A study conducted in Costa Rica exposed an issue rarely discussed in Latin America: pesticides also accumulate in urban wastewater, even after passing through treatment plants.

The research detected 29 compounds present in everyday household, commercial, and industrial products. These chemicals appeared both at the entry and exit of four treatment systems in the Central Valley, the region where the country’s largest population is concentrated.

The most concerning compounds were cypermethrin, diazinon, cinerin II, diuron, and terbutryn. In all cases, their levels exceeded safe limits for aquatic organisms, implying a constant risk for rivers and streams receiving these effluents. Additionally, they belong to chemical families associated with harmful effects on the nervous system and hormonal disruptions.

The most unexpected finding was the detection of five pesticides that had never been recorded before in urban wastewater. Among them are cinerin II, flubendiamide, dichloran, biphenyl, and 1,4-dimethylnaphthalene. Many originate from domestic insecticides, fungicides for ornamental plants, and preservatives used in foods.

Tools to measure toxicological impact

To assess the real extent of the problem, researchers applied a hazard coefficient that compares pesticide concentrations with safe levels for aquatic fauna. Additionally, they conducted tests with three sentinel organisms: a freshwater crustacean, a luminescent bacterium, and lettuce seeds. 

The results were conclusive: 17 substances represented a high environmental risk. However, the overall picture showed an even more complex scenario. The combination of all compounds present in each sample increased the total toxicity, both in raw water and in treated effluent.

The mixture of chemicals acted cumulatively, generating an environmental risk greater than that of each pesticide separately. This work is pioneering in the region by analyzing more than 400 active ingredients used in commercial pesticides.

The advancement represents an important step in understanding how cities contribute invisible pollutants to ecosystems. The study also opens the door to new evaluations that consider the combined effect of chemical residues.

Structural limitations in water treatment

Water resource experts warn that this situation is not exclusive to Costa Rica. Most treatment plants in Latin America were designed to remove organic matter, not pesticides or highly persistent compounds.

Therefore, the effluents end up releasing substances that the systems are not prepared to retain or degrade. Part of the pesticides even increased their concentration at the exit of some plants.

This could occur when chemicals detach from the retained solids or when they transform during the treatment process. The phenomenon reveals deficiencies that require an urgent review of current purification methods.

In Costa Rica, less than 15% of the population is connected to wastewater treatment systems. This means that most urban waste reaches rivers without undergoing any purification process. The combination of low coverage and insufficient technology exacerbates the risk to freshwater ecosystems.

A regional issue advancing faster than regulation

The report points to a major challenge: the rapid incorporation of new molecules into the market. The industry introduces compounds at an increasing speed, while regulatory frameworks take years to evaluate, control, or ban dangerous substances.

Thus, chemicals withdrawn in developed countries continue to circulate in markets with weaker controls. This regulatory lag has direct effects on the urban environment and water bodies.

Pesticides are massively used in gardens, buildings, businesses, and public spaces without clear information about their impacts. The result is a continuous flow of pollutants into drainage systems and, ultimately, into rivers.

The magnitude of the problem demands dialogue with authorities responsible for sanitation and urban planning. Decisions about infrastructure determine which substances reach watercourses and which could be retained or treated. The lack of technological and regulatory updates leaves cities exposed to a silent but persistent pollution.

Benefits of this scientific initiative for the region

The study opens an opportunity to transform urban environmental management in Latin America. Its main contribution is to highlight a previously ignored problem: the pesticides we use in our cities also pollute ecosystems.

Having concrete data allows for the promotion of more effective policies adapted to the regional reality. Additionally, the developed methodology facilitates the early identification of emerging chemicals.

This can help update regulations, ban highly toxic substances, and improve requirements for household products. It also offers tools to improve the design of treatment plants and promote technologies capable of retaining persistent pollutants.

The research encourages cooperation between universities, local governments, and environmental organizations. With a solid scientific base, the region can promote prevention strategies, citizen education, and permanent monitoring. In the long term, these advances could reduce the chemical load received by urban rivers and strengthen the health of ecosystems.