Mercury speciation and exchanges at the air–water interface of a tropical artificial reservoir, French Guiana

The distribution and speciation of mercury (Hg) in air, rain, and surface waters from the artificial tropical lake of Petit-Saut in French Guiana were investigated during the 2003/04 period. In the air, total gaseous mercury (TGM) at the dam station averaged 12 ± 2 pmol m− 3 of which > 98% was gaseous elemental mercury (GEM). GEM distribution depicted a day–night cycling with high concentrations (up to 15 pmol m− 3) at dawn and low concentrations (down to 5 pmol m− 3) at nightfall. Reactive gaseous mercury (RGM) represented < 1% of the GEM with a mean concentration of 4 ± 3 fmol m− 3. Diel RGM variations were negatively related to GEM. In the rain, the sum of all Hg species in the unfiltered (HgTUNF) averaged 16 ± 12 pmol L− 1. Temporal distribution of HgTUNF exhibited a pattern of high concentrations during the late dry seasons (up to 57.5 pmol L− 1) and low concentrations (down to 2.7 pmol L− 1) in the course of the wet seasons. Unfiltered reactive (HgRUNF), dissolved gaseous (DGM) and monomethyl (MMHgUNF) Hg constituted 20, 5 and 5% of HgTUNF, respectively. All measured Hg species were positively related and displayed negative relationships with the pH of the rain. In the reservoir surface waters, dissolved total mercury (HgTD) averaged 3.4 ± 1.2 pmol L− 1 of which 10% consisted of DGM. DGM showed a trend of high concentrations during the dry seasons (480 ± 270 fmol L− 1) and lower (230 ± 130 fmol L− 1) in the course of the wet seasons. Diel variations included diurnal photo-induced DGM production (of about 60 fmol L− 1 h− 1) coupled to minute to hour oxidation/reduction cycles (of > 100 fmol L− 1 amplitude). Finally, calculated atmospheric Hg inputs to the Petit-Saut reservoir represented 14 mol yr− 1 whereas DGM evasion reached 23 mol yr− 1. Apportionment among forms of Hg deposition indicated that up to 75% of the total Hg invasive flux follows the rainfall pathway.