Horizontal and vertical variability of mercury species in pore water and sediments in small lakes in Ontario

Mary Lake, St. George Lake, and Philips Lake are located in the Greater Toronto Area, Ontario, Canada. These lakes are relatively small and have no direct inflow and outflow channels. Mercury (Hg) input to the lakes comes mainly from atmospheric deposition. Sediment cores from the points of the maximum lake depth and surface sediment samples from the points of maximum lake depth to the bank of each lake were collected in October 2005. Total and methyl mercury concentrations in the pore water and sediments of these samples were determined. In these small lakes with high organic content, there was no correlation between organic content and total mercury (THg) in the samples throughout the entire sediment cores while strong positive correlation between these two parameters was observed in all the surface sediments. Compared with typical methylmercury (MeHg) depth-profiles of sediment cores in other studies, where MeHg concentrations and methylation rates decreased sharply with increasing depth, MeHg distributions in the sediment cores in this study showed that MeHg might have been produced not only in the upper sediment but also in the deeper sediments, which resulted in a larger MeHg reservoir in the sediment. Organic matter, to some extent, affected MeHg distributions in the samples throughout the entire sediment cores. Concentrations of MeHg in all the surface sediments, however, were not controlled by organic matter, whereas they were largely a function of water column depths. Total mercury concentrations in pore water were relatively homogenous in both the sediment cores and surface sediment while MeHg in pore water generally deceased with increasing depth in the sediment cores and increasing distance from the centre of the lakes in surface sediments. Methylmercury contributed 1% to 76% of THg in the pore water samples. Concentrations and distributions of MeHg in overlying water and sediment-surface water in Mary Lake and St. George Lake suggested that both in situ production of MeHg in lake water and the release of MeHg from sediment contributed to high MeHg in deep anoxic water.     

Partitioning of dissolved organic matter-bound mercury between a hydrophobic surface and polysulfide-rubber polymer

This study investigated the role of dissolved organic matter on mercury partitioning between a hydrophobic surface (polyethylene, PE) and a reduced sulfur-rich surface (polysulfide rubber, PSR). Comparative sorption studies employed polyethylene and polyethylene coated with PSR for reactions with DOM-bound mercuric ions. These studies revealed that PSR enhanced the Hg-DOM removal from water when DOM was Suwannee River natural organic matter (NOM), fulvic acid (FA), or humic acid (HA), while the same amount of 1,3-propanedithiol-bound mercuric ion was removed by both PE and PSR-PE. The differences for Hg-DOM removal efficiencies between PE and PSR-PE varied depending on which DOM was bound to mercuric ion as suggested by the PE/water and PSR-PE/water partition coefficients for mercury. The surface concentrations of mercury on PE and PSR-PE with the same DOM measured by x-ray photoelectron spectroscopy were similar, which indicated the comparable amounts of immobilized mercury on PE and PSR-PE being exposed to the aqueous phase. With these observations, two major pathways for the immobilization reactions between PSR-PE and Hg-DOM were examined: 1) adsorption of Hg-DOM on PE by hydrophobic interactions between DOM and PE, and 2) addition reaction of Hg-DOM onto PSR by a complexation reaction between Hg and PSR. The percent contribution of each pathway was derived from a mass balance and the ratios among aqueous mercury, PE-bound Hg-DOM, and PSR-bound Hg-DOM concentrations. The results indicate strong binding of mercuric ion with both dissolved organic matter and PSR polymer. The FT-IR examination of Hg-preloaded-PSR-PEs after the reaction with DOM corroborated a strong interaction between mercuric ion and 1,3-propanedithiol compared to Hg-HA, Hg-FA, or Hg-NOM interactions.     

Potential effects of forest fire and storm flow on total mercury and methylmercury in sediments of an arid-lands reservoir

A study was conducted from July 1995 to June 1996 to examine the spatial and temporal changes of mercury concentrations in sediments of an arid-lands reservoir. Prior to the first sample collection in July, a forest fire burned 2930 ha of mixed conifer and ponderosa pine in the watershed of Caballo Reservoir in south-central New Mexico. The fire was eventually extinguished by summer rains and storm runoff resulting in the mobilization and transport of charred vegetative material into an intermittent tributary (Palomas Creek) that drains the watershed into Caballo Reservoir. Concentrations of total mercury (THg), monomethlymercury (MMHg), and total organic carbon (TOC) in surficial sediments revealed fire, followed by storm runoff, enhanced the transport of mercury and organic matter to the reservoir. Concentrations of THg in sediments increased from 7.5 etag/g in July to 46.1 etag/g by November 1995 at one site (Palomas) nearest the outflow of Palomas Creek. No other spatial or temporal trends were observed for THg at other sites throughout the remainder of the study. Concentrations of MMHg in sediments at the Palomas site increased from 0.428 etag/g in July to 12.46 etag/g by October 1995 compared to concentrations in sediments at the remaining sites which ranged from 0.11 to 1.50 etag/g throughout the study. The ratio of MMHg to THg (a gross index of methylation activity) was greatest in sediments from the Palomas site (5.4-33.8%) compared to the remaining sites (0.01-3.60%). The ratio was mirrored by elevated TOC in sediments at the Palomas site (2.5-11.8%) that remained elevated throughout the study. Fire and subsequent late-summer rains may have had a twofold effect on mercury concentrations in Caballo Reservoir. The storm-driven runoff following the forest fire carried mercury complexed to organic matter which resulted in elevated levels of mercury as well as providing a carbon source for microbial methylation processes in sediment.     

Distribution of total and methyl mercury in sediments along Steamboat Creek (Nevada, USA)

In the late 1800s, mills in the Washoe Lake area, Nevada, used elemental mercury to remove gold and silver from the ores of the Comstock deposit. Since that time, mercury contaminated waste has been distributed from Washoe Lake, down Steamboat Creek, and to the Truckee River. The creek has high mercury concentrations in both water and sediments, and continues to be a constant source of mercury to the Truckee River. The objective of this study was to determine concentrations of total and methyl mercury (MeHg) in surface sediments and characterize their spatial distribution in the Steamboat Creek watershed. Total mercury concentrations measured in channel and bank sediments did not decrease downstream, indicating that mercury contamination has been distributed along the creek's length. Total mercury concentrations in sediments (0.01-21.43 microg/g) were one to two orders of magnitude higher than those in pristine systems. At 14 out of 17 sites, MeHg concentrations in streambank sediments were higher than the concentrations in the channel, suggesting that low banks with wet sediments might be important sites of mercury methylation in this system. Both pond/wetland and channel sites exhibited high potential for mercury methylation (6.4-30.0 ng g(-1) day(-1)). Potential methylation rates were positively correlated with sulfate reduction rates, and decreased as a function of reduced sulfur and MeHg concentration in the sediments. Potential demethylation rate appeared not to be influenced by MeHg concentration, sulfur chemistry, DOC, sediment grain size or other parameters, and showed little variation across the sites (3.7-7.4 ng g(-1) day(-1)).     

Total mercury and methyl mercury levels in British estuarine sediments—II

Results from a large-scale study of Mersey Estuary, U.K. sediments are discussed. Correlations are drawn between total mercury, methyl mercury, silt and organic carbon contents of the sediments. These correlations are compared to earlier results, and coefficients range from 0.55 to 0.94.The influence of redox potential and the sulphide content of a sediment on both the ambient level of methyl mercury and the extent of a growth and decay effect in methyl mercury levels in sediments after sampling is discussed. The results show that in an esturary (Clyde, U.K.) where sulphide levels vary from 0 to 8.5 mg g⁻¹ there is a maximum level of methyl mercury found at approximately 5.8 mg g⁻¹ sulphide, suggesting that sulphide concentrations may control methyl mercury levels.     

Influences of iron, manganese, and dissolved organic carbon on the hypolimnetic cycling of amended mercury

The biogeochemical cycling of iron, manganese, sulfide, and dissolved organic carbon were investigated to provide information on the transport and removal processes that control the bioavailability of isotopic mercury amended to a lake. Lake profiles showed a similar trend of hypolimnetic enrichment of Fe, Mn, DOC, sulfide, and the lake spike ((202)Hg, purity 90.8%) and ambient of pools of total mercury (HgT) and methylmercury (MeHg). Hypolimnetic enrichment of Fe and Mn indicated that reductive mobilization occurred primarily at the sediment-water interface and that Fe and Mn oxides were abundant within the sediments prior to the onset of anoxia. A strong relationship (r(2)=0.986, n=15, p<0.001) between filterable Fe and Mn indicated that reduction of Fe and Mn hydrous oxides in the sediments is a common in-lake source of Fe(II) and Mn(II) to the hypolimnion and that a consistent Mn:Fe mass ratio of 0.05 exists in the lake. A strong linear relationship of both the filterable [Fe] (r(2)=0.966, n=15, p<0.001) and [Mn] (r(2)=0.964, n=15, p<0.001) to [DOC] indicated a close linkage of the cycles of Fe and Mn to DOC. Persistence of iron oxides in anoxic environments suggested that DOC was being co-precipitated with Fe oxide and released into solution by the reductive dissolution of the oxide. The relationship between ambient and lake spike HgT (r(2)=0.920, n=27, p<0.001) and MeHg (r(2)=0.967, n=23, p<0.001) indicated that similar biogeochemical processes control the temporal and spatial distribution in the water column. The larger fraction of MeHg in the lake spike compared to the ambient pool in the hypolimnion suggests that lake spike may be more available for methylation. A linear relationship of DOC to both filterable ambient HgT (r(2)=0.406, n=27, p<0.001) and lake spike HgT (r(2)=0.314, n=15, p=0.002) suggest a role of organic matter in Hg transport and cycling. However, a weak relationship between the ambient and lake spike pools of MeHg to DOC indicated that other processes have a major role in controlling the abundance and distribution of MeHg. Our results suggest that Fe and Mn play important roles in the transport and cycling of ambient and spike HgT and MeHg in the hypolimnion, in part through processes linked to the formation and dissolution of organic matter-containing Fe and Mn hydrous oxides particles.     

The poison chain for mercury in the environment

The toxic effects of mercury in specific population groups, for example following occupational exposure or therapeutic application, has long been known, but mercury as a general environmental hazard has followed comparitively recent technological development. Contamination with both inorganic mercury and organic mercury compounds has resulted from industrial waste and from the use of the latter compounds as fungicides. Deposited eventually in rivers and lakes, this has led to raised levels of mercury in fish. Under appropriate conditions, methylation of mercury may occur in the natural environment, converting less toxic inorganic and aryl mercury compounds into the more toxic alkyl mercury form. In certain predominantly fish eating populations this has led to an increase in the body burden for organic mercury, resulting in localized outbreaks of methyl mercury poisoning. Raised levels of mercury in eggs and bird tissues have resulted from feeding on mercury contaminated fish and on organomercury treated seed. This has contributed to a reduction in some species of birds in certain areas. In addition to the direct toxic action of mercury, limited evidence suggests that methyl mercury compounds may possibly exert mutagenic and teratogenic effects, at levels below those usually associated with poisoning. Methyl mercury concentration should be monitored in exposed groups and where this is raised, epidemiological studies performed to evaluate a possible hazard.     

Tests of the fidelity of lake sediment core records of mercury deposition to known histories of mercury contamination

There has been recent controversy over the discrimination between natural and anthropogenic loadings of mercury to lakes. Sediment core profiles have been interpreted as evidence that inputs to lakes have increased. Some investigators have argued, however, that mercury may be sufficiently mobile in sediments to generate profiles that are misinterpreted as historical records. This argument can be tested where the histories of inputs of mercury are known independently from other kinds of information. We have such cases in Canadian lakes and we have been able to assemble sediment core records for comparison with known source histories. Three cases are represented by Clay Lake in Ontario where the source was a chlor-alkali plant with a known history of mercury discharges, Giauque Lake in the Northwest Territories where mercury was used at a gold mine to extract gold from ore, and Stuart Lake in British Columbia where a mercury mine operated for a known period at Pinchi Lake, the lake immediately upstream from Stuart Lake. In these cases lake sediment cores were dated using lead-210 and cesium-137 and then slices were analysed for mercury. The histories of mercury deposition derived from the cores agreed well with the known histories of inputs.     

The role of suspended solids in the estuarine geochemistry of mercury

The behaviour of mercury in total water (soluble + particulate fractions) and suspended solids of the surface waters of the Wyre estuary, northwest England, has been studied during a tidal cycle. The estuary is contaminated by mercury derived from chlor-alkali effluents and other sources. The results from eight stations in the estuary show that quantitatively mercury in suspended solids dominates the mercury budget of the estuarine waters. At high chlorinity periods in the tidal cycle the mercury concentrations in suspended solids decreased significantly, and can be related to changes in the specific surface area and organic carbon contents of the suspended matter. A very good correlation between mercury and organic carbon concentrations in the suspended solids exists, and laboratory experiments support the conclusion that mercury is mainly associated with the organic phase of the suspended material and is not easily chemically desorbed during estuarine mixing processes.     

Composition of bacterial and archaeal communities in freshwater sediments with different contamination levels (Lake Geneva, Switzerland)

The aim of this study was to compare the composition of bacterial and archaeal communities in contaminated sediments (Vidy Bay) with uncontaminated sediments (Ouchy area) of Lake Geneva using 16S rRNA clone libraries. Sediments of both sites were analysed for physicochemical characteristics including porewater composition, organic carbon, and heavy metals. Results show high concentrations of contaminants in sediments from Vidy. Particularly, high contents of fresh organic matter and nutrients led to intense mineralisation, which was dominated by sulphate-reduction and methanogenesis. The bacterial diversity in Vidy sediments was significantly different from the communities in the uncontaminated sediments. Phylogenetic analysis revealed a large proportion of Betaproteobacteria clones in Vidy sediments related to Dechloromonas sp., a group of dechlorinating and contaminant degrading bacteria. Deltaproteobacteria, including clones related to sulphate-reducing bacteria and Fe(III)-reducing bacteria (Geobacter sp.) were also more abundant in the contaminated sediments. The archaeal communities consisted essentially of methanogenic Euryarchaeota, mainly found in the contaminated sediments rich in organic matter. Multiple factor analysis revealed that the microbial community composition and the environmental variables were correlated at the two sites, which suggests that in addition to environmental parameters, pollution may be one of the factors affecting microbial community structure.     

Hydrochemistry and mercury cycling in a High Arctic watershed

Mass budgets for total mercury, major ions and nutrients were calculated for Amituk Lake, located on Cornwallis Island, Nunavut, Canada. Total mercury in two distinct snowpacks averaged 1.25 and 4.21 ng L(-1); the discharge-weighted concentration of influent streams averaged 0.76 ng L(-1). The recent and pre-industrial Hg(T) fluxes in atmospheric deposition to the catchment were estimated to be 0.57 and 0.23 microg m(-2) but through retention within the catchment and/or re-volatilization from the melting snowpack, these decreased by 69% in the lake inflow. The spring freshet was the prime conduit for transporting Hg(T) into Amituk Lake. Because of limited mixing of surface runoff with the lake water column during snowmelt, 59% of the Hg(T) input was directly discharged through the outflow, 16% entered the lake water column where concentrations increased from 0.23 to 0.33 ng L(-1) from June to August and 25% was deposited to the bottom sediments producing a sediment Hg(T) flux of 3.1 microg m(-2).     

The competitive role of organic carbon and dissolved sulfide in controlling the distribution of mercury in freshwater lake sediments

The detailed distribution of mercury was studied in sediments and porewaters of two freshwater lakes, which were selected because of the contrasting conditions they present at their respective sediment-water interface (SWI). One lake is characterized by a SWI that remains oxic all year long whereas the other one shows a clear seasonal variation with the evolution of strongly anoxic conditions through the summer season. The results of the study clearly identify the importance of redox conditions on the geochemical behaviour of Hg at the SWI of both lakes but a very limited influence of an oxidized layer enriched in Fe and Mn oxyhydroxides at the top of the sediment of the oxic lake. In both lakes, a competitive effect on the cycling and mobility of the element was observed between natural organic matter and amorphous or organo-sulfide compounds. The proportion of Hg associated to natural organic matter in sediments showed a general increase with sediment depth. A fraction containing elemental Hg and Hg suspected to be bound to iron sulfides and organo-sulfides constituted the other major fraction of solid Hg in the sediments of both lakes. This second pool of Hg was generally larger at the top of the sediment where the production of dissolved sulfides is usually more detectable and it decreases with depth, suggesting that the metal is partially transferred from one pool being the sulfides including amorphous FeS and organo-sulfides to the organic matter pool. Methyl Hg represented less than 1% of the total Hg in sediments of both lakes. Our results obtained at different times of the summer season from two lakes contrasted by their SWI emphasize the competitive or alternating role played by dissolved and solid natural organic matter and sulfides on the fate of Hg in freshwater systems.     

The fate and transport of mercury, methylmercury, and other trace metals in Chesapeake Bay tributaries

Six tributaries to the Chesapeake Bay were analyzed for suspended particulate matter, dissolved organic carbon, particulate organic carbon, mercury, methylmercury, lead, nickel, zinc, cadmium, chromium, and copper. This study examined the importance of flow regime, suspended particulate concentration, and watershed characteristics on the transport of mercury, methylmercury, and other trace metals. Total mercury concentrations were higher under high flow conditions which is consistent with the tendency of this metal to bind strongly to particulate matter. Methylmercury showed less flow rate dependence. Nickel, lead, and zinc concentrations responded strongly to flow rate on the Potomac River, while weaker correlations were found on the other rivers sampled. Cadmium, copper, and chromium concentrations were the least influenced by flow. Partition coefficients calculated in this study were similar to those of other estuaries and overall decreased in the order of Hg > Ni-MMHg > Cr-Pb-Zn > Cd > Cu. Watershed yield estimates and associated retention factors were calculated for the various rivers. These calculations showed that for most of the rivers, mercury was the most strongly retained within the watershed.     

Distributions of total mercury and methylmercury in surface sediments and fishes in Lake Shihwa, Korea

The concentrations of total mercury (THg) and methylmercury (MeHg) in the sediments of Lake Shihwa, an artificial salt lake in Korea located near two large industrial complexes, were determined to investigate the state of Hg contamination in the lake sediments and the effect of local Hg source. THg and MeHg concentrations in the sediments, monitored for 2 years, ranged from 0.02 to 0.28 µg g^− 1 and ≤ 0.026 to 0.67 ng g^− 1, respectively. The overall distribution of Hg in lake sediments showed higher values near industrial complexes and in the central part of the lake. However, the correlations between Hg and environmental factors, such as organic material (OM) content, and acid volatile sulfide (AVS), were weak and did not clearly explain the variation in Hg distribution. The spatial distribution of sediment Hg and monthly precipitation data during the sampling period showed that the amount of runoff following rain events and water gate operation may be additional important factors regulating Hg level and distribution in lake sediments. The levels of THg in fish species in this lake ranged from 9.8 to 35 ng g^− 1, suggesting that the bioavailability of sediment Hg in the lake may be low. Although the THg concentrations in Lake Shihwa sediment were lower than those in other foreign study sites, they were higher than in neighboring coastal regions, and are constantly increasing. This result indicates that the nearby industrial complexes may be the major source of Hg found in the sediments of Lake Shihwa.     

Characterizing nutrient dynamics with relation to changes in partial pressure of CO2 in a tropical sewage‐fed aquaculture pond situated in a Ramsar wetland

The changes in nutrient dynamics (nitrate, ammonium, silicate, phosphate and iron concentration) vis‐à‐vis partial pressure of CO₂ in water [pCO₂(water)] from tropical sewage‐fed aquaculture ponds (East Kolkota Wetlands, India) were analysed by means of a microcosm. A significant relationship between these nutrient’s removal from the system and reduction in pCO₂(water) was observed (with few exceptions). These water bodies acted as significant sources of CO₂ in pre‐monsoon and monsoon seasons despite having substantial quantity of chlorophyll‐a to make it a net autotrophic system. The study revealed that if conditions favourable for optimum photosynthesis can be maintained in these ponds, the CO₂ source character of these ponds can be reversed. In the post‐monsoon season, when the pH of the water column was high, the system acted as sink for CO₂ which suggests the use of lime to prevent these systems from becoming hypereutrophic and carbon source at the same time.     

Denitrification at various carbon to nitrogen ratios

The aim of this research was to examine whether the residual dissolved organic matter, remaining in chemically treated raw sewage would be able to satisfy the carbon demand in a denitrification process. In the first stage of research we investigated the effect of type and amount of organic substrate on denitrification efficiency. The critical weight ratios of methanol and sodium acetate to total concentration of nitrite and nitrate which enable the occurrence of complete denitrification were studied. It was found that when the concentration of the organic matter was expressed as BOD, a critical ratio of (mg BOD/mgΣNO_x-N) = 2.3 ensured 100% denitrification. Lower ratios decreased denitrification efficiencies proportionally. The same critical ratio was found when the chemically treated raw sewage was used as an available organic carbon source. Denitrification-nitrification process was also investigated by recirculating the nitrified effluent into the denitrification reactor, to which effluents from chemical treatment of raw sewage were fed to satisfy the carbon demand. The same critical ratio of BOD/ΣNO_x-N) = 2.3 was found. By increasing the recycling, nitrate concentration in effluent was decreased.     

Phosphate uptake under anoxic conditions and fixed-film nitrification in nutrient removal activated sludge system

Simultaneous enhanced biological phosphate uptake and biological denitrification under anoxic conditions were investigated in a modified lab-scale nutrient removal activated sludge system. The aim of the experiments was to find whether poly-P bacteria are capable of taking up phosphate under anoxic conditions by utilising nitrate as an electron acceptor. The phosphate uptake in anoxic conditions was compared to that in aerobic environment in batch tests. The results of the long-term operation of continuous-flow lab-scale system as well as the results of batch tests showed that the anoxic phosphate uptake with simultaneous denitrification after preceding anaerobic substrate uptake could significantly reduce the extent of competition for organic substrate between poly-P bacteria and denitrifiers. A side-stream nitrification in fixed-film reactor enabled to reduce the losses of organic carbon by aerobic oxidation and to stabilise the slow-growing population of nitrifiers in the system.     

Influence of traditional agricultural practices on mobilization of arsenic from sediments to groundwater in Bengal delta

In the wake of the idea that surface derived dissolved organic carbon (DOC) plays an important role in the mobilization of arsenic (As) from sediments to groundwater and may provide a vital tool in understanding the mechanism of As contamination (mobilization/fixation) in Bengal delta; a study has been carried out. Agricultural fields that mainly cultivate rice (paddy fields) leave significantly large quantities of organic matter/organic carbon on the surface of Bengal delta which during monsoon starts decomposing and produces DOC. The DOC thus produced percolates down with rain water and mobilizes As from the sediments. Investigations on sediment samples collected from a paddy field clearly indicate that As coming on to the surface along with the irrigation water accumulates itself in the top few meters of sediment profile. The column experiments carried out on a 9 m deep sediment profile demonstrates that DOC has a strong potential to mobilize As from the paddy fields and the water recharging the aquifer through such agricultural fields contain As well above the WHO limit thus contaminating the shallow groundwater. Experiment also demonstrates that decay of organic matter induces reducing condition in the sediments. Progressively increasing reducing conditions not only prevent the adsorption of As on mineral surfaces but also cause mobilization of previously sorbed arsenic. There seems to be a cyclic pattern where As from deeper levels comes to the surface with irrigational water, accumulates itself in the sediments, and ultimately moves down to the shallow groundwater. The extensive and continual exploitation of intermediate/deep groundwater accelerates this cyclic process and helps in the movement of shallow contaminated groundwater to the deeper levels.     

Distribution of organic carbon in tropical mangrove sediments (Cochin,India)

Surficial sediments were sampled every month from three mangrove regions around the metropolis of Cochin (India). Sedimentary organic carbon content exhibited wide seasonal fluctuations. The hydrology of the mangrove system is regulated mainly by high rainfall during the monsoon and by tidal inundation. The results indicated the role of tidal activity and sediment texture in the preservation and retention of organic matter.     

生物粉末活性炭/超滤组合工艺处理微污染原水

针对微污染原水中存在的有机物和氨氮等污染物,采用生物粉末活性炭/超滤(BPAC/UF)组合工艺进行处理。结果表明,当进水氨氮浓度较低时,硝化细菌活性较差,无法充分发挥生物降解作用,氨氮去除率较低,同时有机物去除率也较低;当进水氨氮浓度在0. 6 mg/L左右时,可以形成稳定的生物活性炭,组合工艺对氨氮的去除率较高,且对有机物的去除率较为稳定。进水中主要以分子质量<5 ku的有机物为主,组合工艺对这部分有机物的去除率也最高。组合工艺对疏水性物质的去除,主要依靠生物粉末活性炭的吸附降解和膜面滤饼层的截留作用。NaClO强化反冲洗可以很好地降低跨膜压差的增长速度,当NaClO浓度为400 mg/L、反冲洗时间为10min时可达到最佳清洗效果。