Can Sediments Contaminated by Mining be a Source of Mercury in the Coastal Environment Due to Dredging? Evidence from Thermo-Desorption and Chemical Speciation

Abstract

The sediments in the Gulf of Trieste (northern Adriatic Sea, Italy) are contaminated by mercury (Hg) due to historic mining which took place in Idrija (Slovenia). Despite many studies having been done regarding Hg, no information is available on the potential impact of dredging required along the main channel approaching the Port of Monfalcone. Sixteen surface sediment samples were collected along the channel to determine both total Hg concentration and chemical species using the thermo-desorption (TD) technique. Six samples were also chosen to apply a selective sequential extraction (SSE). The TD technique showed the maximum Hg release approximately at 260 and 335°C, corresponding to metacinnabar (β-HgS) and cinnabar (α-HgS), respectively. The SSE demonstrated that Hg was mainly associated with poorly soluble or insoluble compounds (98.7%). A resuspension event over a limited period of time can be considered of negligible impact to the water column due to the scarce Hg mobility from sediments.

The Legacy of the Idrija Mine Twenty-Five Years after Closing: Is Mercury in the Water Column of the Gulf of Trieste Still an Environmental Issue?

“…However, this perturbation lasted approximately 24 h, suggesting that it may have been related to technical operations at the mussel farm such as the lowering of a boat's anchor. The Hg concentration in the investigated surface sediments varied between 0.77 (site P1) and 6.39 (site P6) µg g −1 and the grain-size composition was consistent with previous research focused on the main channel approaching the Port of Monfalcone [30] (Table 2) showing that the surface sediments were dominated by silt, and Hg ranged between 0.30 and 13.5 µg g −1 , decreasing from the offshore area to the innermost sector of the access channel to the port area [30].…”

“…The concentration of Hg in the surface sediments investigated in this study (0.77–6.39 µg g −1 , Figure 3 ) exceeded the Italian regulatory threshold limit of 0.30 µg g −1 (Decrees of the Italian Ministry of the Environment 260/2010 and 172/2015 according to EU Directive 2000/60/EC). Although the results from this study testified to a total Hg concentration in the surface sediments which remains of concern, speciation analyses performed on sediments collected along the main access channel to the Port of Monfalcone recently demonstrated that the element appeared to be strongly associated with the less mobile chemical fractions [ 30 ]. This suggested that most of the Hg in the investigated sediments was not available for MeHg production unless under conditions of anoxia [ 32 ].…”

“…This suggested that most of the Hg in the investigated sediments was not available for MeHg production unless under conditions of anoxia [ 32 ]. Indeed, the methylation rate does not only depend on the total amount of Hg [ 17 , 19 , 30 ] since several factors (e.g., temperature, pH, Eh, dissolved oxygen) may also have a role in MeHg production [ 47 , 64 ].…”

“…In the Gulf of Trieste, the occurrence and behaviour of Hg have been the main topics of several studies focused on coastal sediment contamination [ 19 , 30 ], transport and distribution of Hg associated with the SPM at the Isonzo River mouth [ 22 , 23 ] as well as Hg cycling at the sediment-water interface [ 31 , 32 ]. However, little information is currently available on the occurrence of dissolved (DHg) and particulate (PHg)Hg in the northernmost sector of the Gulf of Trieste (Bay of Panzano), where the main access channel approaching the Port of Monfalcone is located.…”

“…Indeed, over 500 years of cinnabar (HgS) extraction activity at the Idrija mining district (western Slovenia, Figure 1) has led to Hg contamination of multiple environmental matrices both in the areas surrounding the mine and far from this primary source [24,25,[27][28][29]. In the Gulf of Trieste, the occurrence and behaviour of Hg have been the main topics of several studies focused on coastal sediment contamination [19,30], transport and distribu-tion of Hg associated with the SPM at the Isonzo River mouth [22,23] as well as Hg cycling at the sediment-water interface [31,32]. However, little information is currently available on the occurrence of dissolved (DHg) and particulate (PHg)Hg in the northernmost sector of the Gulf of Trieste (Bay of Panzano), where the main access channel approaching the Port of Monfalcone is located.…”