Concentration addition-based approach for aquatic risk assessment of realistic pesticide mixtures in Portuguese river basins.

A two-tiered outline for the predictive environmental risk assessment of chemical mixtures with effect assessments based on concentration addition (CA) approaches as first tier and consideration of independent action (IA) as the second tier was applied based on realistic pesticide mixtures measured in surface waters from 2002 to 2008 within three important Portuguese river basins ('Mondego', 'Sado' and 'Tejo'). The CA-based risk quotients, based on acute data and an assessment factor of 100, exceeded 1 in more than 39 % of the 281 samples, indicating a potential risk for the aquatic environment, namely to algae. Seven herbicide compounds and three insecticides were the most toxic compounds in the pesticide mixtures and provided at least 50 % of the mixture's toxicity in almost 100 % of the samples with risk quotients based on the sum of toxic units (RQSTU) above 1. In eight samples, the maximum cumulative ratio (MCR) and the Junghan's ratio values indicated that a chemical-by-chemical approach underestimated the toxicity of the pesticide mixtures, and CA predicted higher mixture toxicity than that of IA. From a risk management perspective, the results pointed out that, by deriving appropriate programmes of measures to a limited number of pesticides with the highest contribution to the total mixture toxicity, relevant benefits also on mixture impact could be produced.