Seasonal cycling of mercury and methyl mercury in the Venice Lagoon (Italy)

From 1950s to 1980s, the Venice Lagoon was extensively contaminated with mercury (Hg) from chlor-alkali discharges and other sources, prompting researchers to assess the distribution of contamination in sediments and biota. Since the lagoon is shallow and bordered by extensive wetlands, contains high sulfate from the sea, and high nutrient loading from agricultural runoff and urban sewage, it is expected to act as a vigorous microbiological incubator for Hg methylation. Here we report seasonal findings (2001–2003) for total Hg and monomethyl mercury (CH3Hg) in water column, rivers, precipitation, suspended matter, sediments, and biota samples collected using ultra-clean sampling techniques. Aqueous total Hg ranged from about 5–10 pM at the Lido entrance to the Adriatic, to 25–100 pM within the main body of the northern lagoon. A sudden drop in Hg concentration is observed when crossing the Canale dei Petroli,the major deep-water shipping canal which roughly bisects the lagoon) in a southward direction. The less contaminated southern reaches of the lagoon were not studied as intensively, but were found to contain only 10–20 pM total Hg. Up to 250–1000 pM were observed in the industrially impacted areas and urban canals. Most Hg was from re-suspended sediments, as levels are strongly correlated with total suspended solids (TSS) in this very shallow (b1 m) lagoon. Dissolved Hg concentrations were narrowly constrained to the range of approximately 3–15 pM. Monomethyl mercury ranged from about 0.1–2 pM in unfiltered samples and 0.03–1 pM in the dissolved phase. Surface (0–3 cm) sediments varied from about 2.5–12.5 nmol g1, with 0.1–0.3% of that as CH3Hg. Monomethyl mercury levels were about three times higher during the spring and summer, compared to winter. The lagoon appears to be moderately, but extensively contaminated by Hg, showing concentrations of both total Hg and CH3Hg similar to those reported in other urban harbors, but spread over hundreds of km2. The impact of the historic direct chlor-alkali discharges to the lagoon is diffused throughout the system by an array of re-suspension pathways, but may ultimately be mitigated by rapid tidal flushing.