Anthropogenic impact on urban rivers: insights from the Mugnone Creek (Florence, Italy) waters and sediments characterization in the framework of the multidisciplinary SENECA project

Urbanization is a striking phenomenon, responsible for the development of cities as complex and highly dynamic systems. One of the most pressing issues in urban areas is water cycle management, which directly influences the availability and the quality of this resource. Urban streams are highly vulnerable to the impacts resulting from the increasing urbanization and they have often lost most of their pristine, natural character.

We hereby present the preliminary results of the multidisciplinary StrEams urbaN Ecological City plAnning (SENECA) project, which is not only but mainly devoted to characterizing the chemical features of stream waters and sediments of the Mugnone Creek (MC). MC is a typical example of an urban stream, crossing the city of Florence (Italy) and eventually discharging to the Arno River, that has suffered an intense denaturation due to the urban sprawl, such as several diversions, canalizations and rectifications, alterations of the stream bed and riparian habitat, widespread concrete revetment of the banks and burial of short stream tracts.

Different sites along the 17-km long MC were investigated for water and sediment geochemistry from upstream of the Florence urban area (“blank” sites) to the MC outlet (Cascine Park), passing through variably urban-impacted areas, including sites located along traffic-congested roads, close to the new tramway construction sites and under the railway line. Stream waters were sampled twice a year (June and December 2019 and 2020) to account for seasonal variability. Stream discharge at three representative points was simultaneously determined during water sampling to allow mass load calculations of contaminants. In details, major dissolved ions (Ca, Mg, Na, K, HCO₃, SO₄, Cl, NO₃) and reduced nitrogen (NH₄), trace elements (TEs: Sb, As, Cu, Zn, Cd, Co, Ba, Li, Pb, Ni) and microplastics (MPs) in stream sediments and waters of MC were determined. Major dissolved ions and NH₄ were analysed via acidimetric titration and IC. TEs were determined in both sediments and waters (0.45 μm filtered and unfiltered) by ICP-OES and ICP-MS, respectively. MPs (1 μm-5 mm) were identified in sediments and waters by HD binocular stereomicroscope.

Chemical composition (major element constituents) of water reflected both the seasonality and the progress of the water-rock interaction acting in MC from its spring toward the outlet. In addition, the pattern of abundances of Cu, Zn, Sb and Pb and especially their mass loads reflect the increasing anthropogenic impact on the water and sediments, even if is important to stress that the contents are within the limits defined by law, except for stream sediments at a few sites. For MPs, up to 10⁹ particles, coming mainly from the NW side of Florence, are estimated to be discharged daily to the Arno River during the winter season, much higher than creeks with similar urbanization contexts worldwide. Increasing data collection will make it possible to identify and better understand what type of sources and processes are responsible for the anthropogenic impacts in the MC and could help in better safeguarding the creek within the framework of the urban social-ecological systems of Florence.