Comparison of Water Turbidity and Suspended Particulate Matter Concentration and Mineral Composition for the Terek, Sulak and Mzymta River Plumes Based on Satellite Data and In-Situ Measurements

The paper summarizes the results of a 2019–2024 study of river plumes focusing on the determination of water turbidity and suspended particulate matter (SPM) concentration from satellite data, their validation by in situ measurements, and estimation of mineral composition of the SPM. The objects of the study were the plumes of the Sulak and Terek rivers, which flow into the northwestern part of the Caspian Sea, and the plume of the Mzymta River, which flows into the northeastern part of the Black Sea. The satellite data used were high spatial resolution data from Sentinel-2 MSI (Multispectral Instrument) and Landsat-8, -9 OLI/TIRS (Operational Land Imager / Thermal Infrared Sensor) sensors. Synchronously with the satellite observations, in situ measurements using water turbidity sensors and water sampling were made to determine plume water turbidity, SPM concentration and mineral composition. X-ray phase analysis of samples collected from different parts of the plumes was carried out to quantify the SPM mineral composition and its change with river water spreading out to sea. Having similar SPM mineral compositions, mainly comprising three groups of minerals: anhydrous aluminosilicates, clay minerals and carbonates, the plumes are characterized by significantly different SPM particle size distributions in the mouth zones. According to the averaged data for the entire observation period, the change in the mineral composition in different parts of the plume is best manifested for the Terek and Mzymta rivers. Toward the edge of their plumes, the relative proportion of clay minerals increases, while the content of quartz and feldspars becomes significantly lower. No such pattern is observed for the Sulak River. The ratio of clay and aluminosilicate material either practically does not change in different parts of its plume or changes in an arbitrary direction. A comparative analysis between turbidity and mineral composition was attempted. Changes in SPM mineral composition are accompanied by significant changes in water turbidity determined both in situ and from satellite data.