Physico-chemical monitoring of stream water at the Strengbach watershed outlet

1) Field sampling: The natural solution is sampled in polypropylene bottles. The vials are pre-cleaned according to the following protocol: rinse, 10% HCl acid bath 24h, rinse, H2O bath 24h, rinse, 2nd H2O bath 24h, rinse and dry at 60°C. In the field, sampling bootles are conditioned twice with the sample. 2) Sample filtration: Samples are filtered through 0.45µm cellulose acetate filters in Teflon filtration units. To prevent contamination, the filters are rinsed with 2x500ml of suprapur water. 3) Laboratory analysis: Analyses are carried out at the Institut Terre et Environnement de Strasbourg (ITES) water analysis laboratory. This laboratory is part of the PACITE platform (Plateforme d'Analyse Chimique et Isotopiques en Sciences de la Terre et Environnement). The parameters monitored are: pH, conductivity, alkalinity Na+, K+, Ca2+, Mg2+, Cl-, NO3-, SO42-, Dissolved Organic Carbon DOC, Si. pH and Conductivity are analyzed with a Hach Lang HQ440D multi-parameter titrator. COD, with Shimadzu TOC-Vcph automatic analyzer. Alkalinity is measured with Titrando Metrohm automated titrator, Aquatrode LL Plus pH electrode, temperature probe, propeller stirrer. Si is analysed with ICP/AES. Thermo Scientific ICAP6000 spectrometer with cyclone nebulizer, fed by Argon gas at 6 bar. Na+, K+, Ca2+, Mg2+, Cl-, NO3-, SO42- are determined by Ion chromatography. ICS-5000+ Dionex Thermo Scientific, AS18 5x300mm and CS16 5x300mm separative columns. Use of ASRS600 4mm and CSRS600 4mm suppressors. Detection cell is a conductivity electrode. Mobile phases: NaOH 34mM, Methanesulfonic Acid 45mM, 1ml/min. The analytical measurement equipments are calibrated and certified standard samples are run regularly (PERADE-17/River water, Rain-12/Open field precipitation, BIGMOOSE-14/Lake water). The water analysis laboratory supplies the results in the form of an excel file. 4) Data processing: The data are then compiled in a global excel file, including all analyses, by study site and in chronological order. A data verification and validation stages are then carried using python (calculation of ionic balance, search for outliers, temporal monitoring, study of trends). Once the data has been validated, a csv file is generated, which is then fed into the BDOH database. 5) Historical background. These data have been acquired since 1986, thanks to the involvement of many people - technicians, engineers and researchers - who have worked to ensure the continuity and quality of the measurements, both in the field and in the laboratory. We cannot name them all here. The Observatory was initiated by research projects conceived and led by Bruno Ambroise, Bertrand Fritz and then Anne Probst.A brief summary of the history of work on the Strengbach watershed is available on the website: https://ohge.unistra.fr/lobservatoire/historique.