Prospective aquatic risk assessment for chemical mixtures in agricultural landscapes

Environmental risk assessment of chemical mixtures is challenging due to the multitude of possible combinations that may occur. Aquatic risk from chemical mixtures in an agricultural landscape was evaluated prospectively in two exposure scenario case studies: at field scale for a program of 13 plant protection products applied annually for 20 years, and at a watershed scale for a mixed land use scenario over 30 years with 12 plant protection products and two veterinary pharmaceuticals used for beef cattle. Risk quotients were calculated from regulatory exposure models with typical real-world use patterns and regulatory acceptable concentrations for individual chemicals. Results could differentiate situations when there was concern associated with single chemicals from those when concern was associated with a mixture (based on concentration addition) with no single chemical triggering concern. Potential mixture risk was identified on 0.02% to 7.07% of the total days modeled, depending on the scenario, the taxa and whether considering acute or chronic risk. Taxa at risk were influenced by receiving water body characteristics along with chemical use profiles and associated properties. This study demonstrates that a scenario-based approach can be used to determine whether mixtures of chemicals pose risks over and above any identified using existing approaches for single chemicals, how often and to what magnitude, and ultimately which mixtures (and dominant chemicals) cause greatest concern. This article is protected by copyright. All rights reserved.