Use of ADCP acoustic backscatter to estimate seawater total suspended solids in continuous along a transect. Test case in a near shore area (Ligurian Sea, La Spezia Gulf)

Acoustic Doppler Current Profilers (ADCPs), as a secondary output, allow to estimate the concentration of Total Suspended Solids (TSS) in the water column through the analysis of the acoustic backscatter signal. This method was applied during a dedicated ENEA campaign in a near-shore area of the Gulf of La Spezia. ADCP measurements were performed throughout the cruise and joint measurements of TSS (discretely sampled) and Turbidity (by nephelometer) were performed along the water column in one station. The application of the simplified sonar equation made possible to calibrate the ADCP vs TSS by means of a linear regression model, whose correlation coefficient resulted reasonably good (R2 was in fact equal to 0.87), with slope Kc = (0.239 ± 0.008) dB count-1 and intercept Ck = (-69.88 ± 0.59) dB, respectively. The method, whose reliability was proved by comparison with an independent TSS profile, was then applied to obtain a continuous quasi-synoptic mapping of TSS values along the water column throughout the cruise. The overall uncertainty budget of the method was estimated, too: the relative combined standard uncertainty resulted to be less than 30% for TSS values higher than about 3 mg l- 1. The method, although not completely exhaustive due to the multiplicity and unpredictability of the conditions (above all, the type and size and actual distribution of scattering particles), is to be considered valid for a quick acquisition of local situations, allowing to obtain fast and almost zero cost TSS mappings. Results prove that this consolidated approach can be applied in the near-shore area, where ENEA performs its monitoring campaigns and where the interest in TSS measures is also motivated by the presence of fish and mussel farms, the transit of container ships and the proximity of the estuary of the Magra river. 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.