Mercury speciation and total organic carbon in marine sediments along the Mediterranean coast of Israel.

Along the Israeli Mediterranean Coast, three areas are considered "hot spots" of mercury (Hg) pollution: (1) Northern Haifa Bay (NHB), (2) the lower Qishon River at the southern part of Haifa Bay, and (3) a marine outfall of activated sewage sludge at the southern coast off Palmachim (sewage-sludge disposal site [SDS]). Even though the total Hg (HgT) concentrations in the sediments at the three areas are of the same order of magnitude (250-500 μg kg(-1)), Hg was shown to bioaccumulate in fish and benthic fauna from Haifa Bay but not in benthic fauna or in commercial fish caught along the southern Mediterranean Coast of Israel near the SDS outfall. The primary goal of this study was to measure the concentrations of Hg species (HgT, methyl-Hg [MeHg], and Hg in different biogeochemical fractions)-in conjunction with organic carbon-in sediments of NHB and the lower Qishon River to assess its impact on Hg transitions among the species as characterized by different bioavailability and bioaccessibility. HgT concentrations in NHB and the Qishon River ranged from 249 to 347 and 165 to 667 μg kg(-1), respectively. MeHg was significantly higher in the Qishon River (6.3-34.0 μg kg(-1)) than in NHB (0.22-0.70 μg kg(-1)) as were total organic carbon (TOC) concentrations (average 2.5 vs. 0.13 %). The relative Hg distribution in the biogeochemical fractions in NHB was 2.3 % in the most bioaccessible fractions (F1 + F2), 55 % in the organo-chelated species fraction (F3), 42 % in the strong-complexed species fraction (F4), and 0.7 % in the mercuric-sulfide fraction (F5). In the Qishon River, the bioavailable F1 + F2 and F3 fractions were lower than in NHB (<0.01 and 23 %, respectively) and the more refractory F4 and F5 fractions higher (73 and 3.3 %, respectively). The fractionation of Hg in Qishon River sediments was similar to the distribution found in polluted stations at the SDS. TOC and MeHg were positively and negatively correlated, respectively, in Qishon River and NHB sediments. Methylation depended on TOC availability when its concentration was in the range of 2-4 wt%. It is possible that TOC in the sediment controlled Hg speciation: Hg in F3 decreased and in F4 increased with increasing TOC concentrations. In contrast, MeHg/HgT was significantly positively correlated with TOC and Hg in the stable F4 fraction and negatively correlated with Hg in the F3 fraction. It was therefore assumed that higher TOC concentrations enhanced microbial activity and decomposition of organic matter. Hg was released from the F3 fraction and was either transferred to the F4 fraction or made available for methylation processes.