Cumulative Ecological Risk of Pesticide Mixtures in an Agricultural River of the Pampa Region: Identifying Critical Contaminants for Practice and Policy Reform

Abstract Agricultural breadbaskets are vital for global food security, yet reliance on pesticides poses a threat to regional aquatic ecosystems. This study evaluates the cumulative ecological risk of pesticide mixtures in a river system of Argentina's Pampa region, a global agricultural heartland. Over one year, 66 pesticide residues were monitored monthly at four sites. All samples contained complex mixtures of 11 to 30 pesticides. The detected pesticides largely are commonly used in the region, and their spatiotemporal variability aligns with local agricultural practices. Despite their widespread use, many of the pesticides identified are reported for the first time in Pampean waters. Three risk assessment approaches were compared to evaluate potential impacts to aquatic ecosystems: A Risk Quotient (RQ) based on predicted no-effect concentrations (PNEC), a Risk Quotient based on the Sum of Toxic Units (RQSTU), and the multi-substance Potentially Affected Fraction (msPAF). All three risk assessment methods indicate that pesticide mixtures pose significant year-round toxic pressure to the aquatic community, with arthropods being the most vulnerable though risk to primary producers and fish was also evident. The RQ method identified 21 potential risk agents. A more refined analysis using msPAF pinpointed six key pesticides (flumioxazin, flurochloridone, chlorpyrifos, b-cyfluthrin, bifenthrin, fenvalerate) as the primary drivers of the mixture risk. However, the RQSTU approach identified only four of these six pesticides, indicating a potential underestimation. Our results demonstrate that a tiered approach, using RQ for initial screening followed by msPAF for definitive assessment, is the most effective and efficient strategy, with the intermediate RQSTU analysis of limited value. Despite widespread contamination, mitigation is feasible and targeting the identified set of six pesticides for regulatory review could reduce ecological risk. This study provides a validated, actionable strategy for evidence-based environmental management in intensive agricultural systems worldwide, highlighting the feasibility of targeted interventions for sustainable agricultural practice.