Using Polar Organic Chemical Integrated Sampler (POCIS) to monitor pesticide fluxes in Alpine streams

In recent decades, the issue of water quality has become of great concern worldwide. Anthropogenic pressures threatening freshwater and riparian ecosystems also include agricultural activities, which are often located near watercourses and lead to increased levels of nutrients and pesticides in rivers due to surface runoff from rainwater or from irrigation. One of the most widely used approaches to assessing pesticide concentrations in water is to analyse a single volume of water collected at a specific time, i.e. by “grab" sampling. Although results provided by this sampling method are accurate, they are only representative of the conditions at the time the sampling was carried out. Alternative approaches have been proposed over the past 20 years based on passive samplers, including the Polar Organic Chemical Integrative Sampler (POCIS). POCIS is a simple and robust passive sampler, composed of a receiving phase enclosed between two polymeric membrane sheets, like an Oasis HLB phase and by two polyethersulfone (PES) membranes as used in the present work. The sampler adsorbs pesticides flowing in the stream during its deployment period of time in stream water, which are then extracted with a solvent and analysed by gas chromatography (GC-MS/MS) and liquid chromatography tandem mass spectrometry (LC-MS/MS) systems. The present Ph.D. thesis focuses on the use of POCIS for pesticides monitoring in Alpine steams. The use of POCIS is relatively recent and several operational and scientific challenges are still posed by its application and data interpretation. In the majority of the works reported in literature, the analytes were extracted only from the receiving phase, considering the membrane only as a physical filter. Instead, recent works show that the PES membrane is able to adsorb some pesticides, but many aspects on its behaviour are still unknown. In addition, the interpretation of the measurements of pesticide concentrations by POCIS in streams still presents several open challenges issues, which limit the samplers’ use in environmental monitoring. In this context, this Ph.D. thesis addresses three main issues: the study of the functioning of the POCIS and the adsorptive capability of its components (Oasis HLB phase and PES membrane) against a wide number of pesticides, the field validation of a POCIS calibration method, and using POCIS to assess pesticide pollution at a watershed scale and to assess the effectiveness of two pesticides mitigation strategies for running waters. The first results chapter illustrates a set of laboratory experiments, which showed that the extraction of pesticides from both the sampler compartments (Oasis HLB phase and PES membrane) allows the detection of almost all the investigated compounds (224 over 226), covering a wide range of polarities, from -2 to 7 LogKow units. More than half of these compounds were adsorbed more effectively by the PES membrane than the Oasis HLB phase. The affinity of these pesticides towards the two sampler compartments can be explained only partially by the polarity of the compounds. However, a significant adsorption of the most hydrophobic compounds by the PES membrane was noticed, especially for values of LogKow higher than 4. In the second results chapter, the validation of a POCIS calibration model was reported. The samplings were carried out in the field under real-life conditions of use on alpine streams. The calibration was obtained from the comparison of the total amount of pesticides adsorbed by POCIS with the time-weighted average concentration (TWAc), obtained from several grab samples. The validated model can be used to estimate TWAc concentrations for the studied compounds for future environmental monitoring conducted with POCIS under conditions similar to those under study, such as exposure time, flow velocity, temperature. The third results chapter reports the study of the effectiveness of two possible pesticide pollution mitigation strategies analysed with POCIS. The case studies were a natural wetland and a washing plant for agricultural equipment serving local farmers. To this end, sampling campaigns were carried out at the wetland tributary and effluent streams and at a manhole of the white wastewater network near the washing plant, before and after its recent opening. In both cases samplings performed using POCIS allowed to quantify the ability of the two mitigation solutions to reduce pesticide concentrations in water for majority of the compounds studied. In conclusion, the work has improved our knowledge of the POCIS for pesticides monitoring, showing its value for monitoring watercourses, as it is capable of adsorbing a wide number of compounds. In order to optimise the performance of this sampler, the work indicates that it is important to advisable to extract the pesticides also from PES membranes and not only from the phase. The use of POCIS to study the effectiveness of mitigation solutions made it possible to carry out sampling in a simple way, limiting the number of field samplings, without the need for a power supply near the site and ensuring continuous monitoring of pesticide flow; results that would be more difficult to be achieved with the grab sampling method. Finally, the results obtained from this Ph.D. work are able to provide important operational information for the quantitative assessment of pesticides in Alpine watercourses, which can be useful to the environmental protection agencies and stakeholders working in this sector.