Assessing pesticide pollution in the Río de la Plata: A multi-tiered approach to ecological risk evaluation.

Agricultural intensification across the Argentine Pampas is reshaping the contaminant load delivered to the Río de la Plata. This study presents a basin-scale appraisal of pesticide occurrence and ecological risk along the Argentine margin, integrating monitoring at 22 surface-water sites with two sediment campaigns and targeted risk characterization. Eighty pesticides were analyzed; 34 were detected in water above detection limits. Occurrence was dominated by herbicides (glyphosate, atrazine and metabolites), alongside frequent fungicides and insecticides. In sediments, glyphosate and AMPA were ubiquitous across campaigns, whereas pyrethroids and triazole fungicides increased in the summer survey, consistent with seasonal application patterns. Environmental risk was evaluated via a multi-step approach: (i) comparison of measured concentrations with national and international water-quality guidelines (WQGs), (ii) site-level Risk Quotients (RQ), and (iii) derivation of substance-specific guideline values. RQ screening indicated multi-residue risk at 19/22 sites. Risk was disproportionately driven by a small subset of compounds (carbendazim, chlorantraniliprole), whereas frequently detected herbicides generally remained below their WQGs. Following a national framework, species sensitivity distributions (SSD) yielded protective values for imidacloprid (67 ng/L), carbendazim (145 ng/L), and glyphosate (51,165 ng/L); chlorantraniliprole was derived using an assessment-factor approach (29 ng/L). Exceedance analysis showed that adopting SSD-based values materially changes frequency: for imidacloprid, exceedances increased from ∼32% under a conventional benchmark to ∼92% under the SSD-derived value. Findings indicate persistent background contamination overlaid by event-driven peaks, underscore the value of SSD-based derivations for context-specific protection goals, and actionable priorities: periodic revision of WQGs using transparent SSD/AF procedures and integration of sediments in monitoring designs.