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It has been demonstrated that drainages feeding the Baccatoio stream contain extremely

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Academic year: 2021

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ABSTRACT

This Thesis is focused on the potential pathways of environmental impact and fate of toxic and potentially toxic elements related to the past mining practice in the Alta Versilia abandoned minesites of Mt. Arsiccio and Pollone (Tuscany Region, Italy), with particular reference to the effects on surface waters of the Baccatoio stream, from the mining area up-stream to the mouth on the Versilia coastland. The aim is to contribute to the proactive detection of environmental threats, in order to prevent negative impacts to the ecosystem and to human health, and to contribute in planning mitigation requirements. Special attention has been devoted to thallium contamination.

The study has been conducted in the framework of the geologic and hydrogeologic settings of the sites, including the evaluation of the spatial and temporal trends in water quality. It has been highlighted that the main source of contamination comes from the oxidative weathering of sulfide minerals, in particular pyrite, and sulfosalts that produces the metal-laden acid-sulfate water that feed the Baccatoio stream. It has been shown that the sulfide oxidation process is strongly enhanced by bacterial activity (autotrophic thiobacilli and related species), through the Fe(II) oxidation to Fe(III) in acidic conditions, including jarosite formation, and the role of ferric iron in increasing the sulfide oxidation rate.

It has been demonstrated that drainages feeding the Baccatoio stream contain extremely

high iron concentration (up to 14 g/L); arsenic (up to 33 mg/L), antimony (up to 19

mg/L), cadmium (up to 2 mg/L) and thallium (up to 9 mg/L) (in addition to other

metals). During their flow in the superficial environment, stream waters undergo an

increase in pH through the admixing of calcium-bicarbonate waters and by the

interaction with the carbonate sediments that characterize the stream bed. Extensive

iron hydroxides and iron-oxyhydroxysulfate precipitation occurs, depending on pH,

acting as efficient scavengers for most harmful elements, in particular arsenic,

significantly reducing the impact on the aquatic ecosystem. However, adsorption and

precipitation processes seem to have a limited effect on the removal of thallium aquatic

species (mostly tallous ions Tl

+

) that maintain a concentration of concern throughout

the course of the stream to the coast, in particular during high flow. Iron-rich deposits,

that are widespread in the riverbed, may act as potentially sources of contaminant in

case of desorption of dissolution processes.

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During low flow the Baccatoio stream infiltrates in sub riverbed upstream, and waters continue to migrate underground. In these conditions, the input of good-quality waters from springs tapping carbonate aquifers becomes dominant improving the overall quality of waters.

The results of a preliminary survey indicate that the impact of infiltration processes on waters from the phreatic aquifer in the Plain seems negligible, with the exception of some species including the relatively high concentration of zinc.

Oxygen and hydrogen isotopic data allowed the different contribution to the Baccatoio waters to be determined, in particular during high flow. The data underline a major contribution from drainages and groundwaters even during high raining events, and not directly from rain, suggesting that piston flow-like dynamics activate.

The comparison of conservative parameters such as oxygen isotopes and sulfate content

with thallium concentration reveals that this element behaves almost conservatively

during high flow. It is hence suggested that thallium contamination represents the major

threat for the ecosystem and requires a careful investigation including the monitoring of

drinking water supplies.

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