Geochemistry of mercury in the Chao Phraya River Estuary

Abstract

Concentrations of dissolved reactive mercury, (Hg-R)D, total dissolved mercury, (Hg-T)D, and suspended particulate mercury, Hg-P, in the Chao Phraya River estuary were <0.15-5.29 ng/L, 2.16-18.1 ng/L, and 0.09-3.49 microg/g (dry weight) (10.2-112 ng/L) respectively. These are in a similar concentration range to those found in major estuaries elsewhere. The seasonal variation of mercury distribution in the water column was observed between the dry and the wet season. Total dissolved Hg in the wet season was about 30% of the combined dissolved and particulate concentrations, while in the dry season it was only 15% of the combined Hg. Most of the dissolved Hg was in the non-reactive Hg [(Hg-NR)D] form, which accounted for more than 85% of the total dissolved Hg.Suspended particulate mercury was the main species of Hg in the estuary and was associated with permanently suspended particles. The partition coefficients (KD) between particulate and dissolved forms were higher in the high turbidity zone during both seasons and the KD value of the dry season was higher than in the wet season. The higher KD for non-reactive dissolved Hg in the dry season suggested that the behavior of (Hg-NR)D was different between the dry and the wet season. Increase of KD with increasing salinity was also found in the Chao Phraya estuary. Laboratory experiments showed that adsorption and desorption of mercury occurred during mixing along a salinity gradient. Resuspension experiments, in the laboratory, showed that mercury desorbed from sediments rapidly, however, the readsorption occurred within 6 hours.The Hg concentration of surface sediments ranged from 0.2 to 0.77 microg/g (dry weight). The vertical profile of Hg in core sediment indicated that the Hg concentrations in surficial sediment (about upper 10 cm) were about three to five times lower than concentration found 20 years ago due to the use of mercury in the production process of some industries during that period