FIRST EXPERIENCES OF CORRELATION BETWEEN PHYSICAL AND CHEMICAL WEATHERING AT BASIN SCALE. THE CASE STUDY OF NICCONE BASIN

Rivers' sediment load can be classified as dissolved, suspended and bedload. Dissolved Sediment (TDS) is the portion of the total sediment that is transported in solution, in the form of ions, mainly produced by chemical weathering in the hydrological basin. Suspended Sediment (TSS) consists of small particles (clay, silt, and fine sands) uplifted by the fluid's flow and rarely in contact with the river bed. The bedload is composed by the largest particles (gravels and pebbles, but also coarse sand) transported along the river bed. Chemical weathering of carbonates and silicates is an important sink of atmospheric CO2. In the literature weak correlation was observed between seasonal variation of river water discharge and TDS, and little attention was paid to the possible correlation existing at the time scale of a single meteorological (extreme) event. At the same time, recent studies paid attention on the correlation between atmospheric CO2 consumption, caused by chemical weathering, and physical weathering (erosion), which can be estimated monitoring, respectively, TDS and TSS. We started to investigate the relationship between TDS and TSS within hydrological basins having different lithological properties. The aim of the present contribute is to describe the first activities and results obtained in the catchment of the Niccone stream, a tributary of Tiber river, a basin characterized by flysch deposits, with layered sandstones interleaved with marls and pelitic levels. In this watershed, rainfall forecasts, rainfall measurements and water stage are monitored through, respectively, the analysis of the COSMO-I5 Limited area model data, the regular ingestion (5-minute interval) of information from the Civil Defense Department's radar platform and the continuous measurements (15-minute interval) obtained from two different water level radar sensors. This continuous monitoring enables the planning and execution of sampling activities, measuring TDS and TSS sampling different stages of stream flood. TDS was measured considering the electrical conductivity (EC) and the river water alkalinity - i.e. bicarbonate ion (HCO3) concentration in river water - as proxies of river dissolved load. EC was determined by using a multi-parameter portable meter, and alkalinity by using portable dosimeters that perform acid titration with HCl 0.1 N using methyl orange as indicator. Regarding TSS measurements, we used the DH-59 sediment sampler that consists in a streamlined bronze casting weighing approximately 10 kg that encloses the sample container. With a pulley, we lowered the sampler from the bridges where hydrometers are installed. The stream velocity, mandatory data to define the sampling time interval, was measured by a portable "radar gun". The stream samples containing TSS have been successively filtered and weighed in laboratory. The experimental activities carried out within the Niccone Basin represent a first step in understanding the extent to which atmospheric CO2 consumption processes by chemical weathering are influenced by meteo-climatic events and subsequent erosional phenomena.