Gaseous mercury at the air–water interface of a highly turbid estuary (Gironde Estuary, France)

Abstract

The present work aims at better understanding dissolved gaseous Hg production and dynamics in surface water of highly turbid estuaries, where light penetration and photochemical processes in the maximum turbidity zone (MTZ) are very limited. Measurements of dissolved gaseous mercury (DGM) and total gaseous mercury (TGM) in surface water of the MTZ and at the mouth of the Gironde Estuary were carried out at different seasons. Diurnal cycles of DGM concentrations in surface water at both sites, showing variations by a factor of up to 60, may be attributed to photoreduction of Hg(II) by solar radiation. During low-turbidity situations at the estuary mouth, DGM concentrations paralleled the daily evolution of solar radiation. Under highly turbid conditions, DGM concentrations paralleled the daily evolution of the ratio between solar radiation and suspended particulate matter concentrations (rad/SPM) in surface water (~ 200–2000 mg L − 1 ), rather than solar radiation level alone. These results clearly suggest that high SPM levels may constitute an important factor influencing the DGM production in surface water by limiting light penetration. Despite of the generally very high SPM levels in the Gironde estuary, seasonal trends in maximum daily DGM concentrations, saturation and fluxes to the atmosphere were observed. Surface water appeared to be Hg super-saturated most of the time, especially during high radiation periods (300–1500%; May and July) resulting in Hg evasion. However, saturation levels down to 10% were observed during the night and even during the day in winter, when rad/SPM was sufficiently low. Under these conditions, Hg flux from atmosphere to the under saturated surface water may occur. Most of the year, hourly fluxes of Hg° from water to the atmosphere were positive during the day (up to 0.064 nmol m − 2 h − 1 ; 12.8 ng m − 2 h − 1 ) and were negative during the night, but in winter they were negative during the whole diurnal cycle (− 0.01 nmol m − 2 h − 1 ; − 2 ng m − 2 h − 1 ). Integration of the hourly fluxes over 24 h for the different campaigns, and extrapolation to the whole estuary scale suggest that the Gironde Estuary constitutes a Hg° net source to atmosphere during most of the year, with an annual Hg° net flux of ~ 22 mol y − 1 . However, this net Hg° flux may probably be counterbalanced by (dry + wet) Hg deposition.