Abstract
A detailed multi-scale investigation of the geotechnical, chemical and mineralogical properties was conducted on contaminated sediment samples collected within the Mar Piccolo, a marine basin in the south of Italy. The basin is located close to one of the most important industrial sites in Europe, which has also been declared ‘at high risk of environmental crisis’ and prioritised for remediation activities. A multidisciplinary investigation campaign showed that the samples collected close to the sea floor are characterised by high values of both heavy metals and organic pollutants and by the presence of significant amount of organic matter. Several samples in the top layer exhibited peculiar geotechnical properties, in terms of plasticity and activity indexes, compressibility and hydraulic permeability. While the prime suspect for such unconventional behaviour was chemo-mechanical coupling between the soil skeleton and contaminants, it turned out that the biogeochemical degradation of organic matter and the presence of microfossils and diatoms is likely to affect significantly the micro to macro behaviour of polluted marine sediments.
Highlights
The research presented in this paper has been prompted by the emblematic case of the contaminated Mar Piccolo (MP) basin in Taranto, where the high degree of pollution that has been recorded in the clayey sediments at the sea bottom (Cardellicchio et al, 2007; Petronio et al, 2012), affects the water quality (ARPA, 2014) and promotes bio-accumulation of pollutants in fish and mussels (Giandomenico et al, 2016)
The main results of the present research contribute to illustrate the extent to which the geotechnical properties of marine sediments may not depend only on the soil granulometry and mineralogy, especially when they have been recently deposited in the ecosystem location of a biocenosis rich in organic compounds
In the case of the MP system, the tests on the sampled sediments have shown that the anthropogenic pollutants, either heavy metals or organic compounds, the abundance of which has been widely documented at the site (e.g. Cardellicchio et al, 2007; Cotecchia et al, 2021), do not impact significantly on the sediment’s geotechnical properties
Summary
The research presented in this paper has been prompted by the emblematic case of the contaminated Mar Piccolo (MP) basin in Taranto (southern Italy, Fig. 1), where the high degree of pollution that has been recorded in the clayey sediments at the sea bottom (Cardellicchio et al, 2007; Petronio et al, 2012), affects the water quality (ARPA, 2014) and promotes bio-accumulation of pollutants in fish and mussels (Giandomenico et al, 2016). The multi-scale data give evidence of the extent to which the bio-chemical processes in real soils may influence their index properties and mechanical parameters, supporting the appropriate engineering characterisations of sediments located in complex ecosystems and the selection of the parameter values to be input in the design of remediation measures, when the sediment pollution is a source of environmental risk. From the pore fluid salinity, the amount of OM, measured by total organic carbon (TOC; DIN EN 15936 (DIN, 2012); section S2·1 in the online supplementary material), varies greatly in the shallow samples and tends to decrease with depth down to the ASP (TOC = 0·05%; Fig. 7(b)). An unpolluted typical MP sediment sample of clayey silt of medium plasticity and high activity (spiked sample, Fig. 3) was mixed with two PCB and PAH congeners and subjected to liquid limit (wL) measurements (BSI, 1990), in order to investigate the effect of the organic pollutants on the soil plasticity. Other tests from the literature (Muththalib & Baudet, 2019) report similar results for ASP samples artificially mixed with single heavy metal ions: either Cu, or Pb, or Zn, in concentrations even higher than those found in the S6-S sample
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