershed is a systems-level insight: herbicide occurrence, transport, and ecological risk in subtropical agricultural regions are jointly governed by agricultural practices, compound-specific environmental behavior, and hydrological processes. None of these factors can be addressed in isolation. The authors position their work as a scientific basis for targeted herbicide monitoring, runoff control, and watershed-based pollution management, and the data support concrete interventions, from timing applications to avoid pre-monsoon windows to prioritizing the southwestern hotspot zones for best-management practices. As herbicide use intensifies across the subtropical belt to feed growing populations, studies like this one provide the evidence base needed to keep crop protection from becoming waterway contamination, and they make clear that in intensively farmed watersheds, the two are currently separated by a very thin line.

Subject of Research: Occurrence, seasonal and spatial distribution, and aquatic ecological risk of sixteen current-use herbicides in a subtropical agricultural watershed in South China

Article Title: Occurrence, spatiotemporal distribution, and risk assessment of herbicides in a subtropical agricultural watershed

Article References: Liu, L., Mo, G., Zhao, Y., Rasheed, U., Li, Y., Hu, M., Qin, Q., Qin, Y., Tan, H., & Shan, B. (2026). Occurrence, spatiotemporal distribution, and risk assessment of herbicides in a subtropical agricultural watershed. Environmental Monitoring and Assessment, 198(10), Article 1094. https://doi.org/10.1007/s10661-026-15948-0

Image Credits: AI Generated

DOI: 10.1007/s10661-026-15948-0

Keywords: herbicides, diuron, ametryn, imazapic, butachlor, mesotrione, agricultural runoff, watershed, ecological risk assessment, risk quotient, surface water contamination, subtropical agriculture