Mercury isotopes to explore sources and fate of Hg in the environment: the case study of the Augusta Bay (southern Italy)

The Augusta Bay is a nearly closed marine basin located on the southeastern coast of Sicily (southern Italy) which has suffered, since the early 60s, the effect of the uncontrolled chemical discharge of Hg from the most important chlor-alkali plant in Italy (Le Donne and Ciafani, 2008), resulting in high Hg concentrations in sediments, seawater, atmosphere and fishes (Bagnato et al., 2013; Bonsignore et al., 2013; Sprovieri et al., 2001; Ficco et al., 2009; ICRAM, 2005; Ausili et al., 2008; Environ International Team, 2008; Di Leonardo et al., 2007, 2008). For the first time, here we report the results obtained by the measurements of mercury isotopes in sediments, fish muscles and human hair of resident populations, in order to trace sources of Hg and processes driving the biogeochemical cycle of this metal in the environment. Actually, Hg stable isotopes demonstrated an excellent tool for identifying sources and transformation processes of this element in different environmental compartments. Specifically, a positive relationship (r2=0.94; slope: 1.07±0.01) was found between Δ201Hg and Δ199Hg (MIF) for all the environmental matrices coming from the Augusta Bay. This primarily suggests the crucial link between the main Hg source in the Augusta Bay (sediment) and the final receptor (the man) related to fish consumption. Sediments display MIF (expressed as Δ201Hg and Δ199Hg) close to zero and negative MDF (expressed as δ202Hg) that we consider biologically mediated MeHg production. The lower δ202Hg values recorded in the southern area of the basin demonstrated the higher microbial activity occurred just in the zone with the highest mercury concentration in sediments. Furthermore, MIF fractionations in fishes plot on a single line (r2=0.84; slope 1.00±0.03) clearly testifying photochemical reduction of Hg (II) prior of the intake in the marine food web. Moreover, Hg fractionates according to fish ecology with MIF and MDF higher in pelagic fishes and lower ratios in demersal and benthic species. This observation suggests crucial effects of Hg(II) photoreduction processes in surface waters and relevant effects of microbial methylation in the deeper part of the basin. Finally, hair samples show a positive relationship on the base of Δ201Hg and Δ199Hg comparison (r2= 0.93; slope 1.00±0.14) and that we ascribe to the effects of photoreduction of Hg(II) in presence of organic substances. The role of sediments as primary source of Hg for the marine biotic compartment is testified by the overlapping δ202Hg values in fishes and sediments, while the 2‰ difference in δ202Hg values between fishes and hair samples appears consistent with a metabolic fractionation (Laffont et al., 2009).