A geo-chemo-mechanical study of a highly polluted marine system (Taranto, Italy) for the enhancement of the conceptual site model

The paper presents the results of the analysis of the geo-chemo-mechanical data gathered through an innovative multidisciplinary investigation campaign in the Mar Piccolo basin, a heavily polluted marine bay aside the town of Taranto (Southern Italy). The basin is part of an area declared at high environmental risk by the Italian government. The cutting-edge approach to the environmental characterization of the site was promoted by the Special Commissioner for urgent measures of reclamation, environmental improvements and redevelopment of Taranto and involved experts from several research fields, who cooperated to gather a new insight into the origin, distribution, mobility and fate of the contaminants within the basin. The investigation campaign was designed to implement advanced research methodologies and testing strategies. Differently from traditional investigation campaigns, aimed solely at the assessment of the contamination state within sediments lying in the top layers, the new campaign provided an interpretation of the geo-chemo-mechanical properties and state of the sediments forming the deposit at the seafloor. The integrated, multidisciplinary and holistic approach, that considered geotechnical engineering, electrical and electronical engineering, geological, sedimentological, mineralogical, hydraulic engineering, hydrological, chemical, geochemical, biological fields, supported a comprehensive understanding of the influence of the contamination on the hydro-mechanical properties of the sediments, which need to be accounted for in the selection and design of the risk mitigation measures. The findings of the research represent the input ingredients of the conceptual model of the site, premise to model the evolutionary contamination scenarios within the basin, of guidance for the environmental risk management. The study testifies the importance of the cooperative approach among researchers of different fields to fulfil the interpretation of complex polluted eco-systems.


Overview of the geological and environmental features of the Mar Piccolo I Bay
Geological and environmental setting. Taranto occurs about the eastern border of the Bradano Trough, just west of the Apulian Foreland (Fig. 3). In the study area, the Mesozoic carbonate basement of the Apulian Foreland underlies the Bradanic succession, formed of Plio-Pleistocene transgressive deposits. In turn, the succession is covered by Pleistocene to recent fine-grained sediments, of alluvial, to transitional, to marine origin 4,5,[25][26][27][28][29] . The geological formations outcropping in the study are shown in the geological map reported in Fig. 3.
Based on geophysical data 30 , reported that, at the Mar Piccolo sea floor, a thick stratum of soft Holocene sediments overlies the Pio-Pleistocene marine succession of the Bradano Trough ( Fig. 4) 5,[26][27][28][29][31][32][33][34][35][36] . Such Pio-Pleistocene marine succession is formed of the Gravina Calcarenite (GRA, Plio-Pleistocene), overlain by the Sub-Apennine Clays (ASP, Pleistocene). GRA is a shallow water bioclastic calcarenite, of medium to high permeability. The Sub-Apennine Clays (ASP; Fig. 3c), which outcrop extensively in the Bradano Trough, are very stiff silty clays, rich in calcareous fossils. The upper part of this formation is highly weathered, due to physical-chemical processes 31,37 and richer in sand levels. The ASP overly the Altamura Limestone (CA, Late Cretaceous), which is from calcareous to dolomitic and from medium to fine-grained, and it is permeable due to karst dissolution phenomena. Its top deepens from the northern (on average 20 m below sea floor, b.s.f.) to the southern part of the Mar Piccolo I Bay (reaching 80 m b.s.f.; Fig. 4). In both the Mar Piccolo Bays, alluvial and marine clayeysilty sediments deposited during the Late Pleistocene and the Holocene above the ASP. These are very soft at the sea floor 35 .
The morphology of both the Mar Piccolo bays resulted from Late Pleistocene to Holocene erosion processes alternating with sedimentation in marine-coastal, to river, to lagoonal and continental a deposition environments 4,34,38 . During the Last Glacial Maximum (MIS2 about 20 ka BP), the two bays were location of river erosion, before a new start of sedimentation occurred since the last Holocene marine transgression till present 4,5,29,39 . In recent times, also anthropogenic actions have recurrently caused local disturbance and remoulding of the very top soft sediments, affecting their sequence, especially in the southern part of the I Bay.
The ASP are characterized by a low coefficient of saturated permeability, i.e. k ≅ 10 -10 m/s at the field scale 40,41 and may underlie coarser soil deposits inland (e.g. Marine Terraced Deposit, MTD in Fig. 3 www.nature.com/scientificreports/ phreatic shallow aquifers. At the same time, given their very low permeability, the ASP confine the groundwater flowing within the deep karst limestone aquifer, hosted in the CA platform. Such aquifer is recharged by the rainfalls infiltrating in the CA outcroppings inland and it is subjected to seawater intrusion in the coastal areas 42 . In the Taranto area, the piezometric heads in such deep karst aquifer drop from about 8 m above the sea floor (a.s.f.) at 10 km distance from the coastline, to about 1 m a.s.f. in the Mar Piccolo area (Fig. 4a) 30,43 As shown in Fig. 4a, within such aquifer the freshwater flows towards the Mar Piccolo, where it is discharged through submarine springs (Figs. 1, 2, 3, 4) called 'citri' , which occur at the seafloor where the ASP were eroded. This is, for example, the case of the submarine spring 'citro Galeso' shown in Fig. 4b 10,25 . According to the investigation commissioned by the Special Commissioner, the average flow over the year of 0.75 m 3 /s is measured at citro Galeso, whereas an average flow of 0.35 m 3 /s is found at citro Citrello (Figs. 2 and 4) 3,36,44,45 . Several other minor submarine springs occur in the I Bay, mostly in the north-eastern portion. In addition, various small tributary rivers discharge water in both the I and II Bays, through catchment areas location of either CA, GRA or ASP outcroppings. The Galeso river is the most important one discharging water in the I Bay, with a mean flow of 50,000 m 3 /day 3,36,45 , whereas the Canale D' Aiedda-Leverano D' Aquino provides the biggest river discharge in the II Bay ( Fig. 3) 6,29,36 . Since solely the I Bay is connected to the open sea through two channels (Figs. 1 and 2), on the whole the Mar Piccolo can be considered a sheltered sea of limited water circulation, where the tidal excursions do not (*) sites equipped by piezocone tests. The map was obtained by co-authors through QGIS-software (version 3.14.16); https ://eur03 .safel inks.prote ction .outlo ok.com/?url=https %3A%2F%2Fwww .qgis.org%2Fen%2Fsit e%2F&amp;data=04%7C01%7C%7Ca2e ff60f e9454 19600 8a08d 8b0b2 0556%7C5b4 06aab a1f14 f13a7 aadd5 73da3 d332%7C0%7C0%7C637 45362 59383 25668 %7CUnk nown%7CTWF pbGZs b3d8e yJWIj oiMC4 wLjAw MDAiL CJQIj oiV2l uMzIi LCJBT iI6Ik 1haWw iLCJX VCI6M n0%3D%7C100 0&amp;sdata =J6w9D Tllpa V5Eyn faYk%2B2iG RAyDA imzXd Oh6u1 0%2FXJQ %3D&amp;reser ved=0), license Creative Commons. Attribution-Share Alike 3.0 licence (CC BY-SA) integrated with ESRI World Imagery. The onset of contamination. Since the second half of the XIX century, the town of Taranto and its coastline have become location of an intense industrialization. In particular, since 1889 the I Bay has hosted the larg-   1 and 2). Furthermore, just west of the I Bay, a major oil refinery, ENI, and one of the largest cement and concrete plants in Southern Italy, CEMENTIR, have been operating since the second half of the XX century ( Fig. 1). Lastly, since 1965 one of the biggest steel factories in Europe, named ILVA, has been active just 1.5 km north-west of the I Bay, pumping thousands of cubic metres of water off its north-western shore. On the whole, the industrial, naval and urban activities have caused huge discharge of contaminants in the basin 13,15,51,52 , due to the scarcity of appropriate sewage treatments. Furthermore, chemicals deriving from the agricultural farming in the surroundings of the Mar Piccolo have been for long released in both the bays 3 . In particular, 14 uncontrolled sewage pipes discharging waste liquids in the basin have been identified during the recent investigations coordinated by the Special Commissioner.
Major chemical characterization campaigns were conducted in the basin between 2004 and 2013 16,24 . Their results revealed a high level of contamination for the Mar Piccolo sea water and sediments 51,52 . The map in Fig. 5 shows the distribution of both the organic and the inorganic contaminants recorded solely within the shallow sediments of the northern and central portions of the I Bay, by depths ranging from 0.5 to 3 m, in the campaign carried out in 2010 by the Italian Institute for Environmental Protection and Research (ISPRA), during which the southern portion of the bay was not explored. The contaminant concentrations have been mapped in the figure accounting for the corresponding threshold values set by the different regulations. In particular, the contaminant thresholds set by the regulations will be distinguished as follows: the threshold set for the Taranto Site of National Interest 53 will be recalled as yellow; the threshold set for all the Italian industrial sites (National Environmental Legislative Decree 54 , not applying to TPH) as red; the threshold of Total Petroleum Hydrocarbons set within the National law 55 as violet.
The mapping in Fig. 5 concerns the average contaminant concentrations logged in 2010 within the first 0.5 m depth at each of the investigation sites. Each coloured hatch is indicative of the degree of contamination for each contaminant, according to the figure legend. A uniform hatch covers multiple nearby sites when these were location of similar contamination degree. Conversely, when the contamination degree at a given site resulted different from that of the surrounding sites, a given hatch covers only a small area around the site. Such mapping procedure shows that high concentrations of one or more contaminants, either inorganic (Hg, As, Zn, Pb, Cr, Cu, Ni, Cd), or organic (PAHs, PCBs, TPH), exceeding the Taranto Site yellow threshold were present in large areas of the I Bay in 2010. In particular, Hg was the most widespread contaminant, even exceeding the National Environmental Law red threshold in the central portion of the I Bay. These high Hg concentrations were locally logged even down to 1.2 m depth at some sites. Moreover, there was a recurrent concentration above the yellow threshold for Pb, Cu and Zn. For the organic contaminants, concentrations exceeding the yellow threshold were logged for PAHs, in the South-West, Centre-West and North-East part of the I Bay, and for PCBs in the central and the Northern areas. Furthermore, PCBs concentrations exceeding the red threshold were logged in the North-Eastern part. The data testified also the widespread presence of high concentrations of TPH, even The 2010 investigation included also the measurement of the sediment organic content in terms of TOC. TOC values up to 8% were found within the shallow sediments in large part of the basin, as typically of coastal areas and estuarine muds (10%). However, such values are much higher than those typically measured in the shallow sediments of the Adriatic and Ionian Sea (i.e. less than 2%) and in the clay muds lying at the floors of deep seas (1-2%) 56 .
The hazardous contaminants were also found to impact the living species in the Mar Piccolo, since concentrations (Cd, Pb, PCBs and dioxins) exceeding the thresholds set by the European Community Regulation were recorded in fishes and mussels 17 .
A geotechnical characterisation of the soil profiles down to 18 m depth was carried out within a limited portion of the I Bay in 2013 16   www.nature.com/scientificreports/ properties of the sediments occurring down to about 4-6 m depth were found to differ from those expected according to the soil skeleton composition. Such observation has suggested that the significant presence of organic matter and of either organic or inorganic contaminants in the sediments may be prompting coupled chemo-mechanical phenomena in the sediments, which modify their hydro-mechanical properties. Furthermore, the geotechnical investigation provided evidence of the presence of a few metre thick stratum of sediments of slurry consistency at the sea floor, within which remoulding processes could easily cause either the migration of contaminants from the sea floor down to few metre depth, or their resuspension in the water column. Since the 2010 investigation ( Fig. 5) had been strictly dedicated to the measurement of the degree and typology of contamination in the very top layers of the sediment deposit and did not provide indications about the potential migration over time of the contaminants towards the different environmental sectors of the system (i.e. the water column, the deeper sediment strata, the aquifers and the living species), the Special Commissioner issued the execution of the above cited new investigation campaign in 2017. Such campaign was planned to be multidisciplinary and to explore the whole sediment deposit (down to large depths) and its boundary conditions, in order to widen the knowledge about the distribution, mobility and availability of the contaminants across the soil deposit in the I Bay and, hence, reach the goals (i) to (iv) cited in the introduction section.

The new multidisciplinary investigation
The off-shore sampling campaign. The campaign was meant to investigate the sedimentological, geochemical, mineralogical and geotechnical properties of the I Bay seabed soil deposit, as well as the contaminant concentrations down to large depth. To start with, a geophysical survey of the basin was carried out before sampling, in order to achieve high resolution seismic profiles indicative of the variability of the sediments across the deposit and to steer the distribution of the sampling sites in I Bay. The geophysical survey allowed also to feature out the top of the ASP formation 29 , found to range between 5 m depth in the north of the I Bay (in land sampling site, named "Tes1", Fig. 2), up to more than 40 m depth in the southern part. The ASP were found to be locally absent in some northern portions of the I Bay, due to erosion.
Based on the geophysical survey results, 19 continuous coring sites were selected (Figs. 2 and 5). One of the main concerns was to take high quality sediment samples complying with the different testing standards holding in the different investigation fields. To this aim, the sampling techniques ( Supplementary Fig. S2) and the sample storage were designed on purpose. Different samples were assigned to the different laboratories according to the outlined shown in Supplementary Fig. S3. For each of the sites, continuous coring was carried out down two twin boreholes, named A and B ( Supplementary Fig. S3), 10 m far from each other. The sediments collected from each A borehole were tested in the environmental technology laboratory to investigate the effects of different remediation technologies for the contaminated sediments, after characterizing their contamination [59][60][61] . The sediments taken down each B borehole were used for the other tests. In particular, the continuous coring down the B boreholes was interrupted at given depths to retrieve undisturbed geotechnical samples 62 . In addition, at each site one undisturbed geotechnical sample was retrieved within the shallowest sediments by scuba divers (short borehole M in Supplementary Fig. S3), using on-purpose designed samplers ( Supplementary Fig. S2).
The continuous coring machine was installed on an offshore platform ( Supplementary Fig. S2), fixed in place through four poles of 40 cm diameter, stuck within the shallowest seabed sediments. Polycarbonate liners of 1.5 m long (chosen to avoid cross contamination phenomena) were used to sample the sediments subjected to sedimentological, geochemical, mineralogical and chemical tests. When brought to the laboratory, the liners were cut longitudinally in two parts; one portion (red colour in Supplementary Fig. S3a,b) was then used to retrieve the chemical samples, whereas the other portion (green colour in Supplementary Fig. S3a,b in SI) was used for the other tests. The undisturbed geotechnical samples were, instead, collected by means of samplers selected according to the sediment depth and consistency. Within the least consistent shallow sediments, of very high liquidity index 35,63-65 , thin-walled tube samplers (made of transparent polycarbonate), were pushed into the soil by the scuba diver (M boreholes in Supplementary Fig. S2a), whereas the sediments of medium consistency, at depths larger than 1.5 m, were sampled using the Osterberg hydraulic piston sampler. The length of the boreholes varied from 11.6 to 45.5 m; the bottom of the boreholes was always located at 1.5-3 m depth below the top of either the ASP or of the GRA .
Sample storage, testing programmes and methods. Soon after sampling, the liners and the geotechnical samples were stored in a fridge on the off-shore platform, at + 4 °C, and then transferred to the laboratories. There, the liners were split as outlined above and stored at − 1 °C, for the geochemical, sedimentological and mineralogic tests, and at − 20 °C for the chemical tests.
Soon after extruding the sediment from either the liner or the geotechnical sampler, both pH and redox potential were measured. In addition, the organic matter content, OM, was determined for the sediments retrieved along the whole B boreholes, down to the ASP formation. For the chemical tests, the samples by 1.5 m depth were split in three sub-samples of 0.5 m height. The three sub-samples were mixed to form a representative homogenised and composite sample, which was subjected to the chemical testing. The chemical analyses were carried out also on small portions of the undisturbed geotechnical samples. The geotechnical samples were stored in the fridge in the Geotechnical laboratory.
The geological and sedimentological analyses were based on visual inspection of the sediment cores. The core description was performed focusing on the lithology, colour (Munsell soil colour chart), sedimentary structure, biological content (type and concentration of shells and organic material). High resolution photos were taken for each 20 cm long core, using a camera set in a fixed position, with an overlap of about 25% between subsequent photos. The results were recorded in stratigraphic logs, which were, thereafter, compared with the results of the www.nature.com/scientificreports/ geophysical investigations cited before, and with stratigraphic data from the literature 29,39 , in order to assess the geometry and the distribution of the sedimentary units across the deposit, and to interpret their geological origin. The geotechnical testing programme included the characterization of: the soil granulometry, the geotechnical index properties, indicative of either the soil composition (Atterberg limits: liquid (w L ) and plastic (w P ) limits and soil specific gravity, G s ), or the soil state (soil unit weight, γ, water content, w 0 , void ratio, e, degree of saturation, S r ), the mechanical and the hydraulic parameters, such as the shear strength parameters, c′ and ϕ′, the deformability parameters, the undrained shear strength, S u , and the coefficient of saturated permeability, k. In particular, S u was determined on the off-shore platform through pocket penetrometer tests on both the top and the bottom bases of the soil cores, when still in the liners, in order to assess the class of consistency of the cored sediments (Table 1) 66,67 . Furthermore, S u was measured also through in-situ piezocone tests, performed at ten of the coring sites (reported with * in Fig. 2). All the tests were carried out according to the standard procedures set by American Standards Test Methods (ASTM) and British Standards (BS), adapted to take account of the presence of brackish fluid and contaminants in the soil, as well as of fragments of shells and fossils 58,68,69 . An integrated system (hardware and software), named GeoLab, for the geotechnical measurements was also developed 70 in order to increase speed, accuracy, and productivity during testing. The developed software ensured the communication with different platforms used for data acquisition and allowed to follow remotely the testing results minimizing the exposition of the operators to the contaminated sediments.
The geotechnical characterisation of the sediments was aimed at the definition of the geotechnical model of the basin (first conceptual, quantitative thereafter), of use for the prediction of: the sediment susceptibility to either remoulding or resuspension; the hydrodynamic dispersion of the contaminants through the sediments (by means of numerical modelling); the sediment settlements for different possible remediation measures (by means of numerical modelling). In the following, though, the discussion will concern solely those soil features and parameters useful to: define the conceptual geotechnical model of the system; foresee the attitude to remoulding and resuspension of the sediments; foresee the attitude of the contaminants to undergo advective mobility through the deposit; estimate the qualitative features of the sediment responses to alternative remediation strategies, such as dredging and capping. To these aims, the discussion will deal solely with the sediment granulometries, index properties, coefficients of permeability and undrained shear strengths. The discussion of all the other geotechnical parameters, together with both the numerical modelling of the hydrodynamic dispersion of the contaminants through the sediments and the modelling of the geotechnical response of the system to different remediation interventions, will be covered in a subsequent paper, given the space that the presentation of these modelling applications and results requires.
The chemical tests were addressed to the determination of the concentrations in the sediments of those contaminants found to be of high concern in the previous investigations 13,15,24,51,52 . In particular, the concentrations of the metals and metalloids: Al, As, Cd, Cr, Cu, Fe, Hg, Mn, Ni, Sb, Ni, Pb, and V, of the persistent organic pollutants: PAHs, PCBs, PCDD/PCDFs, VOCs, TPH, polybrominated diphenyl ethers (PBDEs), organotin (OTs) and organo-chlorinated pesticides, and of the organic matter (OM), were measured, according to the appropriate standardized procedures. EPA methods 3052 (ICP/MS) 71 and 8270D (GC/MS) 72 were used for metals and PAHs, respectively. The concentrations of PCBs, PCDD/PCDFs and PBDEs were measured according to the methods CEN 15308 73 , EPA1613B 73 and UNIEN16377 74 respectively, and the tests were performed through gas chromatography/tandem mass spectrometry (GC/MS-MS). VOCs were determined according to EPA8015D method 75 , while for TPH (C 12 -C 40 ) the method was ISPRA 75/2011 (GC-FID) 76 . Pesticides were measured according to Environmental Protection Agency (EPA) Method 8270D (GC/MS/MS) 72 , and OTs were determined through ICRAM 2001 77 . Finally, the OM contents were characterized through the Loss On Ignition method (LOI, i.e. combustion in an unheated muffle furnace, according to EPA 160.4 78 ). In the following, the discussion about the chemical data will focus only on the concentration of nine trace elements (As, Cd, Cr, Cu, Hg, Ni, Pb, V and Zn) and of PAHs, (in terms of sum of 16 EPA PAHs congeners), PCBs (in terms of sum of 31 PCB congeners), TPH and OM contents (Supplementary Table S1).
The geochemical analyses were carried out through tests performed soon after opening the liners. The concentrations of As, Cu, Cr, Ni, Pb and Zn were measured using an on-purpose designed Portable X-Ray Fluorescence (PXRF), calibrated according to standards NIST [79][80][81][82] ; furthermore, the magnetic susceptibility was also measured. The results have allowed to correlate the metal concentrations and the magnetic susceptibility with the lithostratigraphic features observed through both the geological and the sedimentological analyses. The geochemical data obtained by PXRF were compared with the metal concentrations obtained using the EPA methods 71 , with the aim at finding evidence of how, for each given metal, the chemical concentration measured as average over a depth interval (EPA methods) derives from the distribution of local metal concentrations across the sediment. www.nature.com/scientificreports/ Mineralogical analyses were carried out by means of X-Ray Powder Diffraction (XRPD) on several geological samples, as well as on the specific sediment intervals where geochemical anomalies had been detected. Handling and treatment of sediment samples for the mineralogical analyses were performed according to the United States Geological Survey (USGS) protocol. The data acquisition and interpretation complied with what reported by Hiller 83,84 . The composition of the clay fraction was analysed in the form of random mount, as well as of oriented mounts, subjected to various treatments (air drying, glycolation, heating at 400 °C and at 550 °C). Figure 6 exemplifies the combination of the results of all the different tests cited above.

Discussion of the results
The Litho-technical characterization of the deposit. The litho-technical characterisation of the sediments has resulted from: the geological inspection of the cores in the liners and of the undisturbed geotechnical samples; the paleogeographic reconstruction of the soil deposition 29,39 ; the soil geotechnical index properties; the geochemical and the mineralogical analyses. Here-forth, Fig. 7a reports the litho-technical section N-N′ whose trace is shown in Fig. 7b.
A First litho-technical unit, hereafter 1 st LTU (light yellow colour in Fig. 7a), of about 1.5 m thickness, has been found to cover the whole deposit. It is formed of either clay with silt, or sandy to slightly sandy silt with clay, deposited in recent times up to present, according to the sedimentology and paleogeographic studies. The corresponding grading curves (Fig. 8) show that its clay fraction, CF, varies in the range 27-53%, its silt fraction, MF, in the range 39-57%, and its sand fraction, SF, is minor, except for site S1, close to the Porta Napoli channel (Fig. 2). It is rich in organic matter and the pocket penetrometer S u data (S u < 20 kPa) prove its largely fluid consistency ( Table 1). The coefficient of permeability, K, measured by oedometer testing on the samples taken by scuba divers within this unit (M boreholes), varies in the range 10 -8 -5 × 10 -9 m/s. Given the very low consistency of the 1 st LTU sediments, it is very likely that these have been either remoulded, or resuspended all way through their history, due to the navigation activities in the south of the I Bay and to the dragging of ship anchors (Figs. 1 and 2). Furthermore, Mastronuzzi et al. 7 provided evidence of the occurrence of important flooding events in the last two centuries (1883, 1996 and 2005), causing resuspension and redeposition of the 1st LTU sediments. Given so, the exact age of the sediments within this unit cannot be assessed.
The underlying Second litho-technical unit, hereafter 2nd LTU (light orange colour in Fig. 7a), is on average 6 m thick and it is formed of grey-coloured sandy or clayey-sandy silt, or clay with silt, of consistency varying from fluid to soft (S u = < 40 kPa) and permeability, K, on average about 10 -9 m/s, if a sand layer occurring in the western part of the N-N′ section is excluded (Fig. 7a). Accordingly, the grading curves of this unit show that SF ranges between 3 and 24%, whereas CF and MF vary in the intervals 22-39% and 43-59%, respectively For this sand level, from medium-dense to loose, an average K = 10 -6 m/s has been measured by means of permeameter testing 85 . According to both the paleo-geographic and the sedimentological analyses, also the 2nd LTU sediments, slightly coarser than the 1 st LTU sediments, are either present day or recent and derive from the erosion of calcarenites and parent coastal-alluvial, or marine formations in-land. At larger depths, several boreholes cross a Third litho-technical unit, hereafter 3rd LTU (light to dark blue colours in Fig. 7a). This is formed of silt, clayey silt or silty clays (Fig. 8), including local sandier levels, gravel levels and peat. It is of lower permeability than the overlaying units (K = 10 -9 -5 × 10 -10 m/s) and includes different sub-units. The lower portion of the 3rd LTU is interpreted to be the result of a high energy fluvial deposition, most probably occurred during the Last Glacial Maximum (33-14 ka) 86 . Above the fluvial sediments, the recognition of levels of peat with pulmonated gastropods suggests that part of this unit was deposited within a transient continental environment, most likely at the beginning of the Holocene (11-10 ka). This part of the 3rd LTU passes gradually to upper silts and clays containing lagoonal shells, which mark the beginning of the marine transgression, within a sheltered marine environment. About 9 ka years ago, this sheltered marine basin was affected by the fall of volcanic ashes, deriving from the Pomici di Mercato eruption of Vesuvius. As a result of such event, a whitish porous tephra layer, 5 to 40 cm thick, is found to occur locally within this unit, at about 19-21 m depth, as confirmed by the results of PXRF analyses discussed later. (2) top of the calcareous bedrock according to 30 (3) bathymetry (Port authority 1947-1978); (4) significant content of organic matter; (5) fishing net (anthropogenic material); (6) coastline; (7) stratigraphic contact; (8) 1stLTU; (9) 2ndLTU, of consistency from very soft to soft and occasional presence of sand or silty sand, from very loose to loose (a); (10) 3 rd LTU, of consistency increasing with depth, from very soft to soft (a), from soft to firm (b), firm (c), stiff (d) 66,67 , and occasional layers rich in sand (e), gravel (f) and peaty levels (g); (11) Possible disturbed top layers of the ASP formation; (12) ASP formation, with clayey silt or silty clay of very stiff consistency, and sandy levels (S u = 200-500 kPa) (a), or Grey-bluish marly-silty clay (S u > 500 kPa) (b).  Fig. 7a; S u < 20 kPa; Table 1), and higher at medium depth, in the soft second sub-unit (blue in Fig. 7a; S u = 20-40 kPa; Table 1). Underneath, the sub-unit shown as dark blue in Fig. 7a is of firm consistency, i.e. S u = 40-75 kPa, and overlays the deepest stiffest sub-unit, i.e. S u = 75-150 kPa (Table 1), shown in cobalt blue. Boreholes S6 and S7 (Fig. 7a) do not cross the 3rd LTU, since the 2nd LTU overlies directly the ASP.
The ASP have been found at the bottom of all the boreholes in the section (Fig. 7a). They are grey-bluish silty clays (Fig. 8), from very stiff (S u = 150-300 kPa, light grey, Table 1) to hard (S u > 300 kPa, dark grey, Table 1), of low permeability (k = 10 -11 m/s laboratory measurement, k = 10 -10 m/s field measurements). Their top is deepest in the southern part of the I Bay and deepens to the west of the section (Fig. 7a) 29 . The irregular surface of the ASP top represents the result of river erosion during the Last Glacial Maximum, before the deposition of the overlying units 29,39,87 . Neither the GRA, or the CA (Figs. 3 and 4) were reached in the drilling operations along section N-N′; the top of CA reported in Fig. 7a (dashed green line) is that inferred by 30 through geophysical surveys.
The XRPD analyses, carried out on the S1 to S7 corings have shown that all the sediments have similar mineralogical composition to the ASP clays 41,88-92 . As an example, Supplementary Fig. S4 illustrates the diffraction XRPD patterns for samples from borehole S2. The main mineralogical phases detected in the analyses are: clay minerals, quartz, carbonates (mainly calcite and aragonite), plagioclase and feldspar. Minor phases also occur, but they are not ubiquitous and are not homogeneously distributed within the sediments. The vertical profiles of mineral content in Fig. 9 testify an increase of clay mineral content with depth. This ranges from 24 to 40% in the 1st LTU, and from 40 to 60% in the 3rd LTU. In the 2nd LTU, the clay mineral content reaches an out of trend peak of 56% in a sample at 5 m depth b.s.f. in borehole S1, where an amorphous component is abundant and impacts the accuracy of the determination. The clay minerals are a mixture of illite, chlorite, kaolinite and inter-stratified illite-smectite phases (I-S); the smectite minerals are not detectable in the diffraction patterns as a single mineralogical phase.
The pore water salinity has been found to range between 30 g/L and 36 g/L within the 1 st LTU, and to be about 32 g/L within the deep sediments of the 3rd LTU 58 . Therefore, the salinity of the pore water in th Holocene sediments remains high with increasing depth.
The limited variability in both the granulometry and the mineralogy of the sediments forming the different units, except for the coarser sediments locally interbedded in confined levels, is not consistent with the significant differences among the values of some geotechnical index properties recorded for the different units. This is the case for the values of the liquid limit, w L 93 , the plasticity index, PI (PI = (w0 − wP)) 93 and the activity index, A = PI/CF, measured for the 1st LTU and 2nd LTU on one side, and those characterizing both the 3rd LTU and the ASP, on the other. Such differences are evident in Fig. 10, reporting the data in the Casagrande plasticity chart (Fig. 10a) and the activity chart (where PI is plotted versus the clay fraction CF, to characterize the activity index A; Fig. 10b). It is worth remarking that since w L , PI and A are the geotechnical indices most closely related to the amount and mineralogy of the clay fraction, CF, and to the pore water salinity, their values should vary little among the 1st to the 3rd LTUs and the ASP. Conversely, the samples belonging to the 1st LTU are characterised by values of w L (70-113%), PI (35-66%) and A (> 1.1) much higher than those recorded for the 3rd LTU and  For some of the geotechnical properties, Fig. 12 reports the profiles grouped together in a single plot, as well as a plot of all the OM content profiles. It is evident the ubiquitous presence in the section of a top layer, up to 2 m thick, which roughly corresponds to the 1st LTU, where the sediments are of highest liquid limit (w L = 70-113%), plasticity index (PI = 35-66%) and activity index (A up to 2.5), and lowest soil specific gravity (G s = 2.54-2.66), much lower than that typical for the clay minerals forming the skeleton of the sediments within this layer (e.g. G s = 2.75-2.78 for montmorillonite, G s = 2.74 for illite and G s = 2.62-2.66 for kaolinite) 94 . The low specific gravity is recognisably due, at least in part, to the major OM content in this layer, 15-18% (Fig. 14e), much higher than that usually measured in the sediments at the sea floor of open marine basins 56 . In addition, the soils in this top layer have the highest void ratio, e 0 = 2.12-3.98 (Fig. 12f) and water content, w 0 = 72-157% (Fig. 12g), even higher than the corresponding liquid limit (w L ). Hence, their liquidity index LI: is higher than 1 (Fig. 12h), as confirmed by the very low S u values (Fig. 11). Therefore, the combined analysis of the data in Fig. 11 demonstrates that the very soft 1st LTU (S u < 20 kPa), in the interface between the sediment deposit and the seawater column has geotechnical properties highly affected by the high OM content and, possibly, by the presence of the significant contaminant concentrations discussed in the following and leads to suppose that it is highly prone to remixing and resuspension.
Within the 1st LTU, only at site S1 the shallowest sample, which is of higher sand fraction (SF = 29.4%), has lower organic content (OM = 8-12% in Fig. 12e) than all of the other shallow samples in the section. This is likely to be due to the hydrodynamic conditions of the channel area where the site is located 95,96 . In addition, the ratio of organic carbon to organic nitrogen, C org /N tot , measured in the I Bay in previous studies 24,97 , reaches very high values close to both the sampling sites S1 and S2 (15 < C org /N tot < 65), suggesting that the organic matter in the channel area is largely allochthonous 98 .
As to the physical-chemical properties, the sediments within the 1st LTU are characterised by neutral to slightly alkaline conditions, with pH values in the range 7.3-8.3 (Fig. 13b), and Eh in the range from − 400 to − 200 mV (Fig. 13c), indicative of a far more reducing environment with respect to that at the sea floor in both the Adriatic and the Ionian Sea, where positive Eh values are measured 56 . The negative redox potential reveals a high rate of oxygen consumption in the I Bay, even before sediment deposition, which is likely to be due to aerobic microbial-mediated redox-processes, that can reduce the redox potential over − 300 mV and cause the total depletion of oxygen, used as terminal electron acceptor. After deposition, anaerobic mediated www.nature.com/scientificreports/ redox-processes are activated, such as anaerobic sulphate reduction (SO 4 2− to S 2− ) or anaerobic methanogenesis, this latter converting the CO 2 produced by the mineralisation of the organic matter, into CH 4 .
The chemical data (Supplementary Table S1) plotted in Fig. 11 reveal that in the 1 st LTU, down to 1.5 m depth, some of the contaminants, either organic or inorganic, exceed either the red or the yellow thresholds cited above. In 4 of the 6 sampling sites along the section, S2, S3, S4 and S6, the concentration of metals Hg, As, Pb, Cu and Zn, either approach or exceed the Taranto Site (yellow) threshold (Fig. 11, Supplementary Table S2). In addition, values of Hg above the National Environmental Law (red) threshold are recorded at both sites S3 and S6, in the  High metal concentrations are also recorded between 0.5 m and 1.5 m depth, although lower than in the top 0.5 m depth (Fig. 11). Unlike the other metals, the concentrations of Cr, Ni and V are well below the Taranto Site (yellow) threshold (Fig. 11) in the whole 1st LTU. The highest concentrations of As, Pb, Zn and Cu were recorded to occur within the top 0.5 m sediment layer of the 1st LTU also through the PRXF technique (Fig. 14). Since this technique detects the concentrations within spots of 6 mm diameter, very high concentrations of some metals were detected through its application also down to 4.0-5.0 m depth, as shown in Fig. 14.
Within the first 0.5 m stratum of the 1st LTU, the highest concentrations of the organic contaminants were recorded too (e.g. the overall sum of either the congeners of PCBs, or the congeners of PAHs), found to exceed even the National Environmental Law (red) threshold at several sites (Fig. 11). In particular, the highest values were recorded at S2 for ΣPAH 16EPA (9775 ppb), at S6 for ΣPCB 31 (6828 ppb), and at sites S2, S4, S6, S3 for TPH (750 ppm; Fig. 11). The concentration of the organic contaminants reduces below 0.5 m depth.
In the 2nd LTU and 3rd LTU, both the geotechnical indices PI and A reduce with increasing depth (Figs. 11 and 12), tending to the values typical for the Pleistocene ASP clays 33,41,99,100 . Also the void ratio and the liquidity index decrease, due to both the compression of the sediments under burial and the reduction in content of either the OM or the contaminants.
However, in the 2nd LTU the soil void ratio, the liquidity index and he OM content of the fine sediments may be still quite high by 5 m depth (Figs. 11 and 12). As expected 97,101 , a lower OM content is recorded in the sandy layer in the western part of the section (OM = 2.5%), which is, therefore, less capable to trap the contaminants, with respect to the surrounding finer soils. As to the physicochemical properties, in the 2nd LTU the pH values (Fig. 15b) change among the different sites, although they keep being from neutral and slightly alkaline in the eastern sites, S6 and S7, and tend to increase in the sandy layer intercepted in the western sites. The negative www.nature.com/scientificreports/ Eh values (Fig. 13c) of the 2nd LTU suggest that the process of degradation of the organic matter occurs under anaerobic conditions in both the first two units. Contamination is heterogeneous within the 2nd LTU. A significant decrease in concentration of the metals with respect to the 1st LTU is detected, except for few local high values. For example, at about 2.25 m depth at sites S1, S2 and S3, Hg concentrations above the Taranto Site threshold (yellow) have been recorded. In addition, concentrations exceeding the Taranto Site threshold are recorded for: Zn at 2.5 m depth at site S3; Pb, down to 4 m depth at both sites S1 and S3; Cu, at 6 m depth at site S2. Furthermore, high concentrations of PAHs and PCBs have been recorded at sites S1 and S3, even exceeding the Taranto Site threshold (e.g. ΣPAH 16EPA ≅ 4592 ppb at 2.25 m b.s.f. in S1; ΣPCB 31 > 190 ppb at 3.55 m b.s.f. in S6). In addition, in S3, at 2.25 m b.s.f. the concentration of TPH (> 750 ppm) exceeds the National Environmental Law threshold.
The 3rd LTU, from 10 to 40 m b.s.f. (Figs. 7a and 11), is characterised by a low variability of the geotechnical index properties indicative of the soil composition ( Fig. 11; average values: w LAV = 50%, PI AV = 28%, A AV = 0.65 and G sAV = 2.67), which are all very close to the values characterizing the ASP clays. The void ratio reduces with increasing depth (e 0 = 1.38-0.88), in a way compatible with the compression of the soil under burial (LI < 1; Fig. 12h). However, the OM content recorded in the soft to firm portion of the 3rd LTU (i.e. down to 35 m depth; light blue in Fig. 12e) is still quite high (OM = 6-12%), whereas it reduces to 5-6% only in the deepest firm portion (blue in Fig. 12e). Therefore, on the whole, the OM profile (Fig. 12e) suggests that the decomposition of the organic matter buried within the 3rd LTU has been anaerobic and so slow as to preserve high OM contents at depth long time after deposition. Such hypothesis is validated by the measured values of the redox potential and the pH in this unit, which are still typical of a reducing environment in large part of the unit (− 100 mV; Fig. 13), and tend to zero, or to positive values, only when the sediment becomes firm at large depth (blue in Fig. 13c). The high OM content of this unit justifies the G s values slightly lower than those typical for the ASP clays (for ASP G s = 2.73 33,41,99,100 ).
In the 3rd LTU, though, the concentrations of the metals and of the organic contaminants are very low at all sites (Fig. 13), with the only exceptions of the lithogenic metals Cr, Ni, Zn and V, whose concentrations keep being high in most part of the unit. At sites S1 and S2, both Cr and Ni even exceed the Taranto Site threshold, from 11 m b.s.f. and 17 m b.s.f. downwards, respectively (Fig. 11). Also the PRXF profiles (Fig. 14) indicate that the concentration of either Ni or Cr increases at the depth of transition between the 2nd LTU and the 3rd LTU. They also show an excess of As, Pb and Zn down boreholes S1, S2 and S3, at about 19-21 m depth, probably relating to the presence of the thin layer of volcanic soils (tephra) recognized through the geological analyses.
Finally, the samples belonging to the ASP formation (Figs. 11 and 12) are characterised by values of the index properties (i.e. w LAV= 48%, PI AV = 24%, A AV = 0.66, G sAV = 2.71: w 0AV = 27%, e AV = 0.74, LI AV = 0.15) consistent with the average values usually measured for such clays either in land, or at depth below the Mar Grande seafloor 32,33,41,99,100,102 . The OM content of the ASP clays is lower than that of the overlying units (OM = 6-8%), irrespective of depth (Fig. 12e) and the redox potential becomes positive (i.e. about 50 mV, Fig. 13c). As in the 3rd LTU, high concentrations of Cr, Ni, and V are recorded in the ASP clays, which demonstrate that the content of such metals recorded in both the 3rd LTU and the ASP clays is part of the skeleton of the sediments in these units, as further discussed in the following.

The conceptual site model
The data presented above allow for an insight into the relations between the distribution of the OM content and of the contaminants, either inorganic or organic, across the I Bay seafloor deposit, and the variability in geotechnical properties of the soils. Such insight allows for the construction of the conceptual model of the contaminated system, which represents the tool for the prediction of the fate and mobility of the contaminants and the assessment of the contamination hazard. The conceptual site model refers here to the southern part of the Mar Piccolo I Bay, but a three-dimensional conceptual model of the whole I Bay will be covered in a following paper according to the same methodology presented in this paper.
As recognized before, the variability in sediment granulometry and state parameters (w 0 , e 0 , LI) reduces with increasing depth, since the soil geotechnical properties get close to those typical for the ASP clays by the mid depth of the 2nd LTU. The OM is high in the 1st LTU and in the top portion of the 2nd LTU, but it reduces progressively with increasing depth, to become quite constant in the 3rd LTU. The concentrations of the organic contaminants and of several metals, such as Hg, As, Pb, Cu, Zn, are highest in the 1st LTU and remain locally high also in the 2nd LTU. Conversely, Cr, Ni and V are found to increase in both the 3rd LTU and the ASP clays.
The high levels of OM, together with the high concentration of either the organic or the inorganic contaminants in the 1st LTU, appear to activate coupled chemo-mechanical processes which confer to the sediments in this unit values of the geotechnical properties indicative of the soil composition which are not consistent with the soil granulometry and mineralogy. High OM content appears to correspond to higher values of w L , PI and A, and lower values of G s, with respect to those applying to the corresponding inorganic soil (Figs. 17, 18), as already observed in the literature for other organic soils [103][104][105] .
The high OM content in the 1st LTU and in some portions of the 2nd LTU is likely to be partly the remain of peat brought in the basin during Holocene transgression stages, as usual in coastal environments. Otherwise, it may be effect of either the biogenic accumulation of "algal carpets" at the sea bottom 29,39 , or the long-lasting mussel farming activities carried out in the bay. The recognition that the very high w 0 and LI values within the top sediments are effect of coupled chemo-mechanical processes is confirmed by the observation that such sediments are not of the liquid consistency expected for uncontaminated soils 106 . This is why these sediments could be sampled, irrespective of their high w 0 , and exhibited a higher cohesion, which is typically conferred to soils by high OM contents 107 . Nonetheless, as consequence of the high LI values, the 1st LTU sediments and those in the upper part of the 2nd LTU are prone to be remoulded and resuspended when impacted by the dragging of ship www.nature.com/scientificreports/ anchors and the navigation activities. Such finding suggests that the recorded local presence of contaminants at depths as high as 2.25-3 m may have been caused by the sediment remoulding. Furthermore, the persistence of a low consistency of the sediments down to depths higher than usually expected (e.g. 4-5 m depth), is recognized to be a finding of great interest in the design of the contamination remedial measures, since it poses additional challenges to the success of either standard capping measures, or traditional dredging activities.
The measured values of Eh and pH within both the 1st LTU and the 2nd LTU are typical of a reducing environment and suggest that the organic matter buried during the building up of such units has undergone a slow anaerobic degradation, leaving significant part of the buried OM only partially degraded. This is why the OM keeps being relatively high down to medium-large depths. It is worth recalling that the fate of the OM in the sediments impacts the fate of the contaminants 101,108 , since the degradation of the OM may trigger leaching phenomena and the concurrent release of the contaminants from the sediments to the water column. Figures 15,16,17,18 reveal also the influence of the sediment granulometry on the sediment capacity to trap contaminants 18,20,95,96,101,[108][109][110][111][112] . In particular, the transition from the fine sediments to the coarser ones (more sandy) in in the western part of the 2nd LTU (sites S1 and S2 in the N-N′ section) is clearly characterized not only by an abrupt variation in the sediment geotechnical properties, but also by a reduction in concentration of all the contaminants and in OM content. However, it is worth recalling that in addition to granulometry, also the sediment textural features and mineralogy influence the quantity and the state of the OM and of the contaminants in soils. This is because the clay minerals exhibit a high capacity of contaminant adsorption, through short-range chemical forces, such as ionic or covalent bonding 109,112 . Furthermore, the finest sediments hinder the diffusion of oxygen at depth, thus favour the preservation of the organic matter 101 and, consequently, the absorption of organic particles onto the charged surfaces of the clay minerals. On the other hand, such preserved OM, trapped in the clayey sediment, owns terminal charges OH − and COO − , which can further either complex or adsorb contaminants (insoluble inorganic particles and organic colloids). As a consequence, the combination between the capacity of the clay minerals in the fine sediments to retain contaminants, and that of the significant OM trapped by the clay particles to absorb/complex contaminants, makes the I Bay seafloor sediments prone to   18,97,110,113 . This is confirmed by the high concentration of contaminants found in the 1 st LTU, when either OM or CF increases, and by the depletion of contamination recorded, instead, within the sand stratum in the 2nd LTU (SF = 80%). Needs to be mentioned that just above the sand stratum in the 2nd LTU (Sites S1 and S3), a local peak in contaminant concentration is recorded (2.25-3 m depth), where the concentrations of As, Cd, Cu, PCBs come close to the Taranto Site threshold, whereas Hg, Pb, Zn and PAHs (in terms of ∑PAH 16EPA ), even exceed such threshold and TPH exceeds the National Environmental Law threshold. This peak suggests that mixing processes take place in the low consistency sediments of the 1 st LTU, causing the contamination of the top of the sand level. The occurrence of such mixing represents one of the main causes of the contaminant mobility in the basin, which is here shown to be possibly directed not only towards the water column, but also towards the deeper sediments. This aspect of the site conceptual model is of major impact in the selection of the mitigation strategies, since it implies that the contaminated soil volumes in the basin are larger than the very top 0.5-1 m depth stratum, and may evolve with time, depending on the external actions.
At larger depths, within the fine soils of either the 3rd LTU or the ASP, the concentrations of Cr, Ni, V and Zn are found to be high, even exceeding the Taranto Site threshold (Figs. 11 and 15, 16, 17, 18). Such high concentrations may be justified by the geological origin of the deep sediments, since they derive from the erosion www.nature.com/scientificreports/ of the ASP clays outcropping inland, whose mineralogy has been discussed in "The Litho-technical characterization of the deposit" section. In particular, for the main clay minerals present in these clays, Zn, Ni, V, Cr are generally present in the crystal structure 114 . As such, they are found in the 3rd LTU, which has mainly resulted from the sedimentation of such eroded soils in the late Pleistocene and early Holocene. Therefore, the Taranto Site thresholds 53 for Zn, Ni, Cr should be revised in light of the geological origin of the sediments, i.e. of the site-specific geochemical background. Furthermore, since Zn, Ni, Cr and V are part of the soil skeleton, they are characterised by a lower degree of mobility than the other metals and should not be then considered priority targets in the remediation strategies. It is interesting to notice that, at large depth, also the concentration of Cu and, to a less extent, Cd, follow variations similar to those of Ni, V and Cr, suggesting that, in this case, also these two metals might be of lithogenic origin. The remaining metals, As, Hg, Cd, Pb, and Zn for the top unit, follow a spatial variation pattern which is, in first approximation, similar to that applying to the organic contaminants PCBs, PAHs, TPH; therefore, all these either inorganic or organic contaminants represent priority targets for the remediation strategies 115 . Their concentrations reduce significantly by 2.25-3 m depth. The similarity between the contamination patterns of the metals and of the organic contaminants suggests that they comply with a similar history of deposition, most likely triggered by anthropic activities.
The excessive concentration of PAHs by 3 m depth suggests the prevalent anthropic origin of such hydrocarbons. The molecular ratio among the different PAH congeners and isomers needs to be assessed for an insight into their origin, which may be either petrogenic (fuel loss), or pyrogenic (fuel combustions), or diagenetic (natural 111 ). On the whole, the concurrence of high concentrations of Pb, PAHs and TPH, according to the contaminant profiles in Figs. 13 and 17, 18, suggests that lead-enriched fuel losses may have largely caused the contamination of the sediments, given the intense ship's navigation in front of the Navy area. In addition, organometallic antifouling residues may have been the source of the high concentrations of Cu, Zn, Cd, probably used for ship maintenance in the near shipyard areas 116 . www.nature.com/scientificreports/ The concentration of PCB is high not only in the 1st LTU, but also in local spots down to 2-3 m depth. Given the anthropogenic origin of such contaminants their local presence at such depth may be due to the remixing of the top sediments, where usually these contaminants are found. Finally, it is worth highlighting the low contaminant concentrations recorded at both sites S1 and S7, located close to channel areas. At site S1, the contaminant concentrations in the top layer of the 1 st LTU resulted to be unexpectedly lower than those recorded in the deeper sediment envisaged that the peculiar hydrodynamic conditions applying to the areas where these sites are located impact the distribution of the contaminants, since the finest sediments of the 1st LTU are prone to be resuspended and transported by the marine currents 95,96 .

Conclusions
The multidisciplinary study presented in the paper has proven to be successful in achieving an insight into the distribution of the contaminants in the southern part of the Mar Piccolo I Bay, as well as into their origin, mobility and fate, which provide rational indications about the hazard of the site. Such insight has been pursued thanks to the innovative aspects of the study with respect to traditional characterization approach of the polluted sites and consists in the tight collaboration among researchers of different fields in a join interdisciplinary analysis of the investigation results, as well as the deterministic assessment of the pollution hazard as the main target of the innovative campaign. Given such target, the investigation and testing have been designed to measure, aside the contaminant concentrations, all the data characterizing the factors influencing the contaminant mobility and their fate within the seafloor deposit, essentials in providing knowledge about the current state and the evolution with time of the site pollution. In order to tackle the challenge of characterizing all these factors, the seabed of the site has been explored down to large depths and the boundary conditions of all the chemical and hydro-mechanical processes that may impact the contaminant mobility have been deeply analyzed.
It has been shown that in the southern portion of the I Bay, the concentrations of either organic or inorganic contaminants reach the highest level within the fine sediments of the very shallow layers, that resulted to be the most exposed to the anthropogenic contaminations. Nevertheless significant concentrations are locally recorded at larger depths. The fine sediments that are results of the inland erosion of marine formations and illitic clays (ASP), tend to trap the contaminants not only for their high clay fraction, but also for the significant content of organic matter, revealing that the coupling phenomena between textural features and the cycle of the organic matter within sediment control the distribution of pollutants and their fate. The multidisciplinary research study has demonstrated also that in some cases (i.e. Zn, Ni and Cr), the background contaminant thresholds need to be revised in light of the site-specific geochemical variability of metals having lithogenic origin.
Furthermore, the OM resulted poorly degraded down to few metres depth and, as such, confers to the sediments much higher plasticity and cohesion than expected according to their composition. Consequently, the sediments are fluid at the sediment-water interface, and of very low consistency below, even when buried at few metres, as effect of chemo-mechanical coupled phenomena. It follows that, being prone to undergo remoulding and re-suspension due to external actions, they represent by themselves a predisposing factor for the spatial and vertical migration of pollutants, processes that need to be taken into account in the selection of the proper remediation strategy.
The investigation reveals also that the mobility of the contaminants as effect of an advective upward flow from the deep karst artesian aquifer is likely to be minor, due to the large thickness of sediments of very low permeability covering the artesian aquifer, which is characterized by a piezometric level of 1 m above sea level. Therefore, the hydrogeologic regime of this portion of the bay is not expected to contribute to the site contamination hazard. However, further studies are ongoing to numerical modelling the hydro-dynamic dispersion of the contaminants, by using as input data all the chemo-hydro-mechanical parameters collected in the present investigation. The numerical simulations are entailing also the loading and unloading processes at the base of traditional remedial strategies, like capping or dredging, and account for the ultra-soft consistency of the top sediments that makes such interventions peculiarly challenging.
The several insights into the features and possible sources of the contaminants in the system under study confirm the efficiency of the cooperative approach promoted by the Special Commissioner, that can be largely used by decision makers as premise of the selection of the most sustainable mitigation measures and as support to the risk management process.

Data availability
The datasets generated during the current study are not publicly available and are used through a specific clearance issued by the Special Commissioner for urgent measures of reclamation, environmental improvements and redevelopment of Taranto. The data are available at the reasonable request and with permission of the Special Commissioner for urgent measurements of reclamation, environmental improvements and redevelopment of Taranto.