ENEA internal best practices for seawater total suspended solids measurement: experimental assessment of measurement uncertainty and correlation with turbidity. Test case in a near shore area (Ligurian Sea, La Spezia Gulf)

Total Suspended Solids (TSS) is considered a very important oceanographic quantity to describe the status of the seawater column. The ENEA Marine Environment Research Centre of S. Teresa, devoted since the ‘70s to the monitoring, analysis and comprehension of physical, chemical and biological processes in marine environment, has therefore developed an historical experience also in TSS measurement. Aim of the present work is to collect ENEA expertise and internal procedure related to experimental TSS measurement, formalizing in particular the assessment of TSS measurement uncertainty in accordance with reference guidelines and standards. The contribution of mass and volume in the laboratory-based gravimetric method to measure TSS (by filtering process) has been analysed, together with the effect due to repeatability proper of this type of experimental activity. TSS combined standard uncertainty has been finally evaluated, and just the repeatability contribution proved to be the most relevant one (about 10 % in terms of relative standard uncertainty). The link between TSS and Turbidity, continuously optically measured along the water column, is also discussed. Turbidity of a water sample can in fact be used to infer the mass of particles in suspension by means of a sort of a calibration curve that relates TSS to Turbidity itself. This relation can be reasonably approximated by a linear model, whose slope is sample-dependent: consequently, the conversion from a Turbidity profile to a TSS one has to be determined each time. Results prove that this consolidated approach can be applied in the near-shore area, where ENEA performs its monitoring campaigns. The present technical report is intended to serve as a basis for developing procedures more and more compliant to international metrological standards, with the aim of further guaranteeing the metrological traceability of oceanographic quantities.