Rain or shine, flowing from the spring to the outflow: geochemical evolution of stream waters across a rural-to-urban transect (Greve River, Tuscany, Italy)

Stream ecosystems are experiencing dramatic upheavals during the Anthropocene as a consequence of multiple stressors including pollution, urbanization, and climate change. Being topographical low points, they collect sediments and runoff from the surroundings, resulting in disproportionately high impacts on biodiversity and water quality. Moreover, changes in precipitation regimes, resulting in prolonged droughts interspersed with concentrated and intense rain events, induce augmented perturbations and pose challenges to the resilience of stream ecosystems undermining their self-purification capability (i.e. a combination of physical, chemical and biological processes able to remove pollutants from water, either immobilized in particulate forms or released as gaseous compounds). In this study, the results of multiple sampling campaigns, conducted under diverse climatic conditions and aimed at collecting and analyzing stream waters and suspended solids along the Greve River (GR; Tuscany, Italy) are presented. GR rises in the Chianti hills and flows across rural landscapes covered by vineyards and olive groves, encountering minor urban settlements. The final part of the stream course crosses an intensely industrialized and urbanized area in the eastern outskirts of Florence until the confluence with the Arno River. Six sampling campaigns were performed between May and October 2022 for a total of 71 water samples and 39 suspended sediments. Water samples were analyzed for (i) major and minor dissolved ionic species, (ii) nutrients (total phosphorus and total nitrogen), (iii) chemical oxygen demand (COD), (iv) trace elements, (v) dissolved gases, and (vi) stable water isotopes. Suspended sediments were gravimetrically quantified and subjected to leaching tests in CO2-saturated water to investigate the potential release of trace elements under intense interaction with meteoric waters. Stream waters experienced a geochemical evolution along the stream course, mainly affecting the total dissolved solids, nutrients and COD. The comparison among the different sampling campaigns highlighted the augmented stressed conditions suffered by the watercourse during drought periods, producing oxygen depletion. Nutrients and trace elements were affected as well as the concentrations of dissolved CH4 with potentially augmented fluxes of this greenhouse gas to the atmosphere. Trace elements recovered from leaching tests on suspended materials mimed the variations of their dissolved counterparts, testifying that their origin was mainly related to water-rock interaction and sediment transport through surface runoff. Both spatial and temporal variability was observed in dissolved metals in response to land use and precipitation regime. Rural sites were enriched in Cr, Cu and Se, whereas urbanized sites were enriched in V, As, Sb. Seasonality and weather conditions particularly affected the levels of Sb, Cr and Pb.