Dissolved gaseous mercury behavior in shallow water estuaries

The formation of dissolved gaseous mercury (DGM) can be an important pathway for mercury removal from an aquatic environment. DGM evasional fluxes from an aquatic system can account for up to 95% of atmospheric Hg and its deposition pathways. While this makes DGM an important species of mercury to investigate, the difficulty of accurately analyzing DGM has prevented many from studying it. In this study, DGM was measured in two different types of estuarine environments and with two different methods, discrete and continuous analysis. The discrete technique works reasonably well and is reproducible, but it does not allow one to observe rapid changes in DGM concentration due to long analysis times (~2 hr per sample). When used in this study, the discrete sampling technique agreed well with the continuous technique for Offatts Bayou, Galveston, Texas, and Georgiana Slough in the California Bay-Delta region. The average DGM concentration during the March continuous study at Offatts Bayou was 25.3 + 8.8 pg L-1. This is significantly higher than the average DGM concentration from Georgiana Slough during late March 2006 (9.6 + 6.6 pg L-1). DGM seemed to correlate best with photosynthetically active radiation (PAR) data in every study, suggesting that the primary control of its formation is solar irradiation. Stronger positive correlations with PAR were seen when DGM data was shifted back one hour, indicating that mercury photoreactions take time to complete. DGM also correlated positively with wind speed in most instances. However, increased wind speed should enhance air to water transfer of elemental mercury, thus one would expect a negative correlation. DGM co-varied negatively with salinity during the continuous studies, suggesting that the DGM pool is reduced in surface waters by chloride mediated oxidation. Three predictive flux models were used in the study to assess the potential for DGM water to air transfer. For both the Georgiana Slough and Offatts Bayou studies, the predicted flux dropped to or below zero after sunset. This study does contribute to the understanding of DGM cycling in aquatic environments as there are few studies that have made continuous DGM measurements in estuarine environments.