Mercury distribution and methylmercury mobility in the sediments of three sites on the Lebanese coast, eastern Mediterranean.

Mercury (Hg) contamination in coastal sediments has been widely studied in clay deposits; however, equivalent results on carbonated sediments are scarce. This article aims to study Hg distribution in Lebanese carbonate coastal marine sediments (Eastern Mediterranean) in order to characterize their contamination level and to explore the postdepositional mobility of methylmercury (MeHg) in the deposits. Vertical distribution profiles of total (HgT) and MeHg have been established for the solid phase of sediment cores collected in various near-shore environments chosen for their hypothetical various degrees of anthropization. In addition, dissolved MeHg was determined in sediment pore waters to test its mobility and potential availability for biota. Three sites on the Lebanese coasts--Akkar, Dora, and Selaata--were selected. Akkar is far from any direct contamination source, whereas Dora, located near the Beirut harbor, is a heavily urbanized and industrialized zone including a huge dump site, and Selaata is near a chemical plant that produces phosphate fertilizers. Particulate HgT concentrations in the sediments varied between <0.04 and 0.65 µg/g, with a proportion of MeHg lower than 1%. Based on a sediment quality guideline (MacDonald et al. 2000), we concluded that Dora bay sediments are heavily contaminated by Hg, with concentrations exceeding the "consensus-based threshold effect" level (0.17 µg/g) and almost reaching the "effects range-medium" level (0.71 µg/g). In spite of the low HgT concentration in Akkar and Selaata sediment (similar to natural carbonated sediment: 0.04 µg/g according to Turekian and Wedephol (1961), a closer analysis of the sediment core vertical profile allows one to observe an anthropogenic impact. This impact might be toxicologically insignificant; however, it allows tracing the time increase of Hg diffuse deposition. On the other hand, dissolved MeHg concentrations ranged from 0.04 to 0.09 and from 0.04 to 8.76 ng/l in the Selaata and the Dora sediments, respectively; MeHg vertical profiles in interstitial water enabled us to calculate diffusive fluxes of MeHg from the sediment varying from 0.3 to 1.0 ng/m(2)/day. Thus, the deposited sediments constitute a measurable source of bioavailable Hg for epibenthic organisms.