Validation of a calibration model able to estimate the concentration of pesticides in an alpine stream through passive sampling (POCIS) monitoring

Environmental context. Pesticides and other contaminants released by agricultural activities negatively impact aquatic biota and water quality, but are not always measured. The Polar Organic Chemical Integrative Sampler (POCIS) is among the most suitable tools for identifying pesticides and other organic compounds in water bodies. However, determination of a mean concentration (over the duration of the deployment) is still a significant challenge, which is addressed in the present work using field observations and a computational model. Rationale. The Polar Organic Chemical Integrative Sampler (POCIS) is a passive sampler composed of a receiving phase, Oasis HLB, enclosed between two polyethersulfone (PES) membranes. It is used for monitoring organic compounds in surface water and in groundwater. The measurement of pesticide concentrations by passive samplers in streams still represents an open challenge, which limits the samplers’ use in environmental monitoring of the quality status of water bodies in accordance with the EU Water Framework Directive. Methodology. We addressed these limitations by calibrating a POCIS with concentration measurements performed on samples collected by means of an automatic sampler from the same section of a small alpine river where a passive sampler was deployed. In the majority of published studies, the analytes were extracted only from the receiving phase, but recent works show that some compounds were also adsorbed by the PES membrane, suggesting the importance of extracting the analytes from both the phase and the membrane. Results. The POCIS was calibrated for 31 compounds, by a comparison between the total amount of pesticides adsorbed by POCIS (Ms) and the time-weighted average concentration (TWAc) obtained from several grab samples, and by estimation of the sampling rate (Rs), which allows linking the Ms with the TWAc over time. The data showed an increasing Rs trend with hydrophobicity for the most hydrophobic and hydrophilic compounds, while compounds with 1.5 ≤ log Kow ≤ 3.5 assumed a value of Rs independent of log Kow. The contribution of PES membranes allows expansion of the calibration of Rs to the most hydrophobic compounds and allows monitoring of pesticides that would hardly have been detected if they were extracted only from the Oasis HLB phase, such as fluazinam. Discussion. The calibration was then verified on the same stream and the model extended to a different sampling site; in both cases, the calibration allowed estimation of a value of TWAc that fits with the reality within a factor of 5, and in the majority of cases within a factor of 2. If this level of accuracy is accepted, the Rs reported here can be used to estimate TWAc from sampling with POCIS.