2b-3: Sources and cycling of mercury in freshwater ecosystems

Understanding the hydrological and biogeochemical controls on mercury mobility in terrestrial upland and wetland landscapes is imperative for accurately simulating terrestrial mercury inputs to aquatic ecosystems. Despite decades of research investigating the relationships between hydrological processes and the biogeochemical cycling of mercury, landscape heterogeneity is still a major source of uncertainty in generating reliable simulations of mercury transport to receiving waters across scales and landscapes. Within the hydrological community, the concept of hydrologic connectivity is viewed as an organizing principle that underlies landscape complexity. While the phrase biogeochemical connectivity has not been frequently used in a similar manner, many approaches to understanding diffuse pollution risk at the catchment scale have focused on sources and mobilization with a basic representation of the effect of connectivity between the landscape and receiving waters. Recently, a cohesive mathematical framework to quantify the occurrence, frequency, duration, and magnitude of hydrologic and water-mediated material connectivity has been proposed to bring these concepts together. Here, we show how this framework can be applied to better understand the hydrological and biogeochemical conditions necessary for inorganic mercury to be transported from forested upland catchments to receiving waters. By providing a means to detect, quantify, understand and model the water-mediated connectivity of material, such as mercury, across different landscape units, we demonstrate that this framework will also be useful for site inter-comparisons, as well as have the potential to inform environmental policy and management decisions that are dependent on quantifying connectivity.

The Cache Creek Settling Basin (CCSB) is a 1600-hectare floodplain area in Yolo County, bounded by levees since its construction in 1937 and expansion in 1993. The CCSB traps sediment to reduce accumulation in the Yolo Bypass, a conveyance for Sacramento River flood waters to the SacramentoSan Joaquin Delta, protecting the city of Sacramento. The CCSB traps particulate mercury (Hg) eroded from the Cache Creek watershed, which has several natural and anthropogenic Hg sources: wastes from inactive Hg mines and gold mines where Hg-amalgamation was used historically, thermal springs, and erosion of geologic materials with naturally elevated Hg. Analysis of filtered water and suspended particulate samples for total mercury (THg) and methylmercury (MeHg) in stream-flow entering and exiting the CCSB during 201016 indicated that: ~65% of particulate THg and MeHg were trapped in the CCSB; there was no net change in filtered-water THg load; and filtered-water MeHg load increased ~30% in basin outflows relative to inflows. The long-term average rate of sediment accumulation in the CCSB was ~2 cm/yr based on analysis of 137Cs in 6 cores. Principal habitats mapped within the CCSB were: open water (stream channels and canals), riparian (along stream channels), non-agricultural floodplain, and agricultural (row crops, primarily corn and tomato). Analysis of shallow sediment (02 cm) collected at 93 locations within the CCSB on four occasions during 201315 indicated that agricultural and open-water habitats had consistently lower MeHg than riparian and non-agricultural floodplain habitats, along with lower concentrations of total reduced sulfur (TRS) and organic matter. Within each habitat, there was a consistent increase along the direction of stream flow in MeHg concentration and the ratio MeHg/THg (%MeHg). A similar spatial variation was found for TRS and organic matter, which increased in the flow direction, and median grain size, which decreased. A similar pattern of increasing MeHg with downstream distance was found in tissue of western mosquitofish (Gambusia affini), for caged (30-day exposure) and uncaged samples. Monthly sampling of shallow sediment at selected sites during 201516 revealed increased MeHg during wetter conditions (December through May), and increased reactive Hg(II), a precursor to MeHg, during drier conditions (June through November). The finding that riparian and non-agricultural floodplain habitats had significantly higher MeHg than agricultural (row-crop) habitat could have broad implications for resource management, given that these habitats are typical of managed floodplains in the SacramentoSan Joaquin Delta region and elsewhere.

India is the largest consumer (5.31 × 105 kg in 2002) as well as emits 3.10 × 105 kg mercury on large scale. It has gained scientific attention due to its increasing riverine mercury concentrations and discharges at its ocean margins. In northern India, high levels of mercury have recently been reported in rain (3.13 ppb), ground (15.58 ppb) and river water (9.9 ppb) as well as in fishes (152 ppb) and lichens (4112 ppb) of the Ganga Alluvial Plain. To understand the export dynamics of riverine mercury from the Ganga Alluvial Plain, the Gomati River (a 900-km long alluvial tributary of the Ganga River) was selected for the present study. The river drains 30,437 km2 area of the alluvial plain and experiences a warm and humid sub-tropical climate with four distinct seasons: the monsoon, the post-monsoon, the winters and the summer. Chandwak (25°35’N; 83°00’E) sampling site has been identified from distal part to represent the entire drainage area. Dissolved mercury (Hgd) flux in the Gomati River was obtained from the observations (n=36) of water discharge and Hgd concentration at an interval of ten days for the period of June 2009-May 2010.

The average Hgd concentration and flux were recorded during the monsoon (12.8 ppb; 3.73 × 104 kg) the post-monsoon (11.2 ppb; 2.06 × 104 kg), the summer (8.1 ppb; 0.79 × 104 kg) and the winter (6.9 ppb; 0.26 × 104 kg) seasons. The estimated annual Hgd flux was 6.84 × 104 kg to the Ganga River, out of which 85% move during the monsoon and post-monsoon seasons and strongly correlated to water discharge. It is two orders of magnitude higher than Yenisei, Ob, Mackenzie, Nelson and Yukon rivers of the Arctic region. Consequently, all alluvial rivers draining the Ganga Alluvial Plain potentially provide the major mercury to fishes of the Ganga River. Present study underscores the significance of mercury import from thermal power plants besides various anthropogenic sources (electrical wastes containing fluorescent lights, municipal solid wastes, medical wastes, brick kilns, agricultural and animal waste burning, phosphate fertilizer etc.). In the light of ‘Minamata convention 2013’, the transfer and implementation of advanced mercury abatement technology will be a promising way to reduce these mercury emissions. Future research must be a pursuit for the better understanding of impact of mercury on human health (such as cardiovascular disease) in this region which accounts for nearly 7% of world’s human population.

Historic mining practices have left the Carson River and Lahontan Reservoir (CRLR) system contaminated with high levels of mercury (Hg). Specifically, Hg levels in Lahontan Reservoir Sacramento blackfish (Orthodon microlepidotus) exceed federal consumption limits by nearly three-fold. Inputs of Hg to the system are mainly a result of riverbank erosion during high flow and diffusion from bottom sediments during low flow, and the relationships between streamflow and both mercury transport and bioaccumulation are non-linear. This research project attempts to predict the potential impacts of future climate change on the transport and fate (i.e., methylmercury concentrations in reservoir Sacramento blackfish) in the CRLR system.

The United States Bureau of Reclamation has produced future streamflow estimates for time period 2000-2099 for the upstream boundary of the modeled portion of the CRLR system using 112 CMIP3 climate projections and the Variable Infiltration Capacity (VIC) model. VIC results suggest that the hydrology of the system is likely to experience increased frequency of both high and low extreme flows, and the monthly averages of future flows are expected to be higher in the winter and lower in the summer compared to historical values. Predicted upstream river flow values are then used to drive the CRLR Hg transport model (RIVMOD, WASP5, and MERC4) that accurately predicts total and dissolved inorganic Hg along with total and dissolved methylmercury (MeHg) concentrations.

Bioenergetic algorithms and Hg mass balance approaches were combined into a single modeling framework (BioHg) to simulate the bioaccumulation of MeHg in Lahontan Reservoir Sacramento blackfish as a function of dissolved methylmercury (DMeHg) water column concentrations. Model parametrization relies on laboratory and field data to calibrate allometric coefficients related to consumption, respiration, specific dynamic action, egestion, excretion, and spawning. Actual consumption is reliant on feeding strategies, prey selectivity, filtering efficiency, and prey availability. BioHg is able to capture trends in observed growth and MeHg accumulation. The predicted Carson River flow patterns and associated DMeHg concentrations are then input into the calibrated BioHg model to compute potential future Hg levels in reservoir Sacramento blackfish. Predicted bioaccumulation levels increase in both the South (i.e., Upper) and Middle Basins of the reservoir as a result of an increase in DMeHg water column concentrations.

Fish consumption advisories for mercury (Hg) are common in reservoir systems, including Lake Powell, a 300 km long reservoir along the Colorado River in the arid Southwest region of the U.S.. Based on striped bass Hg concentrations, the consumption advisory for Lake Powell is limited to the lower 70 km portion of the reservoir, although the reason for this spatial trend is unknown. This trend coupled with the fact that Hg biogeochemical cycling and bioaccumulation studies in reservoirs have been far more extensive in the eastern U.S., compared to the western U.S., promoted a two-year investigation of Lake Powell Hg dynamics. Samples of surface water (multiple depths from surface to bottom), seston and bed sediment were collected at 12-13 fixed sites during both minimally-stratified / high inflow (May/June 2014) and strongly-stratified / low-flow (August 2015) conditions. These samples were analyzed for a large suite of Hg and non-Hg related parameters. In-situ water column parameters were also measured in continuous profile from 24-25 sites during both sampling events. The totality of results supports a conceptual model in which the concentration and chemical nature of water column particulates transitions from a mixture of inorganic plus autochthonous (phytoplankton) and terrestrially derived organics in the upper end of the system to largely phytoplankton dominant in the lower portion of the system. This transition subsequently results in the deposition of more labile organic matter (phytoplankton) in the lower reservoir, which fuels higher rates of anaerobic microbial activity and higher rates of methylmercury (MeHg) production, which bioaccumulates up the food web and results in the higher observed Hg concentrations in striped bass in the lower portion of the reservoir. Specific supporting observations include increasing rates of microbial MeHg production and microbial biomass in sediment, increasing water column and seston MeHg concentrations, higher phytoplankton biomass, and decreasing fluorescent dissolved organic matter from the upper (riverine input) to the lower (towards the dam) regions of the reservoir. A distinct peak in water column MeHg (as a percent of total Hg) was routinely observed at or near the thermocline, suggesting the possibility of water column MeHg production in addition to benthic production. Molecular analysis of sediment suggests that the elemental sulfur and ferric iron reducing bacterial genus Desulfuromonas sp. may be the dominant microbe associated with MeHg production in Lake Powell.

In Vietnam, mercury has been widely used in industrial production, particularly coal power plants, cement factories, steel plants. Industrial and municipal wastewater is directly discharged into many rivers, whereas the state control and monitoring of Hg has yet to be strictly, making Vietnam facing a high risk of mercury contamination to air and water resources.

In Hanoi capital, the largest city in the Red River Delta, there are more than 90 factories in the city, consuming nearly 148,000 tons of coal and 12,000 tons of petroleum per year, as a big source of mercury to be released to the environment. Analytical data of samples in two rivers (ie. Nhue river and To Lich river) in Hanoi capital revealed Hg concentrations of 0.09 - 3.34 μg/l in river water, 0.078 - 1.136 ppm in river sediment, and 0.036 - 1.567 ppm in suspended particulate matter (SPM), respectively, with a tendency to increase from upstream to downstream. Analytical results for water sample at river mouth of the Red River Delta also demonstrated that most of the Hg is transported into the estuary from upstream sources and Bach Dang river estuary is a significant source of MMHg during the dry seasons (3 g MMHg/day). During high river discharge in the wet season, there was a large estuarine input of total Hg while the longer residence time of the waters during the dry season promotes increasing MMHg formation. Even though high concentration of Hg has been detected to accumulate in water hyacinth, particularly at sampling site with high Hg concentrations detected in water and suspended particulate, there have yet data as evidence for impacts of Hg on human health. After joining the Minamata Convention on Mercury in October 2013, Vietnam has paid serious attention to monitoring, controlling and reducing mercury emissions, and as a way forward, more care on environmental health related to Hg should be taken, in further in-depth research and international collaboration.

Methylmercury (MeHg) is a potent neurotoxin that threats aquatic environment and human health. Human activities can drive more total mercury (THg) and MeHg releasing from land to sea, and riverine pathway contribute the most. To better understand the source and contributions of global riverine THg and MeHg to seas, we quantified the THg and MeHg discharge from Chinese rivers to the adjacent seas, based on some measurements of the 16 largest rivers in Mainland China. Filtered water and suspended sediment were collected in four season of the 7 largest rivers, and dry and wet seasons of other 9 rivers. Both THg and MeHg were divided into dissolved phase and particulate phase, and the measurements followed the U.S. EPA Methods. Totally 80 tons of THg and 1.2 tons of MeHg discharge from rivers in Mainland China to the adjacent seas in 2015. Annual riverine THg discharge was decreasing in recent years, due to the management of Chinese government on Hg release from anthropogenic sources to aquatic environment. This study has provided more detailed information on THg discharge and filled the vacancy of riverine MeHg discharge from Chinese rivers to seas.

Understanding mercury cycling, transport, and fate is essential for assessing and managing water quality at aquatic sites where human influence may have affected natural processes. Water column and near-bed processes vary temporally and spatially due to physical processes such as tidal variability, storm flow events, and hydrodynamic interactions between different ecosystems within an impacted site. This system-wide variability confounds evaluation of mercury sources and transport mechanisms when using conventional laboratory-based analysis of discrete water samples. The OPTically-based In-situ Characterization System (OPTICS) provides for estimation of particulate and dissolved mercury and methyl mercury concentrations in surface water at high frequency (e.g., less than hourly) and over extended (e.g., order of months) durationsupporting efficient, cost-effective collection of an unprecedented amount of data to understand mercury variability and its relationship to the processes that drive mercury fate and transport.

Case studies are presented where OPTICS monitoring was employed to support the evaluation of mercury cycling, transport, and fate at an estuarine site and a riverine site. At the estuarine site, OPTICS data were integrated with physical, biogeochemical, and water quality measurements to demonstrate that marsh-derived organic matter settles to the waterway sediment bed during slack tides to form a thin, benthic layer of low-density material on which mercury and methyl mercury can be sorbed. Thin, benthic layer resuspension during high tidal velocities and storm flows is a primary mechanism for mercury and methyl mercury transport to surface water. Further, the OPTICS results demonstrated a net flux of mercury and methyl mercury from the waterways into the marshes, where the chemicals are retained. The OPTICS results are an important line of evidence to the conceptual site model (CSM), which forms the basis for the evaluation of a remedy for the site.

At the riverine site, OPTICS monitoring enabled evaluation of mercury transport under baseflow and storm flow conditions. Particulate mercury and methyl mercury transport was substantively increased during storm flow conditions due to bank interactions and resuspension of bedded sediment. Dissolved methyl mercury concentrations increased with increasing water temperature, likely due to increased microbiological activity. Baseflow patterns indicate a diurnal pattern potentially consistent with nocturnal bioturbation processes. The results of the pilot test were used to verify and refine the CSM and will be an important consideration to the adaptive management of the site.