Occurrence and behavior of pesticides in wastewater treatment plants and their environmental impact

• Pesticides are poorly removed in WWTPs.
• Urban wastewaters are important sources of pesticides to the aquatic environment.
• Diazinon and diuron: the most relevant pesticides in our study
• New index (ERPWI) to assess the environmental impact of pesticides coming from WWTPs

Reports on pesticides elimination during wastewater treatment are rare since these substances are typically considered of agricultural rather than of urban origin. In this context, the aim of this work was to evaluate the presence, removal and environmental relevance of 22 selected pesticides in three different wastewater treatment plants (WWTPs), paying attention not only to their occurrence and elimination but also to the toxicity of each pesticide against three aquatic micro organisms (algae, daphnia and fish) through the calculation of the so-named Environmental Relevance of Pesticides from Wastewater treatment plants Index (ERPWI). For this purpose, an analytical method based on isotope dilution on-line solid phase extraction–liquid chromatography–tandem mass spectrometry (SPE–LC–MS/MS) was optimized, allowing the determination of the 22 target pesticides in wastewater with satisfactory sensitivity (limits of detection below 30 ng/L), accuracy and precision. Concerning the results, total pesticide levels were in most instances below 1 μg/L but removal in the WWTPs was variable and often poor, with concentrations in the effluent sometimes higher than in the corresponding influent. Possible explanations for these poor or negative removal rates are, among many others considered (e.g. sampling, sample preservation, method biases, atmospheric deposition), deconjugation of metabolites and/or transformation products of the pesticides, hydrolysis, and desorption from particulate matter during wastewater treatment. The most significant pesticides in terms of concentration and frequency of detection were diazinon and diuron. These two pesticides, followed by atrazine, simazine and malathion, were also the most relevant from the environmental point of view, according to the calculated ERPWI.