Hg Concentrations In Sediments Research Articles

Magnetic characteristics of lake sediments in Qiangyong Co Lake, southern Tibetan Plateau and their application to the evaluation of mercury deposition

Heavy metals, one of the most toxic classes of pollutants, are resistant to degradation and harmful to the biological environment. The lakes that have developed on the Tibetan Plateau are ideal regions to investigate historic heavy metal pollution, particularly through the use of the reliable 210Pb dating technique. Environmental magnetism has been successfully applied to estimate heavy metal pollution in different environmental systems due to its characteristics of simple processing steps, good sensitivity, and non-destructibility. However, it has not yet been applied to assess heavy metal pollution in lake sediments on the Tibetan Plateau. A series of environmental magnetic investigations of Qiangyong Co Lake sediments (southern Tibetan Plateau) was therefore conducted to explore the relationship between magnetic minerals and mercury (Hg) concentrations. The results showed that the magnetic mineral species in lake sediments remained stable, with similar levels of four different components from 1899 to 2011. However, the proportion of component 1 (C1, hematite) increased continuously with the corresponding decrease in the proportion of C2 (goethite), while the proportions of C3 and C4 (magnetite) did not change significantly. As a result, the bulk magnetic signals (e.g., SIRM and χlf) were unsuitable for the evaluation of the Hg concentration; however, the proportion of hematite had a strong positive correlation with the Hg concentration. It is possible that the Qiangyong Glacier (the main water supply for Qiangyong Co Lake) has experienced faster melting with global and local warming, and the Hg trapped in cryoconite and ice was released. Hematite, with a large specific surface area, has a strong capacity for absorbing Hg, and both materials are ultimately transported to Qiangyong Co Lake. The proportion of hematite in a sample is therefore a suitable semi-quantitative proxy that can be used to evaluate the Hg concentration in Qiangyong Co Lake sediments. This study confirmed that the variation of magnetic minerals can provide a new method to estimate the variation of Hg concentrations and to study the process of Hg deposition in lakes in the southern Tibetan Plateau on the basis of a detailed environmental magnetic analysis.

Total Mercury in Soils and Sediments in the Vicinity of Abandoned Mercury Mine Area in Puerto Princesa City, Philippines

The abandoned mercury (Hg) mine area in Puerto Princesa City, Palawan was included in the list of abandoned and inactive mines in the Philippines which pose a high risk to human health and the environment, and require rehabilitation. The mine site, operated by Palawan Quicksilver Mines, Inc. (PQMI) from 1953 to 1976, is located approximately 3 km inland from Honda Bay coast and within the catchment of the Tagburos River, which is a local fishery and recreational area. In this study, total Hg levels in soils and sediments were measured to assess the possible release of Hg from the site into the surrounding natural environment. Results showed that total Hg (THg) concentrations found in soils (0.04–67.5 mg kg−1), mine waste calcines (52.7–924.2 mg kg−1), river sediments (1.8–119 mg kg−1), and marine sediments (0.04–12.7 mg kg−1) were elevated compared to the global background of 0.045–0.16 mg kg−1. The high concentrations of Hg in soils and river sediments were influenced by the different pathways for the release of Hg from its mine operations; while Hg in marine sediments was caused by the erosion of mine waste calcines near the pit lake, and calcines used to construct a wharf at the nearby Honda Bay. Mine wastes represent the largest source of Hg contamination in the area, due to the low efficiency of the recovery process during calcination. This work corresponds with the Minamata Convention on Mercury on the updated environmental assessments of abandoned Hg mines as potential source sites of mercury contamination.

Mercury distribution and transfer in sediment-mangrove system in urban mangroves of fast-developing coastal region, Southern China

In urban mangroves affected by surrounding urbanization and anthropogenic activities, the fate of mercury (Hg) in sediment-mangrove system remains unclear. In this study, four urban mangroves with different urban functional zonings in fast-developing Shenzhen of China were selected: Shajing mangrove (SJM) and Xixiang mangrove (XXM) featured with industry district, Futian mangrove (FTM) and Baguang mangrove (BGM) featured with central business district and ecological preserve, respectively. The distributions of Hg in sediment, iron plaque (IP), and Kandelia obovata were determined. The mean concentrations of total Hg in mangrove sediments were SJM > XXM, FTM > BGM. Higher ecological risk was also detected in SJM based on geo-accumulation index, potential ecological risk factor, and contamination factor. The speciation analysis of Hg in non-rhizosphere sediments showed that residual, oxidizable, and volatile Hg acted as dominant fractions in urban mangroves, with volatile Hg to be important for Hg bioaccumulation in K. obovata. IP acted as a natural barrier to limit transfer of Hg from sediment to K. obovate due to lower Hg concentrations in IP compared to that in sediment. In terms of the total and bioavailable Hg, translocation factors (TF) of Hg in mangroves featured with different urban functions were: TFtotal: BGM (0.23) ≈ FTM (0.23) > SJM (0.15) ≈ XXM (0.14); TFavail: BGM (0.81) ≈ FTM (0.74) > SJM (0.40) ≈ XXM (0.37). The reduced TFs of Hg were important for protecting K. obovata from heavy Hg pollution in urban mangroves featured with industry district.

The Amount of Selected Heavy Metals in Water, Sediments and Fish Species from the Rupsha River, Khulna, Bangladesh

The present study was conducted to figure out the heavy metals (Cd, Co, Cu, Fe, Hg, Mn, Mo, Ni, Pb and Zn) concentrations in water, sediments and fish species from the Rupsha River, Khulna, Bangladesh to spot the heavy metal contamination level. Heavy metals are dangerous because they're non-biodegradable and having an extended half-life period. The heavy metal contamination could also be an excellent concern, especially for aquatic life. The metal concentration within the water sample from the Rupsha River was significantly above the rule values of WHO and USEPA respectively. Moreover, the concentration of metal within the sediments was also greater than the respective probable effect concentrations of the sediment quality guidelines. Furthermore, consistent with the fish standards, these studied fish species weren't found to be contaminated by heavy metals. It’s concluded that each of the heavy metals from the whole sample didn't exceed the standard maximum level. This study demonstrated that water of Rupsha River isn't safe for aquatic organisms also like humans in terms of Pb, Cd and Cr contents as accumulated through consumption.

Effects of climate variability on mercury deposition during the Older Dryas and Younger Dryas in the Venezuelan Andes

We present a high-resolution record of Late Glacial and Early Holocene mercury (Hg) accumulation within the sediments of Laguna de Los Anteojos, a small headwater alpine lake in Venezuela. Our sediment core spans the Older Dryas (OD) and Younger Dryas (YD) climate reversals, providing new insight into the effects of abrupt climatic transitions on atmospheric Hg deposition. Sediment Hg concentrations and accumulation rates (fluxes) increased sharply after regional deglaciation and then peaked immediately prior to onset of both the OD and YD. Mercury concentrations and fluxes then decreased through cold OD and YD conditions. Both Hg concentrations and fluxes increased rapidly with warming, rising two-fold in about 100 years after the OD and nearly four-fold in about 500 years during the latter-half of the YD. This lake-sediment record of Hg dynamics during the Glacial–Holocene transition suggests atmospheric Hg cycling is affected by abrupt climate change.

Mercury methylation and demethylation potentials in Arctic lake sediments

Mercury (Hg) transformations in sediments are key factors in the Hg exposure pathway for wildlife and humans yet are poorly characterized in Arctic lakes. As the Arctic is rapidly warming, it is important to understand how the rates of Hg methylation and demethylation (wich determine Hg bioavailability) change with temperature in lake sediments. Methylation and demethylation potentials were determined for littoral sediments (2.5 m water depth) in two deep and two shallow lakes in the Canadian Arctic using Hg stable isotope tracers at incubation temperatures of 4, 8, or 16 °C for 24 h. Compared to sediments from other regions, Hg methylation and demethylation potentials in these sediments are low. The maximum depth of the lake from which sediment was collected exerted a stronger influence over methylation potential than sediment Hg concentration or organic matter content; the shallowest lake had the highest Hg methylation potential. Sediments from the shallowest lake also demonstrated the greatest response to the temperature treatments, with significantly higher methylation potentials in the 8 and 16 °C treatments. Sediments from the deep lakes demonstrated greater demethylation potentials than shallow lakes. The methylmercury to total Hg ratio in sediments supported the measured transformation potentials as the lake with the greatest methylation potential had the highest ratio. This study supports previous works indicating that Hg methylation potential may increase as the Arctic warms, but demethylation potential does not respond to warming to the same degree, indicating that Hg methylation may predominate in warming Arctic sediments.

Five decades of declining methylmercury concentrations in boreal foodwebs suggest pivotal role for sulphate deposition

Widespread declines in mercury (Hg) in fish in pristine lakes in Fennoscandia since the 1970s are unexplained. Interactions between climate, atmospheric deposition, and elemental cycling of carbon (C), sulphur (S) and Hg are complex and affect Hg bioaccumulation. A parallel significant decline in methyl-Hg (MeHg) concentrations in aquatic macroinvertebrates (Chironomidae) was found between 1976–78 and 2004–15 in an intensely studied, pristine boreal lake (Langtjern, boreal Fennoscandia). Monitoring at Langtjern demonstrated a four-fold decrease in aqueous sulphate concentrations (SO4, 50-year record), significant lake browning (30-year records), increasing sediment Hg concentrations (50-year record), warming (45-year record) and increased runoff (40-year record). Contrasting Hg trends in biota (downward) and sediment (upward) indicated a disconnect between lake Hg loading and foodweb Hg bioaccumulation. We suggest that reduced SO4-deposition has 1) constrained substrate availability for SO4-reducing methylating bacteria (causing reduced foodweb MeHg exposure despite increased Hg loading to the lake), and 2), increased the binding affinity between aqueous organic matter and Hg species (leading to reduced MeHg bioavailability). The downward MeHg trend at the base of the foodweb at Langtjern is mirrored at higher trophic levels by strong declines in perch (Perca fluviatilis) and pike (Esox lucius) Hg concentrations in boreal Fennoscandia. A plausible explanation is that declining SO4-deposition, rather than climate change or reduced atmospheric Hg, is currently driving reduced MeHg contamination in northern freshwater foodwebs.

Distribution, bioavailability and contamination assessment of mercury and arsenic in the surface sediments from the Yellow River Estuary, China

The distributions of mercury (Hg) and arsenic (As) in the surface sediments of the Yellow River Estuary (YRE) of China were investigated. The average Hg concentration (54.05 ng g−1) in the YRE sediments was much higher than the background, while the average As concentration (9.53 μg g−1) was comparable with the background. Atmospheric deposition Hg might be the major source of Hg in most studied sediments, while the point source Hg discharge might explain the relatively high levels of Hg in the specific site. The agricultural non-point source pollution and the natural sources might be the main source of As in the YRE sediments. The result of the European Community Bureau of Reference (BCR) sequential extraction experiment showed that the most hazardous fraction (F1) contributed only a small percentage of the total concentrations for both Hg and As while the fraction with low mobility and bioavailability (F4) represented the maximum composition, indicating a low risk of mobility and bioavailability for Hg and As in the sediments. In addition, multiple indices and guidelines for sediment quality assessments were used in this study, results suggested a “good state” regarding Hg and As contamination in the surface sediments from the YRE.

Mercury cycling in Australian estuaries and near shore coastal ecosystems: Triggers for management

Mercury (Hg) sources to estuaries (natural and anthropogenic) as well as Hg concentrations in Australian nearshore marine environment fish are reviewed herein. The question of whether Australian estuaries have a Hg contamination problem is addressed. The Hg concentrations in fish, excluding sharks, tuna, barramundi and some stingrays, in estuaries and near-shore ecosystems with no discernable pollution sources are typically below 0.5 mg/kg wet weight, the level of health concern. There is no relationship of Hg concentration with fish size or age nor any evidence of biomagnification. In locations with historic large discrete Hg input sources (e.g. Derwent Estuary, Tasmania, Princess Royal Harbour WA, Port Phillip Bay Vic, Sydney sewage outfalls NSW), Hg concentrations in some sediment-dwelling fish such as flatheads exceed the health limit of 0.5 mg/kg. In this paper, we also review, within an Australian context, the biogeochemical processes controlling the formation and accumulation of methyl mercury (MeHg). On entering waterways, Hg rapidly partitions to particulate matter and deposits into sediments. The remobilisation of Hg from sediment is dependent on the formation of MeHg by bacteria and ultimately the interplay of S, Fe and Se cycling. Fish species that move and feed in different areas have Hg concentrations that do not reflect the sediment Hg concentrations where they are caught, i.e. there is an uncoupling of diet and potential Hg exposure. Concluding remarks focus on management interventions: source reduction, preventing eutrophication and promoting system biodiversity and biodiverse diets to mediate the accumulation of Hg in marine organisms and limit the intake of Hg by humans when consuming fish.

The spatial legacy of Australian mercury contamination in the sediment of the Molonglo River

Mercury (Hg) contamination is an environmental concern as a by-product of legacy mining in Australia. Here we investigate the spatial and temporal distribution of Hg in the Molonglo River system in New South Wales, Australia, and assess the physical and chemical factors influencing that distribution. Mercury concentrations in sediment cores were measured in conjunction with 210Pb and 137Cs dating to establish historical contamination. This was done at the source mine site of Captains Flat, New South Wales, and the system’s sink in Lake Burley Griffin, Australian Capital Territory. Additionally, surficial sediment Hg concentrations along the Molonglo River were analyzed to determine the spatial distribution of Hg. Analytical results showed the primary physical and chemical factors influencing Hg dispersion to be distance, total organic matter, and the presence of iron oxides and oxyhydroxides. The highest Hg concentrations were near the mine site at Captains Flat and decreased significantly with distance. Sediment core analyses in both Captains Flat and the lake showed reductions in Hg concentrations toward surficial sediment layers. It is suggested government-funded rehabilitation programs are playing a part in reducing the release of metal contamination.

Inter-annual variability of heavy metals pollution in surface sediments of Jiangsu coastal region, China: Case study of the Dafeng Port

The Jiangsu coastal environment in China is affected by pollution, and the Cu, Pb, Cd, Zn, Cr, As, and Hg concentrations in the surface sediment of Dafeng Port during 2011–2017 were analyzed to determine their contamination and ecological risks. The multi-year average of Zn, Cr, Pb, Cu, As, Cd, and Hg concentrations were 50.10, 40.32, 12.41, 11.76, 8.67, 0.08, and 0.04 mg/kg, respectively, which were generally higher than their background values, except for those of Cu and Cr. The spatial distribution of heavy metals were similar, and the highest concentrations were measured near Dafeng Port and coastal river inlets. The geoaccumulation index generally indicated moderate–low ecological risk, except for Cd. Multivariate statistical analyses showed that Pb, Zn, Cr, As, Hg, Cd, and petroleum were strongly influenced by anthropogenic activities, whereas Cu derived from both natural and anthropogenic sources. This study provides important information for the management of heavy metal contamination.

A 1900‐year record of mercury (Hg) from the east continental shelf of Hainan Island, South China Sea

Natural archives are ideal tools for studying sedimentation history of mercury (Hg). However, record of Hg in marine sediments on millennial scales is less documented. This study analyzes a sedimentary core collected from the East Continental Shelf of Hainan Island, South China Sea. From 210Pb and radiocarbon (14C) chronological and geochemical analyses, a sequential record of Hg over the past 1900 years was reconstructed. Integrated analyses of Loss on Ignition (LOI), mean grain size, terrestrial elements aluminium (Al), rare earth element (∑REE) and Hg, allowed us to infer that Hg in the sediments received impacts from atmospheric deposition. Both concentrations and anthropogenic fluxes of Hg exhibit evident changes over the past 1900 years. Specifically, anthropogenic Hg flux in the last ~150 years is substantially high (mean 23.7 μg/m2/yr), which is approximately 11 times greater (in terms of enrichment factor) relative to background. Generally, Hg concentrations in the bulk sediments before 1850 AD can be explained by natural processes, whereas, by contrast, the rapid increase in flux of Hg since the mid‐19th century is related to deposition associated with anthropogenic release. The anthropogenic Hg flux shows a decrease by ~30 μg/m2/yr since 1980s, consistent with the decline in global Hg production. Nevertheless, the anthropogenic Hg flux in the present day remains 6.5 times higher than the baseline (pre‐1850 era).

Effects of drying pretreatments on the analysis of the mercury fraction in sediments.

The geochemical fractions of heavy metals in sediments are crucial indexes for their mobility and bioavailability evaluations. However, different drying processes of sediment pretreatment could change metal geochemical fractions, especially for Hg, which is potentially volatile. In this study, the influence of pretreatment methods including oven-drying, air-drying, freeze-drying, and fresh sediments on the analysis of Hg fractions in sediments was investigated. Results showed that remarkable differences of Hg concentration were observed between fresh sediments and dried pretreatment sediments (P < 0.05). Briefly, the concentrations of the water-soluble and human stomach acid-soluble fractions in oven-dried and air-dried sediments generally showed significant increasing trends compared with those in the fresh sediments, while the organo-chelated fraction exhibited significant decreasing trends. The cause of this phenomenon was primarily the oxidation of organic matter, aging process, and the diffusion of Hg into micropores. The significant loss was also observed at elemental Hg fraction due to its volatilization effect. The freeze-drying posed minor influence on changes of Hg fraction analysis compared with oven-drying and air-drying. Moreover, the total Hg concentrations in pretreated sediments showed a decline of varying degrees compared with those in fresh sediments ascribing to the volatilization of elemental Hg. Finally, Pearson correlation analysis further confirmed that freeze-drying could minimize the errors of the Hg fraction analysis in sediments.

Long-term water and sediment quality of the Elbe River’s oxbow lake near the town of Poděbrady, the Czech Republic

The studied oxbow lake near the town of Poděbrady (the Czech Republic) belongs to unique, less influenced old meanders of the Elbe River that are still completely surrounded by floodplain forests. The research works (2004–2015) included bathymetry, hydrological regime, water and sediment quality, and hydrobiological analyses. Due to the connection with the river, the hydrological regime was significantly determined by the Elbe River. Spatial electrical conductivity differences were found. Values of pH fluctuated (lower ANC4.5) during the year rising with phytoplankton development. Associated parameter of organic matter content (mean CODMn = 5.5 mg L−1) was similar to that in the river. The Elbe represented a source of nitrogen (max. N-NO3 = 5.9 mg L−1) and phosphorus for the lake, where the phosphorus concentrations reached maximum levels in summer (max. TP = 0.220 mg L−1) under low oxygen saturation. The concentrations of Cd, As, Hg, and Pb in sediments exceeded the upper limit values set in the new assessment system developed by the International Commission for the Protection of the Elbe River, and together with a high index of geoaccumulation in the case of Ag, the concentrations of these elements indicate significant contamination. Although the lake was not extremely polluted, high concentrations of nutrients and sediment load could represent a risk for aquatic ecosystems. The comparison of the results with other studies on oxbow lakes situated in different river basins provided new and complex conclusions in a broader context.

Long-term dynamics of mercury pollution of the Bratsk reservoir bottom sediments, Baikal region, Russia

This paper discusses the results of the monitoring of anthropogenic mercury contamination of the upper part of Bratsk reservoir. The paper shows that in the long-term, (1998-2018) there is an ongoing decrease of Hg concentration in the surface layer of bottom sediments. At the same time the study demonstrates that 20 years after the cessation of mercury electrolysis at the Usoliehimprom chemical plant, Hg concentration in the bottom sediments exceeds background levels by 5 to 54 times. Significant part of the Hg in the bottom sediments is stored in organic form, which is potentially dangerous since it may cause secondary contamination of aquatic environment and hydrobionts. The high amplitude of water-level fluctuations in the reservoir is one of the main factors influencing the distribution of mercury in the surface layer of bottom sediments and preventing the sealing of contaminated bottom sediments by terrigenous material.

Tracking Hg historical inputs by Pb-210 geochronology for the Itapessoca Estuarine Complex, Pernambuco, Brazil

The application of geochronology for tracking mercury contamination was remarkably demonstrated in the Itamaraca Estuarine Ecosystem, Pernambuco State, Brazil. Here, two sediment cores were sampled, layered and analyzed by the Gas Flow Proportional Counter technique for determining 210Pb, radionuclide applied to geochronology. Mercury was quantified in sediment samples by Cold Vapor Atomic Absorption Spectrometry, varying from 0.17 to 1.29 mg kg−1 in deep sediments. From 210Pb results, the sedimentation rates of 0.84 (0.07) cm year−1 and 1.03 (0.22) cm year−1 confirmed Hg contamination in Itamaraca since mid-1965. Events of extreme precipitation and drought have been also related to the distribution pattern for Hg concentrations in deep sediments.

Ecological risk assessment and source apportionment of metals in the surface sediments of river systems in Lake Taihu Basin, China.

In this study, the concentrations of Zn, Cr, Ni, Cu, Pb, As, Cd, and Hg in the surface sediments of 94 sites sampled from six water systems in the Lake Taihu Basin in China were measured, and the pollution risks and sources of the metals were identified. The results showed that the mean concentrations of Zn, Cr, Ni, Cu, Pb, As, Cd, and Hg in the riverine surface sediments were 163.6, 102.5, 45.5, 44.7, 37.0, 13.3, 0.5, and 0.1mg/kg, respectively, higher than the corresponding background values (except for Hg). According to the geoaccumulation index (Igeo), the Pb, Ni, Zn, Cu, and Cd concentrations in the riverine surface sediments were generally at low levels of pollution. Based on a pollution load index (PLI) evaluation, the Pb, Ni, Zn, and Cu concentrations in the riverine surface sediments were generally at moderate levels of pollution. According to the thresholds of potential ecological risk, the Cd and Hg concentrations in the riverine surface sediments exhibited moderate potential ecological risks. Multivariate statistical analysis indicated that the Pb in the riverine surface sediments primarily originated from domestic sewage, agricultural wastewater discharge, and petroleum combustion; the concentrations of Cr, Ni, and Zn were influenced by the electroplating and alloy manufacturing industries; the concentrations of Cu and As mainly originated from pesticide use and industrial wastewater discharge; and those of Cd and Hg primarily stemmed from industrial wastewater discharge. This research provides information regarding metallic contamination and the possible associated ecological risks to benthic organisms in the surface sediments of river systems and is useful for developing sustainable strategies for environmental pollution control and management in the Lake Taihu Basin.

Bioaccumulation and growth characteristics of Vallisneria natans (Lour.) Hara after chronic exposure to metal-contaminated sediments.

Metal-contaminated sediments in lakes is a global concern that poses toxicological risk to aquatic organisms. This study performed bioassays using the submerged macrophyte, Vallisneria natans (Lour.) Hara, exposed to contaminated sediments collected from five locations in Dianchi Lake, Yunnan, China. Among the sediments collected, Igeo showed enrichment of As and Cd in Dianchi Lake sediments. In spite of enriched toxic metals at some locations, laboratory bioassays found no significant difference in leaf biomass or leaf photosynthesis rate between the sites. Root biomass and root activity showed significant differences between locations and were negatively correlated with the concentration of As, Cd, Hg, and Pb in sediment but not related to Cr. The above correlations were strongest for Hg and As, respectively. Accumulation of Cd and Pb to leaves of bioassay plants was observed, but this was not evident for As and Cr. Overall, the results indicate that V. natans can be used as a bioassay organism and measures of root toxicity are sensitive to metal concentrations present in Dianchi Lake sediments. Furthermore, the study species holds promise for use as a biomonitor of Cd and Pb sediment metal content.

Mercury and methylmercury bioaccumulation in a contaminated bay

The bioaccumulation and the main source of total Hg (THg) and methylmercury (MMHg) in the deposit-feeding polychaete Neanthes japonica collected in Jinzhou Bay, China, were investigated. Compared with the historical data, THg bioaccumulation in polychaetes collected in sediment of Jinzhou Bay was distinctly higher due to higher sediment THg concentration, but MMHg bioaccumulation was significantly lower. THg accumulation in polychaetes mainly derived from its accumulation in sediment. However, MMHg bioaccumulation in polychaetes did not correlate with Hg concentration in sediment. Besides sediment ingestion, MMHg accumulation in polychaetes may partially source from the process of in vivo transformation. The in vivo Hg methylation may take place in polychaetes, according to the excellent correlation between MMHg concentration and THg and inorganic Hg concentration in polychaetes. The biochemical characters in polychaete body, the oxidation-reduction environment and the microbial activity in polychaete gut may be beneficial to in vivo Hg methylation.

Lake sediment mercury biogeochemistry controlled by sulphate input from drainage basin

Mercury (Hg) and methyl mercury (MeHg) concentrations have never been measured in sediments of coastal lakes of Aquitaine, although high concentrations of Hg have been measured in fish. Our objective was to characterize benthic biogeochemical processes and the distribution of Hg in lake sediments and to connect these results with fish contamination. For this, we mapped and characterized sediments. We measured sediment Hg and MeHg content, and biogeochemical parameters. We identified organic deposits in deep areas, and sandy sediments in shallow areas. Sediments were anoxic below the sediment–water interface. The average Hg concentration in organic sediment was 213 μg kg−1 dry weight. Sandy sediments had an average Hg concentration of 4 μg kg−1 dw. We measured concentrations below 6 μg kg−1 dw in sediments from streams that drain the catchment. Similar concentrations in the four lakes suggest that the source of total Hg was not a point source in a given lake. The highest MeHg concentrations were found in the upper centimetres of organic sediments, where sulphate reduction occurred. MeHg represented 2.53% of total Hg for Lake Carcans-Hourtin, less in other lakes. The proportion of MeHg in sediment followed lake water sulphate concentrations. High sulphate concentrations resulted from agricultural activity in the Lake Carcans-Hourtin catchment. Our results corroborate the hypothesis that Hg methylation is linked to sulphate-reducing activity. High fluxes of sulphate from a drainage basin may induce large proportions of potentially bioavailable MeHg in lake sediment, even in non-polluted areas. The Hg methylation activity in the sediment reflected the exposure of predatory fish to MeHg.