POCIS Calibration for pesticide monitoring : from lab to in-situ experiments
Résumé
In order to estimate the water concentrations of pollutants from accumulated amounts in the sampler, laboratory or in situ calibration data are required in order to estimate the sampling rate (Rs) for each compound. The sampling rate of passive samplers depends on the physicochemical properties of the chemicals (ex. molecular weight, structure and hydrophobicity) and the environmental conditions, such as temperature, water flow rate/turbulences and dissolved organic carbon. The challenge is to obtain TWA concentrations which are sufficiently representative of the real pollution levels in the aquatic medium. This goal is mainly dependent on the calibration of the passive sampler, generally conducted under controlled conditions at laboratory scale. However, as field environment is very different from laboratory conditions, use of inappropriate laboratory derived sampling rates for calculating TWA concentrations from passive samplers exposed in situ could lead to an inaccurate result of the real pollution levels. In order to obtain representative concentrations from passive sampler, it is necessary to correct laboratory sampling rates (Lab-Rs) in order to take into account the exposure conditions. The approach based on the use of performance reference compound (PRC,) can be used to determine in-situ sampling rates more specific to a field conditions, allowing to calculate TWA concentrations with an acceptable trueness The aims of the present work were to study the uptake kinetics in surface water of a range of polar pesticides and metabolites by pharmaceutical POCIS samplers in order to determine sampling rates by in-situ calibration, to compare results with those obtained under laboratory conditions in order to assess the impact of environmental conditions on POCIS field performances. Finally, the objective is to evaluate the effectiveness of POCIS to determine TWA concentrations in the aquatic medium in comparison with the classical spot sampling methodology. The in situ experiment was conducted with samplers deployed in channel pilot system, an artificial irrigation canal bringing water from the Rhône River. Beside the numerous targeted pesticides, 13 compounds were detected in water samples including triazines (atrazine, simazine, terbuthylazine), phenylureas (isoproturon, IPU; diuron, chlortoluron), conazoles (tebuconazole, propiconazole), chloroacetanilides (metolachlor), phenylamides (metalaxyl) and triazines metabolites (deethylatrazine, DEA; deisopropylatrazine, DIA; deethylterbuthylazine, DET), allowing the comparison between lab and in situ experiments. For most of the compounds, the in-situ sampling rates were significantly lower by a factor of 3-5 than those from laboratory experiment, considering that field measured water velocity was 4 time lower than laboratory, the main effect of flow velocity on accumulation capability of POCIS is highlighted. Associated to spot sampling all along passive sampler exposure integrative capabilities of POCIS have been demonstrated on metolachlor pollution event.