Abstract
PurposeThe extensive Hg contamination in soil and sediments occurring along the Paglia River (Central Italy) is the result of the interplay between the geomorphological changes of the river and anthropic activities, primarily associated to the exploitation of Hg-deposits in "The Monte Amiata mining district" (MAMD). The present study determines the implications of the morphological changes that occurred along the Paglia River in the last 200 years on the distribution of Hg along the floodplain and riverbed, which today represent one of the main Hg-reservoirs in the MAMD.Materials and methodsThe temporal changes of the Paglia riverbed and the extent of its alluvial deposits were reconstructed by a GIS-based analysis of the available maps and aerial photos. The Hg-concentration in soil and sediment samples, collected along five transects transverse to the Paglia River channel, was determined by ICP-MS.Results and discussionSamples along the investigated Paglia River segment typically show Hg-contents exceeding the Italian threshold for residential and public green soil use (1 mg kg−1). The distribution of Hg in the Paglia floodplain results from the combination of exceedance of sediment yield to the river during mining activities, that fed the floodplain with large amounts of Hg-contaminated sediments during its braided stage about 100 years ago, and the morphological changes of the river, that led to the evolution from a braided to the present-day single channel river. The magnitude of the extension of Hg-contamination, the river geomorphologic changes, and the processes of transport, deposition, and re-suspension did not allow a natural “clean up” of the river system, which shows a low resilience. Under high flow conditions, and especially in coincidence with intense rain events, large amounts of Hg stored in the overbank sediments are mobilized and redistributed, contributing to make the floodplain a secondary Hg-source. Extreme weather events, expected to intensify as a consequence of climate change, will contribute to the recurrent distribution of Hg-contaminated legacy sediments in the floodplain and along the Paglia river course.ConclusionFrom a water/land management perspective, the variability of the river flow, associated with an increase of extreme flood events driven by climate change, will affect the distribution of Hg-contaminated particles in the Paglia River, contributing to the Hg input into the Mediterranean Sea in the future.Graphical abstract
Highlights
Geomorphic features of riverine systems result from the balance of many parameters, that may in turn be affected by factors such as climate changes and human activities (Grabowski and Gurnell 2016; MarchamaloJournal of Soils and Sediments (2022) 22:656–671 et al 2016; Calle et al 2017; Owens 2020; Vauclin et al 2020)
We will analyze the temporal changes of the Paglia River floodplain, with reference to area variations
In the investigated segment of the Paglia River, the main changes observed during the 1883–2019 timeframe include anthropogenic intervention and modifications in the principal road network, building of an industrial area that occupies part of the river valley, modifications of crop field extension, and other changes in land use
Summary
Geomorphic features of riverine systems result from the balance of many parameters (e.g., water and total sediment load; Schumm and Harvey 1999; Calle et al 2017), that may in turn be affected by factors such as climate changes and human activities (Grabowski and Gurnell 2016; MarchamaloJournal of Soils and Sediments (2022) 22:656–671 et al 2016; Calle et al 2017; Owens 2020; Vauclin et al 2020). Mining-contaminated legacy sediments deposited along waterways may remain stored within river channels and on floodplains for hundreds or thousands of years (Salomons and Förstner 1984; Macklin and Lewin 1989; Pavlowsky et al 2017; Davis et al 2018; Rimondi et al 2019) They become diffuse sources of contamination if re-mobilized, for example, by overbank erosion during flood events, or by human activities (e.g., gravel mining; Macklin et al 1997; Pavlowsky et al 2017; Colica et al 2019). Floodplains play an important role as both sinks and sources of metal contaminants in mined watersheds (Bradley 1989; Horowitz 1991; Lecce and Pavlowsky 1997; Coulthard and Macklin 2003; Ciszewski and Grygar 2016; Pavlowsky et al 2017)
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