Distribution and production of reactive mercury and dissolved gaseous mercury in surface waters and water/air mercury flux in reservoirs on Wujiang River, Southwest China

Abstract

[1] Transformation and distribution of mercury (Hg) species play an important role in the biogeochemical cycling of mercury in aquatic systems. Measurements of water/air exchange fluxes of Hg, reactive mercury (RHg), and dissolved gaseous mercury (DGM) concentrations were conducted at 14 sites in five reservoirs on the Wujiang River, Guizhou, Southwest China. Clear spatial and temporal variations in Hg fluxes, RHg, and DGM concentrations were observed in the study area. Hg fluxes and RHg concentrations exhibited a consistent diurnal variation in the study area, with maximum fluxes and concentrations during daytime. A typical diurnal trend of DGM with elevated concentration at night was observed in a eutrophic reservoir with elevated bacteria abundance, suggesting a bacteria-induced production of DGM in this reservoir. For other reservoirs, a combination of sunlight-stimulated production and loss via photo-induced oxidation and evaporation regulated the diurnal trends of DGM. Seasonal variations with elevated Hg fluxes and RHg concentrations in warm season were noticeable in the study area, which highlighted the combined effect of interrelationships between Hg species in water and environmental parameters. Hg fluxes exhibited much more significant correlations with RHg and THg concentrations and air temperature compared to DGM concentrations and solar radiation. The measured fluxes were significantly higher than those simulated using the water/air thin film Hg 0 gradient model. Aside from the potential limitations of dynamic flux chamber method, this may also suggest the thin film gas exchange model is not capable of predicting water/air Hg flux under low wind speed conditions. Additionally, it is speculated that DGM concentrations might vary significantly in surface waters with depth, and measurements of DGM at a depth of 2-4 cm below the water surface probably underestimated the DGM concentration that should be taken into account in simulations of water/air flux using the thin film gas exchange model. An empirical model of water/air Hg flux was developed, and the simulated fluxes were compared well with measurements. The model yields a mean annual Hg emission of 3.2 AE 1.0 kg in the study area.Citation: Fu, X., X. Feng, Y. Guo, B. Meng, R. Yin, and H. Yao (2013), Distribution and production of reactive mercury and dissolved gaseous mercury in surface waters and water/air mercury flux in reservoirs on Wujiang River, Southwest China,

Gaseous Mercury Exchange from Water–Air Interface in Differently Impacted Freshwater Environments

“…Generally, the gaseous Hg fluxes at the WAI were significantly higher in summer (p < 0.05, Dunn's test) than autumn and spring at all sites (Figure 3). The highest values of gaseous Hg fluxes found in summer are related to the intensity of the incident solar radiation, as observed in several studies conducted in both marine (e.g., [67,68,105]) and freshwater environments (e.g., [19,98,99]); in fact, solar radiation is a key factor in promoting a faster rate of DGM production in warmer periods via the photoreduction of Hg 2+ in surface waters and the subsequent evasion to the atmosphere The highest values of gaseous Hg fluxes found in summer are related to the intensity of the incident solar radiation, as observed in several studies conducted in both marine (e.g., [67,68,105]) and freshwater environments (e.g., [19,98,99]); in fact, solar radiation is a key factor in promoting a faster rate of DGM production in warmer periods via the photoreduction of Hg 2+ in surface waters and the subsequent evasion to the atmosphere [26,[106][107][108]. This was also confirmed in this study, as high DGM concentrations were detected in summer and the lowest in autumn in parallel with UV radiation intensity, which is most effective in systems with low DOC content that in turn allow for higher light penetration [24,26].…”

“…This value is in agreement with those usually reported for lake water ( [35] and references therein). The highest values of gaseous Hg fluxes found in summer are related to the intensity of the incident solar radiation, as observed in several studies conducted in both marine (e.g., [67,68,105]) and freshwater environments (e.g., [19,98,99]); in fact, solar radiation is a key factor in promoting a faster rate of DGM production in warmer periods via the photoreduction of Hg 2+ in surface waters and the subsequent evasion to the atmosphere [26,[106][107][108]. This was also confirmed in this study, as high DGM concentrations were detected in summer and the lowest in autumn in parallel with UV radiation intensity, which is most effective in systems with low DOC content that in turn allow for higher light penetration [24,26].…”

“…In addition, the methods employed for flux measurements add a certain degree of variability. In detail, micrometeorological models generally tend to underestimate the fluxes with respect to flux chamber deployments [17,19,100], but also differences in size, shape, and air turnover time within the chambers used can lead to different results [71]. Thus, our results are directly comparable with those reported using the same experimental approach for the Marano and Grado Lagoon [67], which is downstream from the sites selected in this work and is subject to the same Hg contamination sources (CAP and Hg mining at Idrija).…”

“…This recycling prolongs the lifespan of Hg in surface reservoirs, enhances its global distribution [13], and, particularly in aquatic ecosystems, contributes to limiting the amount of Hg available for the production/bioaccumulation/biomagnification of the neurotoxic species methylmercury (MeHg) [5,[14][15][16]. In natural waters the volatile fraction of Hg, usually referred to as dissolved gaseous mercury (DGM), can account for 1-50% of the total Hg [17][18][19] and is mainly constituted by Hg 0 produced through abiotic (photochemical) or biotic reduction of Hg 2+ . Photoreduction is considered the dominant process in surface waters [20] where it proceeds at higher rates than biotic reduction mediated by heterotrophic bacteria or algae [21,22].…”

Correlation slopes of GEM / CO, GEM / CO&lt;sub&gt;2&lt;/sub&gt;, and GEM / CH&lt;sub&gt;4&lt;/sub&gt; and estimated mercury emissions in China, South Asia, the Indochinese Peninsula, and Central Asia derived from observations in northwestern and southwestern China

“…GEM fluxes from different landscapes in cold seasons (from November to April) were relatively limited. Several studies found that GEM Fu et al (2007Fu et al ( , 2008Fu et al ( , 2010bFu et al ( , 2012cFu et al ( , 2013, Zhu et al (2011Zhu et al ( , 2013, Ma et al (2013), and Liu et al (2014) fluxes from dry farmland, forest soil, and lake waters were about 2.5-40 times (mean = 6.5, n = 18) lower than those in warm seasons Ma et al, 2013;Fu et al, 2010b;Fu et al, 2013). Given the different landscapes and seasonal patterns of GEM fluxes in mainland China, we estimate the annual natural GEM emissions to be 528 t in China.…”