Occurrence and environmental risk assessment of pesticides in urban wastewater in Costa Rica

Environmental contamination with pesticides from agricultural applications in rural areas has been widely studied, however, in urban areas, wastewater represents one of the main pathways for pesticide contamination. This issue has received little attention, particularly in developing countries, where permissive legislation often prevails. This study aimed to quantify pesticide active ingredients in urban wastewater from medium-to large-scale wastewater treatment plants (WWTPs) in Costa Rica using a broad detection protocol (442 compounds) and to estimate environmental risk through hazard quotient (HQ) calculations, ecotoxicological assays and a prioritization approach to generate a local list of high-priority pesticides. In total, 29 pesticides were detected, mainly insecticides (55%), dominated by neonicotinoids, pyrethroids and organophosphates. The most frequently detected compounds were diethyltoluamide (100% of samples), imidacloprid (91%), diuron (81%), cypermethrin (81%) and terbutryn (63%). Considering HQ values, 17 compounds exhibited high environmental risk, the most hazardous being deltamethrin (HQ=93750), diazinon (HQ=6000), cinerin II (HQ=1857), cyfluthrin (HQ=743) and cypermethrin (HQ=549). All influent and effluent samples were categorized as high risk (∑HQ ≥ 1). However, WWTPs with secondary treatment had a lower calculated ecotoxicological hazard in effluents. The prioritization criterion identified cypermethrin, diazinon, cinerin II, diuron and terbutryn as the top prime-concern pesticides (level of ecotoxicological risk = 3.6, 2.8, 2.7, 2.6 and 2.6 respectively). Ecotoxicological analyses revealed that Aliivibrio fischeri was the most vulnerable model organism, followed by Daphnia magna and Lactuca sativa. A significant decrease in the ecotoxicity to D. magna was observed when comparing influents to effluents, whereas A. fischeri exhibited only a slight decrease. Conversely, for L. sativa, some effluents showed higher toxicity. For specific WWTPs, ecotoxicity correlated well with organic matter content (R² 0.581-0.936). These findings provide critical data for regulatory authorities, identifying priority compounds from an ecotoxicological perspective, offering a case study for pesticide risk in urban wastewater in developing nations.