Pesticides used in agriculture can be transported during rain events to surface waters where they can have adverse effect on aquatic organism even at low concentrations. Thus, regular monitoring of pesticides in surface water is an important consideration for safeguarding water quality.
In Europe, pesticides with high application rate, mobility, persistence and toxicity are typically included in monitoring campaigns along with the regulated priority substances as defined in the WFD in order to assess the water quality. As new pesticides are constantly released on the market and the pesticide use pattern can vary strongly on a regional scale, the adjustment of monitoring program to river basin specific pesticides is challenging and relevant pesticides can be easily overseen. The goal of this study was to perform a complete analytical pesticide screening, assess the associated risk for the aquatic environment and compare this with the risk obtained when only the subset of pesticides as required by the WFD is analyzed.
Five rivers impacted by agricultural activities in the Swiss plateau were investigated. Biweekly composite water samples were analyzed over 6 months by a screening method using solid phase extraction (SPE) liquid chromatography coupled to high resolution tandem mass spectrometry (LC-HRMS/MS, Thermo Scientific™ Q Exactive™ MS). The acquired data were processed using a target and suspect screening approach. Using this method, 86% (310) of all registered synthetic organic pesticides in Switzerland (310) could be detected at low concentrations (0.5-10 ng/L).
The results of the complete screening showed that on average 30-40 pesticides were detected per sample. The sum of individual risk quotients following the concept of concentration addition for plants and invertebrates exceeded the critical value by factors of 4-6 and 2-3, respectively. It was found that only 50% of the detected substances would have been measured given a subset of 30-40 investigated pesticides. Approximately 35% of the total risk, especially the risk caused by insecticides, would not have been detected with a reduced monitoring program because relevant substances such as neonicotinoids would not have been included in the subset of analytes. The findings demonstrate that a complete pesticide screening is necessary to reliably represent the water quality of a river.