Observation-Based Mercury Export from Rivers to Coastal Oceans in East Asia

Organic matter content, source, and composition varying with seasons and anthropogenic activities regulate methylmercury dynamics in the Yellow River

Organic matter (OM) in rivers, influenced by natural and anthropogenic events, plays a critical role in regulating methylmercury (MeHg) dynamics. This study investigated the spatial-temporal patterns of MeHg and OM in the Yellow River, through collecting water and sediment samples within the dry and wet seasons and human-induced water-sediment regulation scheme (WSRS) event. During the wet season, the river water was characterized by terrigenous OM, high unfiltered MeHg (UMeHg) concentration (206 ± 83.6 pg/L), and a low UMeHg percentage in unfiltered total Hg (UTHg) (UMeHg/UTHg = 0.37 ± 0.18 %). These findings may suggest that MeHg in the wet season was likely produced in the catchment and imported into the river. In contrast, autochthonous OM predominated during the dry season, probably enhancing in situ MeHg production (UMeHg/UTHg = 1.01 ± 0.50 %), although UMeHg concentration was lower (81.8 ± 25.5 pg/L) due to obviously reduced external inputs. The WSRS dramatically increased water MeHg contents (589 ± 116 pg/L) and UMeHg/UTHg ratios (0.78 ± 0.16 %) probably through direct inputs of upstream reservoirs' MeHg and resuspension of downstream sediments. Exploring the effects of OM on sediment MeHg sources suggested that MeHg in the wet season primarily originated from watershed soil inputs, while in the dry season, it mainly resulted from in situ Hg methylation. However, extremely low total Hg (THg) and MeHg/THg (0.02-0.35 %) indicated that sediment MeHg production may be of minor importance in the Yellow River.

Human activities shape important geographic differences in fish mercury concentration levels

Fish consumption is a major route of human exposure to mercury (Hg), yet limited understanding of how anthropogenic activities drive geographic variations in fish Hg worldwide hinders effective Hg pollution management. Here we characterized global geographic variations in total Hg (THg) and methylmercury (MeHg), compared THg and MeHg levels between the United States and China, and used a structural equation model to link the geographic variability of MeHg in fish to human activities. Despite previously reported higher Hg emissions in China, Chinese fish have lower THg and MeHg levels than fish in the United States owing to a lower trophic magnification slope, shortened food chains and shorter fish lifespans. The structural equation model revealed strong impacts of human activities on MeHg levels in fish. In the future, China may face elevated MeHg levels in fish with the ongoing recovery of food web ecology, highlighting the importance of local policies.

Atmospheric gaseous mercury and associated health risk assessment in the economic capital of India

Mercury cycling in coastal metropolitan areas on the west coast of India becomes complex due to the combined effects of both intensive domestic anthropogenic emissions and marine air masses. The present study is based on yearlong data of continuous measurements of gaseous elemental mercury (GEM) concentration concurrent with meteorological parameters and some air pollutants at a coastal urban site in Mumbai, on the west coast of India, for the first time. The concentration of GEM was found in a range between 2.2 and 12.3 ng/m3, with a mean of 3.1 ± 1.1 ng/m3, which was significantly higher than the continental background values in the Northern Hemisphere (~ 1.5 ng/m3). Unlike particulates, GEM starts increasing post-winter to peak during the monsoon and decrease towards winter. July had the highest concentration of GEM followed by October, and a minimum in January. GEM exhibited a distinct diurnal cycle, mainly with a broad peak in the early morning, a narrow one by nightfall, and a minimum in the afternoon. The peaks and their timing suggest the origin of urban mobility and the start of local activities. A positive correlation between SO2, PM2.5, temperature, relative humidity, and GEM indicates that emissions from local industrial plants in the Mumbai coastal area. Principal component analysis (PCA) and cluster analysis (CA) confirm this fact. Monthly back trajectory analysis showed that air mass flows are predominantly from the Arabian Sea and local human activities. Assessment of human health risks by USEPA model reveals that the hazardous quotient, HQ < 1, implies negligible carcinogenic risk. GEM observations in Mumbai during the study period are below the World Health Organization's (WHO) safe limit (200 ng/m3) for long-term inhalation.

Confounding effects of seasonality and anthropogenic river regulation on suspended particulate matter-driven mercury transport to coastal seas

Riverine mercury (Hg) is mainly transported to coastal areas in suspended particulate matter (SPM)-bound form, posing a potential threat to human health. Water discharge and SPM characteristics in rivers vary naturally with seasonality and can also be arbitrarily disrupted by anthropogenic regulation events, but their effects on Hg transport remain unresolved. Aiming to understand the confounding effects of seasonality and anthropogenic river regulation on Hg and SPM transport, this study selected the highly sediment-laden Yellow River as a representative conduit. Significant variations in SPM concentrations (108 - 7097 mg/L) resulted in fluctuations in total mercury (THg, 3.79 - 111 ng/L) in river water corresponding to seasonality and anthropogenic water/sediment regulation. Principal component analysis and structural equation model revealed that SPM was the essential factor controlling THg and particulate Hg (PHg) in river water. While SPM exhibited equilibrium state in the dry season, a net resuspension during the anthropogenic regulation and net deposition in the wet season demonstrated the impact of SPM dynamics on Hg distribution and transport to coastal regions. Combining water discharge, SPM, and Hg concentrations, a modified model was developed to quantify Hg flux (2256 kg), over 98% of which was in particulate phase.

Climate and industrial pollution determine the seasonal and spatial mercury variations in the China's Weihe River

Natural processes and human activities impact mercury (Hg) pollution in rivers. Investigating the individual contributions and interactions of factors affecting variations in Hg concentrations, particularly under climate change, is crucial for safeguarding watershed ecosystems and human health. We collected 381 water samples from China's Weihe River Basin (WRB) during dry and wet seasons to assess the total Hg (THg) concentration. Results revealed high Hg concentrations in the WRB (0.1-2200.9 ng/L, mean 126.2 ± 335.5 ng/L), with higher levels during the wet season (wet season: 249.1 ± 453.5 ng/L, dry season: 12.7 ± 14.0 ng/L), particularly in the mainstream and southern tributaries of the Weihe River. Industrial pollution (contributing 26.2 %) and precipitation (contributing 33.5 %) drove spatial heterogeneity in THg concentrations during the dry and wet seasons, respectively. Notably, combined explanatory power increased to 47.9 % when interaction was considered, highlighting the amplifying effect of climate change, particularly precipitation, on the impact of industrial pollution. The middle and downstream of the Weihe River, especially the Guanzhong urban agglomeration, were identified as high-risk regions for Hg pollution. With ongoing climate change the risk of Hg exposure in the WRB is expected to escalate. This study lays a robust scientific foundation for the effective management of Hg pollution in analogous river systems worldwide.

Significant impacts of river inputs on the distributions and transports of mercury and methylmercury in nearshore and open seas - Simulation based on field surveys and mass balance modeling

Rivers are important sources of Hg for adjacent seas, and seafood from nearshore waters is a major source of Hg exposure for humans. There is thus a key scientific concern regarding how much riverine Hg inputs influence Hg loads in nearshore waters as well as how far the impact range can extend from the river to the open sea. In addition, it is important to understand the influence of anthropogenic hydro-facilities and activities on Hg levels in downstream seas. Because of the concise mass exchange pattern between the seas and the previously demonstrated intensive Hg inputs under anthropogenic regulation from the Yellow River, the Bohai and Yellow Seas, which are key fishery and marine breeding areas for China, are an ideal research area for exploring the impacts of riverine Hg on nearshore and adjacent open seas. Field surveys were conducted in eight major rivers and two seas, and 433 water samples were collected. The main Hg input and output terms (rivers, ocean currents, underground discharge, sewage, coastal erosion, atmospheric deposition, surface evasion, sedimentation, and fisheries) were quantified in the Bohai and Yellow Seas. Owing to the high inputs from the Yellow and Yalu Rivers, elevated THg concentrations were found. Apart from direct MeHg discharge, riverine nutrients may also seemingly affect nearshore MeHg. Using mass balance models, we found that the Yellow River (9.8 t) was the dominant Hg source in the Bohai Sea, which accounted for more than half of all contributions, and the Bohai Sea played the role of a secondary source of Hg to the Yellow Sea, with a flux of 3.3 t. Anthropogenic hydro-activities in large rivers could significantly influence Hg outputs and loads in the nearshore and even open seas. This study provides useful information for water resource management applications to reduce potential MeHg risks.

Coastal streams and sewage outfalls: Hot spots of mercury discharge, pollution and cycling in nearshore environments

The coastal streams (CSs) and sewage outfalls (SOs) are widely distributed and direct anthropogenic stress on global coastal ecosystems. However, the CS/SO-associated mercury (Hg) discharge, pollution and cycle in nearshore environment are less quantified. Here, we report that total Hg (THg) and methylmercury (MMHg) concentrations in waters of CSs (n = 8) and SOs (n = 15) of the northern China were ∼102 to 103 times of coastal surface waters and 10 to 102 times of major rivers in China and other regions. The CS/SO discharges resulted in the increase of total organic carbon (TOC) contents, THg and MMHg concentrations and TOC-normalized THg and MMHg concentrations in sediments of CS/SO-impacted coasts. The laboratory experiments further illustrated that the CS/SO-impacted sediments characterized with high potentials of dissolved THg and MMHg productions and releases. Our findings indicate that the layout optimization of SOs is able to reduce the Hg risk in coastal environment.

Riverine Discharge Fuels the Production of Methylmercury in a Large Temperate Estuary

Estuaries are an important food source for the world's growing population, yet human health is at risk from elevated exposure to methylmercury (MeHg) via the consumption of estuarine fish. Moreover, the sources and cycling of MeHg in temperate estuarine ecosystems are poorly understood. Here, we investigated the seasonal and tidal patterns of mercury (Hg) forms in Long Island Sound (LIS), in a location where North Atlantic Ocean waters mix with the Connecticut River. We found that seasonal variations in Hg and MeHg in LIS followed the extent of riverine Hg delivery, while tides further exacerbated the remobilization of earlier deposited riverine Hg. The net production of MeHg near the river plume was significant compared to that in other locations and enhanced during high tide, possibly resulting from the enhanced microbial activity and organic carbon remineralization in the river plume. Statistical models, driven by our novel data, further support the hypothesis that the river-delivered organic matter and inorganic Hg drive net MeHg production in the estuarine water column. Our study sheds light on the significance of water column biogeochemical processes in temperate tidal estuaries in regulating MeHg levels and inspires new questions in our quest to understand MeHg sources and dynamics in coastal oceans.

Mercury budgets in the suspended particulate matters of the Yangtze River

Riverine processes are crucial for the biogeochemical cycle of mercury (Hg). The Yangtze River, the largest river in East Asia, discharges a substantial amount of Hg into the East China Sea. However, the influencing factors of the Hg budget and its recent trends remain unclear. This study quantitatively analyzed the total Hg concentration (THg) in suspended particulate matter (SPM) in the Yangtze River and calculated the Hg budget in 2018 and 2021. The results showed that the total Hg concentrations varied substantially along the river, with concentrations ranging from 23 to 883 μg/kg in 2018 and 47 to 146 μg/kg in 2021. The average Hg flux to China Sea in 2018 and 2021 were approximately 10 Mg/yr, lower than in 2016 (48 Mg/yr). Over 70% of the SPM was trapped in the Three Gorges Dam (TGD), and 22 Mg/yr of Hg settled in the TGD in 2018 and 10 Mg/yr in 2021. Hg fluxes in the Yangtze River watershed were driven by various factors, including decreased industrial emissions, increased agriculture emissions, and decreased soil erosion flux. We found that in the upper reach of the Yangtze River changed from sink to source of Hg possibly due to the resuspension of sediments, which implies that the settled sediments could be a potential source of Hg for downstream. Overall, emission control policies may have had a positive impact on reducing Hg flux to the East China Sea from 2016 to 2021, but more efforts are needed to further reduce Hg emissions.

Riverine input of suspended particulate matter controls distribution, partitioning and transport of mercury and methylmercury in the Yellow River Estuary

Riverine mercury (Hg) is the largest global source of Hg in coastal oceans. The Yellow River delivers the majority of Hg to the semi-enclosed Bohai Sea, where Hg contamination adversely affects the surrounding heavily populated provinces in northern China. Mercury distribution patterns in the river-estuary interacting area provides essential information to understand the riverine Hg transport and biogeochemical cycling of Hg in the estuary. Analyzing the spatial distributions of total- (THg) and methyl-Hg (MeHg) in the lower end of Yellow River (∼105 km) and adjacent estuary, we found the dominant role of suspended particulate matter (SPM) in Hg transport, with 99.1% and 86.3% of THg and MeHg being in particulate phase. The SPM dynamics, such as transport, retention, sorting and sedimentation, governs Hg transport with water flow and particle-water partition of Hg. While THg decreased along the water flow to the river mouth with the settlement of particulate THg (about 27.5% onto the riverbed and the rest entering the sea), MeHg and particulate MeHg increased by 110% and 117%, respectively. This study highlights the distinct patterns in THg and MeHg distribution and transport and suggests potential Hg methylation and external MeHg input in the river-estuary mixed zone.

Seasonal variation in mercury and methylmercury production in vegetated sediment in the Dongtan wetlands of the Yangtze River Estuary, China

The seasonal cycling of mercury (Hg) in vegetated sediments in the Dongtan wetlands of the Yangtze River Estuary were determined, and microcosm incubation experiments were conducted to evaluate methylmercury (MeHg) production after Hg input. The results showed that the seasonal variations of total Hg and MeHg were very different. The enhanced activity of methylating bacteria could have been the main contributor to the elevated MeHg in the upper surface layer (0-12 cm), which was supported by the higher copy numbers of the hgcA gene in the surface sediment and the MeHg increase during sediment incubation following litterfall addition. Moreover, the incubation results showed that Hg addition greatly increased net MeHg production and that this increase remained under suboxic conditions, suggesting that the potential health risk of Hg in estuarine wetlands could exist for a long time under changing redox conditions.

Mercury contamination in the water and sediments of a typical inland river - Lake basin in China: Occurrence, sources, migration and risk assessment

In the area affected by non-ferrous metal mining activities, mercury (Hg) contamination in the water and sediments posed potential risks to ecology and human health. In this study, river water and sediment samples were collected in the Zijiang river - South Dongting Lake basin to analyze Hg residues, identify potential Hg sources and evaluate the ecological and health risks posed by Hg contamination. In this study, the average concentrations of THg, PHg, DHg and DMeHg in river water were 38.05 ± 27.13 ng/L, 25.18 ± 26.83 ng/L, 12.88 ± 9.64 ng/L and 0.29 ± 0.07 ng/L, respectively. The THg and MeHg contents in sediments were 234.24 ± 152.93 µg/kg and 0.48 ± 0.16 µg/kg, respectively. The more enrichment of Hg in sediments was observed in the Zijiang River than in the South Dongting Lake, especially in the upstream and midstream regions. Two potential Hg sources in the basin were identified by correlation matrix, principal component analysis (PCA) and positive matrix factorization (PMF) model. The comparable Hg flux with other rivers worldwide was found in the Zijiang River (0.53 Mg/y). Furthermore, it was found by the delayed geochemical hazard (DGH) model that the ecological risk of Hg was more significant in the Zijiang River with more frequent transformation pathways. For different populations, the health risk values caused by Hg were all lower than the USEPA's guideline value. This study provided sound evidence for further control of Hg contamination.

Adsorption and environmental behavior of mercury on the sediment from the Yellow Sea of China

The Yellow Sea (YS) of China is facing severe mercury (Hg) pollution problems while the concentration of Hg in the sediment is relatively low compared to its high discharge intensity, whose mechanisms are still unclear. Here, we performed batch experiments to investigate the Hg adsorption capacity of the YS sediments. Freundlich isothermal adsorption simulation results showed that the parameters K_F (adsorption capacity constant) of the sediments were varied from 3.33 to 2.88 × 10⁴. Correlation analysis of K_F against the physicochemical properties and sequential extraction revealed that organic matter (OM) and particle size influenced the K_F for Hg. In addition, the calculated smaller Kd (distribution coefficient) values in the YS compared to other coastal seas indicate that at the sediment-water interface, Hg tends to be more present in porewater. There is also a significant positive correlation between K_F and Kd. We conclude that the low OM content of YS sediments is one of the main reasons for their weak adsorption capacity. These findings provide a scientific basis for the phenomenon that Hg entering the YS undergoes environmental behaviors to maintain low concentrations in the sediment, deepen the understanding of Hg cycling in the YS and improve long-term risk prediction capacity for Hg in marine environments.

Mass Budget of Mercury (Hg) in the Seawater of Eastern China Marginal Seas: Importance of the Sediment-Water Transport Processes

The Eastern China Marginal Seas (ECMS) have been facing a variety of environmental problems, including mercury (Hg) pollution. Although several previous studies have been focused on mass balance of Hg in the ECMS, the contribution of Hg transport at the sediment-water interface remains unclear. This study was aimed to access and quantify the importance of sediment-water transport processes in Hg cycling. Significantly positive correlations were observed between Hg concentrations in the overlying and bottom water and the diffusion rates of Hg from sediment to the water. Approximately 2-3 times higher of THg concentrations in the entire water column were observed in a winter cruise with strong waves which was supposed to strengthen the resuspension process. The mass budget of Hg in the ECMS further showed that diffusion and resuspension processes accounted for approximate 46%, 60%, and 16% of total input Hg in the BS, YS, and ECS, respectively. These results suggest that the sediment-water transport processes play an important role in Hg cycling in the ECMS. As an important "pool" of Hg in the ECMS, the transport of Hg at the sediment-water interface may affect the long-term risk assessment of Hg in these systems.

Transport of mercury in a regulated high-sediment river and its input to marginal seas

Seafood is a major source of Hg exposure for humans, and rivers are an important source of Hg for adjacent seas. The Yellow River is an extremely high-sediment river that is heavily influenced by anthropogenic water facilities, especially sediment regulations, which might result in significant intra- and inter- year flux variations on pollutants. Overestimations of Hg flux were found in previous studies on high-sediment rivers, and a comprehensive study of Hg transport along such a river has not yet been attempted. In this study, we quantified the fates of Hg and methylmercury (MeHg) in the Yellow River and its main branches, as well as the impacts of anthropogenic activities, including deposition of contaminants in the reservoirs, diversion in the downstream, and sediment regulation, by sampling in 2017 and 2018,and comparing the two typical years with and without sediment regulation. The total Hg (THg) and MeHg concentrations in the Yellow River and its tributaries were high, and extremely high MeHg/THg ratios were found in the three sampled reservoirs. The river discharges substantial amounts of THg and MeHg into the Bohai Sea, and particulate Hg in the humid period accounts for a large proportion. A significant increase in Hg flux into the ocean was found in 2018 compared with that in 2017, which was primarily attributed to the altered hydrology caused by sediment regulation. Substantial amounts of Hg were impounded in the reservoirs, and only a small fraction was discharged downstream of the reservoir in the non-regulation years. Moreover, Hg fluxes to the coastal ocean were impacted by downstream water diversion in non-regulation years, whereas the contributions of diversions in regulation years were small. In conclusion, models showed that in recent 15 years, sediment regulation can enhance the Hg burden and risk to coastal ecosystem of Bohai Sea.

Elucidating sources of mercury in the west coast of Korea and the Chinese marginal seas using mercury stable isotopes

Nearshore systems play an important role as mercury (Hg) sources to the open ocean and to human health via fish consumption. The nearshore system along East Asia is of particular concern given the rapid industrialization, which contributes to significant anthropogenic Hg emissions and releases. We used Hg stable isotopes to characterize Hg sources in the sediment and fish along the west coast of Korea, located at the northeast of the East China Sea. The Hg isotope ratios of the west coast sediments (δ²⁰²Hg; -0.89 to -0.27‰, Δ¹⁹⁹Hg; -0.04 to 0.14‰) were statistically similar with other nearshore sediments (δ²⁰²Hg; -0.99 to -0.30‰, Δ¹⁹⁹Hg; -0.04 to 0.19‰) and overlapped with the industrial Hg source end-member (δ²⁰²Hg; -0.5‰, Δ¹⁹⁹Hg; 0.01‰) estimated from the Chinese marginal seas. Using a ternary mixing model, we estimated that industrial Hg sources contribute 83-97% in the west coast of Korea, and riverine and atmospheric Hg sources play minor roles in the Korean west coast and the Chinese marginal seas. The comparison between Hg isotope ratios of the sediment and nearshore fish revealed that the fish in the most west coast sites are exposed to MeHg produced in the sediment. At a few southwest coast sites, external MeHg produced in rivers and the open ocean water column appears to be more important as a source in fish. This is supported by much higher δ²⁰²Hg (0.74‰; similar to oceanic fish) and lower δ²⁰²Hg (-0.71‰; similar to riverine sources) compared to fish collected from other west coast sites influenced by sedimentary MeHg. The substantial Hg contributions from industrial activities suggest the national policies regulating anthropogenic Hg releases can directly mitigate human Hg exposure originating via local fish consumption. This study contributes to the growing regional and global inventories of Hg fluxes and sources exported into coastal oceans.

Looping Mercury Cycle in Global Environmental-Economic System Modeling

The Minamata Convention on Mercury calls for Hg control actions to protect the environment and human beings from the adverse impacts of Hg pollution. It aims at the entire life cycle of Hg. Existing studies on the Hg cycle in the global environmental-economic system have characterized the emission-to-impact pathway of Hg pollution. That is, Hg emissions/releases from the economic system can have adverse impacts on human health and ecosystems. However, current modeling of the Hg cycle is not fully looped. It ignores the feedback of Hg-related environmental impacts (including human health impacts and ecosystem impacts) to the economic system. This would impede the development of more comprehensive Hg control actions. By synthesizing recent information on Hg cycle modeling, this critical review found that Hg-related environmental impacts would have feedbacks to the economic system via the labor force and biodiversity loss. However, the interactions between Hg-related activities in the environmental and economic systems are not completely clear. The cascading effects of Hg-related environmental impacts to the economic system throughout global supply chains have not been revealed. Here, we emphasize the knowledge gaps and propose possible approaches for looping the Hg cycle in global environmental-economic system modeling. This progress is crucial for formulating more dynamic and flexible Hg control measures. It provides new perspectives for the implementation of the Minamata Convention on Mercury.