Bacterial growth phase influences methylmercury production by the sulfate-reducing bacterium Desulfovibrio desulfuricans ND132

The effect of bacterial growth phase is an aspect of mercury (Hg) methylation that previous studies have not investigated in detail. Here we consider the effect of growth phase (mid-log, late-log and late stationary phase) on Hg methylation by the known methylator Desulfovibrio desulfuricans ND132. We tested the addition of Hg alone (chloride-complex), Hg with Suwannee River natural organic matter (SRNOM) (unequilibrated), and Hg equilibrated with SRNOM on monomethylmercury (MMHg) production by ND132 over a growth curve in pyruvate-fumarate media. This NOM did not affect MMHg production even under very low Hg:SRNOM ratios, where Hg binding is predicted to be dominated by high energy sites. Adding Hg or Hg-NOM to growing cultures 24 h before sampling (late addition) resulted in ~ 2× greater net fraction of Hg methylated than for comparably aged cultures exposed to Hg from the initial culture inoculation (early addition). Mid- and late-log phase cultures produced similar amounts of MMHg, but late stationary phase cultures (both under early and late Hg addition conditions) produced up to ~ 3× more MMHg, indicating the potential importance of growth phase in studies of MMHg production.     

Biogeochemical factors influencing net mercury methylation in contaminated freshwater sediments from the St. Lawrence River in Cornwall, Ontario, Canada

The activity of various anaerobic microbes, including sulfate reducers (SRB), iron reducers (FeRP) and methanogens (MPA) has been linked to mercury methylation in aquatic systems, although the relative importance of each microbial group in the overall process is poorly understood in natural sediments. The present study focused on the biogeochemical factors (i.e. the relative importance of various groups of anaerobic microbes (FeRP, SRB, and MPA) that affect net monomethylmercury (MMHg) formation in contaminated sediments of the St. Lawrence River (SRL) near Cornwall (Zone 1), Ontario, Canada. Methylation and demethylation potentials were measured separately by using isotope-enriched mercury species (²⁰⁰Hg²⁺ and MM¹⁹⁹Hg⁺) in sediment microcosms treated with specific microbial inhibitors. Sediments were sampled and incubated in the dark at room temperature in an anaerobic chamber for 96 h. The potential methylation rate constants (K_m) and demethylation rates (K_d) were found to differ significantly between microcosms. The MPA-inhibited microcosm had the highest potential methylation rate constant (0.016 d⁻¹), whereas the two SRB-inhibited microcosms had comparable potential methylation rate constants (0.003 d⁻¹ and 0.002 d⁻¹, respectively). The inhibition of methanogens stimulated net methylation by inhibiting demethylationand by stimulating methylation along with SRB activity. The inhibition of both methanogens and SRB was found to enhance the iron reduction rates but did not completely stop MMHg production. The strong positive correlation between K_m and Sulfate Reduction Rates (SRR) and between K_d and Methane Production Rates (MPR) supports the involvement of SRB in Hg methylation and MPA in MMHg demethylation in the sediments. In contrast, the strong negative correlation between K_d and Iron Reduction Rates (FeRR) shows that the increase in FeRR corresponds to a decrease in demethylation, indicating that iron reduction may influence net methylation in the SLR sediments by decreasing demethylation rather than favouring methylation.     

Mercury distribution and speciation in Lake Balaton, Hungary

The distribution and speciation of mercury in air, rain, lake water, sediment, and zooplankton in Lake Balaton (Hungary) were investigated between 1999 and 2002. In air, total gaseous mercury (TGM) ranged from 0.4 to 5.9 ng m(-3) and particulate phase mercury (PPM) from 0.01 to 0.39 ng m(-3). Higher concentrations of both TGM and PPM occurred during daytime. Higher concentrations of PPM occurred in winter. In rain and snow, total mercury ranged from 10.8 to 36.7 ng L(-1) in summer but levels up to 191 ng L(-1) in winter. Monomethylmercury (MMHg) concentrations ranged from 0.09 to 1.26 ng L(-1) and showed no seasonal variations. Total Hg in the unfiltered lake water varied spatially, with concentrations ranging from 1.4 to 6.5 ng L(-1). Approximately 70% of the total Hg is dissolved. MMHg levels ranged from 0.08 to 0.44 ng L(-1) as total and from 0.05 to 0.37 ng L(-1) in the dissolved form. Lower Hg concentrations in the water column occurred in winter. In suspended particulate matter and in sediment, total mercury ranged from 9 to 160 ng g(-1) dw, and MMHg ranged from 0.07 to 0.84 ng g(-1) dw. In zooplankton, an average mercury level of 31.0+/-6.8 ng g(-1) dw occurred, with MMHg accounting for approximately 17%. In sediments, suspended-matter- and zooplankton-high Hg and MMHg levels occurred at the mouth of the River Zala, but, in the lake, higher concentrations occurred on the Northern side, and an increasing trend from north-west to north-east was observed. In general, regarding Hg, Lake Balaton can be considered as a relatively uncontaminated site. The high-pH and well-oxygenated water as well as the low organic matter content of the sediment does not favour the methylation of Hg. In addition, bioconcentration and bioaccumulation factors are relatively low compared to other aquatic systems.     

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.     

Mercury accumulation and attenuation at a rapidly forming delta with a point source of mining waste

The Walker Creek intertidal delta of Tomales Bay, California is impacted by a former mercury mine within the watershed. Eleven short sediment cores (10 cm length) collected from the delta found monomethylmercury (MMHg) concentrations ranging from 0.3 to 11.4 ng/g (dry wt.), with lower concentrations occurring at the vegetated marsh and upstream channel locations. Algal mats common to the delta's sediment surface had MMHg concentrations ranging from 7.5 to 31.5 ng/g, and the top 1 cm of sediment directly under the mats had two times greater MMHg concentrations compared to adjacent locations without algal covering. Spatial trends in resident biota reflect enhanced MMHg uptake at the delta compared to other bay locations. Eighteen sediment cores, 1 to 2 m deep, collected from the 1.2 km² delta provide an estimate of a total mercury (Hg) inventory of 2500 ± 500 kg. Sediment Hg concentrations ranged from pre-mining background conditions of approximately 0.1 μg/g to a post-mining maximum of 5 μg/g. Sediment accumulation rates were determined from three sediment cores using measured differences of ¹³⁷Cs activity. We estimate a pre-mining Hg accumulation of less than 20 kg/yr, and a period of maximum Hg accumulation in the 1970s and 1980s with loading rates greater than 50 kg/yr, corresponding to the failure of a tailings dam at the mine site. At the time of sampling (2003) over 40 kg/yr of Hg was still accumulating at the delta, indicating limited recovery. We attribute observed spatial evolution of elevated Hg levels to ongoing inputs and sediment re-working, and estimate the inventory of the anthropogenic fraction of total Hg to be at least 1500 ± 300 kg. We suggest ongoing sediment inputs and methylation at the deltaic surface support enhanced mercury levels for resident biota and transfer to higher trophic levels throughout the Bay.     

The dynamics of alkalinity generation by an anoxic sediment exposed to acid water

The time dependence of the rate of generation of alkalinity by productive sediments treated with acid waters was investigated for various conditions using continuously stirred flowing reactors. Complete mass balances allowed the generated alkalinity to be related to specific processes, none of which were affected by light. The high rate of oxidation of organic material was sufficient to consume all the oxygen, nitrate and sulphate supplied to the vessels during the first few days of incubation, but by 15 days sulphate reduction had ceased and oxygen and nitrate reduction were incomplete. In incubations supplied with anoxic waters there was an initial small release of calcium, but by 10–15 days the generation of alkalinity could be accounted for by nitrate reduction (23%), sulphate reduction (47%), Fe(II) production (23%) and NH₄⁺ production (7%). Iron(II) was exhausted when only 1% of the total amount of iron in the sediment had been released. In incubations supplied with oxygenated waters sulphate was only reduced during the first 10 days while oxygen was completely consumed. By 15 days there was incomplete consumption of oxygen and the generation of alkalinity was accounted for by nitrate reduction (36%), calcium release (53%) and NH₄⁺ production (11%). Comparisons of reaction rates indicate that calcium is released by being replaced by ammonium ions which are generated by decomposition of organic matter. Although, in the longer term, more base is generated by sediment incubated anoxically, sediments incubated with oxygenated water generate base more rapidly for the first few days until the most readily oxidized organic matter is consumed.     

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.     

Mercury methylation rates of biofilm and plankton microorganisms from a hydroelectric reservoir in French Guiana

The Petit-Saut ecosystem is a hydroelectric reservoir covering 365 km² of flooded tropical forest. This reservoir and the Sinnamary Estuary downstream of the dam are subject to significant mercury methylation. The mercury methylation potential of plankton and biofilm microorganisms/components from different depths in the anoxic reservoir water column and from two different sites along the estuary was assessed. For this, reservoir water and samples of epiphytic biofilms from the trunk of a submerged tree in the anoxic water column and from submerged branches in the estuary were batch-incubated from 1 h to 3 months with a nominal 1000 ng/L spike of Hg(II) chloride enriched in ¹⁹⁹Hg. Methylation rates were determined for different reservoir and estuarine communities under natural nutrient (reservoir water, estuary freshwater) and artificial nutrient (culture medium) conditions. Methylation rates in reservoir water incubations were the highest with plankton microorganisms sampled at − 9.5 m depth (0.5%/d) without addition of biofilm components. Mercury methylation rates of incubated biofilm components were strongly enhanced by nutrient addition. The results suggested that plankton microorganisms strongly contribute to the total Hg methylation in the Petit-Saut reservoir and in the Sinnamary Estuary. Moreover, specific methylation efficiencies (%Me¹⁹⁹Hg_net/cell) suggested that plankton microorganisms could be more efficient methylating actors than biofilm consortia and that their methylation efficiency may be reduced in the presence of biofilm components. Extrapolation to the reservoir scale of the experimentally determined preliminary methylation efficiencies suggested that plankton microorganisms in the anoxic water column could produce up to 27 mol MeHg/year. Taking into account that (i) demethylation probably occurs in the reservoir and (ii) that the presence of biofilm components may limit the methylation efficiency of plankton microorganisms, this result is highly consistent with the annual net MeHg production estimated from mass balances (8.1 mol MeHg/year, Muresan et al., 2008a).     

Preservation and storage techniques for low-level aqueous mercury speciation

Although researchers today generally employ appropriate techniques for the storage and preservation of aqueous samples for ambient-level mercury (ppb) speciation, these methods continue to be poorly documented. Numerous experiments were thus conducted to investigate the effects of acidification and bottle type on holding time for various mercury species [elemental mercury (Hg(0)), ionic mercury (Hg(II)), dimethyl mercury (DMHg), monomethyl mercury (MMHg), and dissolved-to-particulate ratio] as well as total mercury (THg). We documented that THg is stable for at least 300 days when stored at 0.4-0.5% acidity in either Teflon or glass bottles. In cases where THg is adsorbed to bottle walls, the addition of BrCl at least 24 h before analysis allowed all Hg to be quantitatively recovered. Polyethylene bottles allowed diffusion of Hg(0) through the bottle walls to or from the sample, depending on the Hg concentration of the sample and storage atmosphere. MMHg in freshwater samples can be stored refrigerated and unacidified for days to weeks with no observed degradation of MMHg. For long-term storage (at least 250 days), samples should be acidified with 0.4% HCl (v/v) and kept in the dark to avoid photodegradation (approximate t(1/2)=6 months). For saltwater samples, preservation with 0.2% (v/v) H(2)SO(4) is preferred to avoid exceeding the optimal chloride concentration if the distillation procedure is used for MMHg determination. For volatile species (Hg(0) and DMHg), samples should be collected in completely full glass bottles with Teflon-lined caps, as these species are lost rapidly (t(1/2)=10-20 h) from Teflon and polyethylene bottles. Because acids can enhance the rapid oxidation of volatile species, these samples should be stored refrigerated and unacidified and processed within 1-2 days if they cannot be purged and trapped in the field. Hg(II) and the dissolved-to-particulate ratio are more stable and can be stored for a period of days to weeks without preservation.     

Mercury and flooding cycles in the Tapajós River basin, Brazilian Amazon: the role of periphyton of a floating macrophyte (Paspalum repens)

Methylmercury (MeHg) increases mercury (Hg) toxicity and is biomagnified in the trophic chain contaminating riverine Amazon populations. Freshwater macrophyte roots are a main site of Hg methylation in different Brazilian environments. Paspalum repens periphyton was sampled in four floodplain lakes during the dry, rainy and wet seasons for measurement of total Hg (THg), MeHg, Hg methylation potentials, %C, %N, δ¹³C, δ¹⁵N and bacterial heterotrophic production as ³H-leucine incorporation rate. THg concentration varied from 67 to 198 ng/g and the potential of Me²⁰³Hg formation was expressive (1-23%) showing that periphyton is an important matrix both in the accumulation of Hg and in MeHg production. The concentration of MeHg varied from 1 to 6 ng/g DW and was positively correlated with Me²⁰³Hg formation. Though methylmercury formation is mainly a bacterial process, no significant correlation was observed between the methylation potentials and bacterial production. The multiple regressions analyses suggested a negative correlation between THg and %C and %N and between methylation potential and δ¹³C. The discriminant analysis showed a significant difference in periphyton δ¹⁵N, δ¹³C and THg between seasons, where the rainy season presented higher δ¹⁵N and the wet period lighter δ¹³C, lower THg values and higher Me²⁰³Hg formation. This exploratory study indicates that the flooding cycle could influence the periphyton composition, mercury accumulation and methylmercury production.     

Wetland influences on mercury transport and bioaccumulation in South Carolina

There are three distinct geological provinces in South Carolina (SC), with the blue ridge/piedmont regions in the west/central portion of the state and the coastal plain region in the central/eastern region of the state. Samples were collected along this gradient to identify potential factors contributing to the concentrations of total Hg and total organic carbon (TOC) throughout the state. Overall, there is a gradient across the state, with water column concentrations of total Hg (9-53 pM) and TOC (80-2721 μM) increasing as one moves from the blue ridge/piedmont region to the coastal floodplain region. Total Hg at all sites in SC is significantly (R² = 0.78; P < 0.001) correlated with TOC in the water samples. This correlation explains 78% of the variance in the data and suggests that mercury is associated with organic matter in water bodies throughout the state. A study of mercury speciation within the coastal plain Waccamaw River indicates that concentrations of total Hg range from 10-68 pM and methyl Hg concentrations range from 1-7 pM. Watershed transport efficiencies for coastal floodplain rivers sampled in this study range from 32-72% for total Hg and 78-477% for methyl Hg. The coastal plain sites are located in watersheds that contain a significantly (P < 0.001) higher percentage of wetlands (16.3 ± 5%) than the blue ridge/piedmont region (1.14 ± 1.6%), suggesting that drainage through wetlands contributes to the increased concentrations of TOC and total Hg found in SC coastal plain rivers. There is a significant correlation between mean fish Hg concentrations in largemouth bass from each watershed and percent wetland area in each watershed (R² = 0.66; P = 0.003). This correlation explains 66% of the variance in the data and suggests that increasing percentages of wetland area contribute to fish Hg concentrations in SC coastal plain rivers.     

Application of physico-chemical amendments for the counteraction of mercury pollution

In order to counteract mercury pollution, due to gold recovery practices, in situ strategies are required. In this study, some physico-chemical additions were tested, in different environmental compartments, to verify their potential to mitigate mercury contamination. Results indicate that the addition of sulfide reduces chemical methylation of Hg(II), but enhances the solubility of Hg0. The use of oxides, phosphate, and organic matter may be effective in the immobilization of Hg(II), depending upon Hg speciation. Calcium was effective in counteracting the solubility enhancement of Hg0 promoted by the presence of Aldrich humic acid.     

Strategic environmental management: time for a new approach

We present a comprehensive analysis of the sensitivity of mercury (Hg) human exposure to environmental variables using a multimedia model of the fate and transport of Hg in the environment. The results of the analysis show that the Hg dose is most sensitive to the lake pH, the burial rate of Hg adsorbed to sediments, and the chemical speciation of Hg emissions to the atmosphere. The lake pH has a strong non-linear effect on the methylation rate and bioaccumulation of Hg in fish. The burial of sediments is a major pathway for removing Hg from the lake cycling. The speciation of Hg emissions is important because Hg(II) is deposited much more rapidly than Hg(0). These results highlight the importance of key variables that should be investigated through well-designed field programs, so that we can minimize the overall uncertainties associated with the modeling of mercury fate and transport.     

The influence of weathering and organic matter on heavy metals lability in silicatic, Alpine soils

We investigated the effect of organic matter and weathering on the lability and solid phase speciation of heavy metals (Cd, Cr, Cu, Ni, Pb, Zn) in two contrasting subalpine regions in the Italian Alps. Cr, Ni and Cu could be linked to weathering. This was not the case for Pb. Since organic matter (OM) influences the solid phase speciation of heavy metals, the total organic C and N content, the C and N content of different density fractions of OM and also of the labile (oxidised by H₂O₂) and stable (H₂O₂-resistant) fractions were determined. Soil OM stocks were high and soils on north-facing slopes had more OM than the south-facing sites to which they were paired. Density measurements and the H₂O₂ fractionation indicated that the higher OM content on north-facing sites was due to an accumulation of weakly degraded organic material. Due to higher weathering intensity on north-facing sites, the abundance of the EDTA-extractable heavy metals was higher than on south-facing sites. All EDTA-extractable heavy metals showed a good correlation to the water-soluble phenolic concentrations which indicates that the metals were probably translocated as metal-organic complexes. Pb and Cu correlate not only to the light (density < 1 g/cm³) and labile, organic fraction but also to the heavy (density > 2 g/cm³) and stable fraction. High-mountain ecosystems like the Alps are sensitive to changing environmental conditions such as global warming. A warmer climate and the more favourable conditions it brings for biological activity, especially at cooler sites, will probably lead in the short- to mid-term to an increased loss of accumulated, weakly degraded OM. As the Pb and Cu content is significantly related to the labile organic matter pools, the risk exists that an increase in OM mineralisation could affect the storage capacity and mobility of these metals in soils.     

Factors controlling Hg levels in two predatory fish species in the Negro river basin, Brazilian Amazon

A number of environmental factors have been shown to influence the dynamics of Hg in aquatic ecosystems. Here we investigate the influence of fish size, pH, dissolved organic carbon (DOC), and the availability of potential methylation sites (floodplain forests and hydromorphic soils) on the concentration of total Hg in two carnivorous fishes: Cichla spp. and Hoplias malabaricus in the Negro River, Brazil. Fish and water samples for chemical analysis were collected from 33 sites in the Negro basin. The percentage of alluvial floodplains and hydromorphic soils (potential methylation sites) in the drainage basin upstream from each sampling point was estimated from radar imagery and existing soil maps with GIS. The average of Hg concentrations were 0.337 ppm (SD = 0.244) in Cichla spp. and 0.350 ppm (SD = 0.250) in H. malabaricus. Although the study area was geographically isolated from most major anthropogenic Hg sources, over 18% of Cichla spp. and 29% of H. malabaricus had Hg concentrations above 0.5 ppm, indicating naturally high background levels of Hg. Hg concentrations increased with size in both Cichla spp. (r² = 0.664, p = 0.000) and H. malabaricus (r² = 0.299, p = 0.000). Hg concentrations in H. malabaricus also increased with percent floodable area (p = 0.006), pH (p = 0.000) and DOC (0,063). In Cichla spp, Hg increased only in relation to percent floodable area (p = 0.000). Hydromorphic soils did not influence fish Hg.     

Sulfide formation in freshwater sediments, by sulfate-reducing microorganisms with diverse tolerance to salt

Understanding how sulfate-reducing microbes in freshwater systems respond to added salt, and therefore sulfate, is becoming increasingly important in inland systems where the threat from salinisation is increasing. To address this knowledge gap, we carried out mesocosm studies to determine how the sulfate-reducing microbial community in sediments from a freshwater wetland would respond to salinisation. The levels of inorganic mineral sulfides produced after 6 months incubation were measured to determine whether they were in sufficient quantity to be harmful if re-oxidized. Comparative sequence analysis of the dissimilatory sulfite reductase (DSR) gene was used to compare the sulfate-reducing community structure in mesocosms without salt and those incubated with moderate levels of salt. The amount of total S, acid volatile sulfide or chromium-reducible sulfide produced in sediments with 0, 1 or 5 g L^− 1 added salt were not significantly different. Sediments subjected to 15 g L^− 1 salt contained significantly higher total S and acid volatile sulfide, and levels were above trigger values for potential harm if re-oxidation occurred. The overall community structure of the sulfate-reducing microbiota (SRM) was explained by the level of salt added to sediments. However, a group of sulfate reducers were identified that occurred in both the high salt and freshwater treatments. These results demonstrate that freshwater sediments contain sulfate reducers with diverse abilities to respond to salt and can respond rapidly to increasing salinity, explaining the observation that harmful levels of acid volatile sulfides can form rapidly in sediments with no history of exposure to salt.     

Modeling metal bioaccumulation in a deposit-feeding polychaete from labile sediment fractions and from pore water

Estuarine sediments are often highly enriched in particle-reactive metal contaminants and because aquatic animals have often been shown to acquire metals predominantly from their diet, benthic animals feeding on deposited or resuspended sediments may also accumulate metals through this uptake pathway. Laboratory experiments were performed in which the surface deposit-feeding polychaete, Nereis succinea, was exposed to As(+ 5), Cd, and Cr(+ 3) in pore water or in estuarine sediments with and without enrichment with algal debris. These experiments generated metal uptake parameters (assimilation efficiency of ingested metal [AE], uptake rate constant of dissolved metal, efflux rate constants following dietary or aqueous metal exposures) used in a kinetic model of metal bioaccumulation. The model showed that > 97% of the body burden of these metals is accumulated through ingested sediment. The kinetic model was further modified to consider the geochemical fractionation of the metals in the sediments because metals bound to some fractions were shown to be unavailable to these polychaetes. The modified model substituted the AE term for each metal by the percentage of metal extracted in neutral and weak acid exchangeable fractions (termed “carbonex” fraction) multiplied by the slope of the regression between the metal AE and its fractionation in carbonex. The modified model generated predictions of As, Cd, and Cr body burdens in polychaetes at three different estuarine sites that matched independent field observations at these sites (r² = 0.84 for sediments without organic enrichment, r² = 0.87 with organic enrichment). Model predictions that relied on total metal concentrations showed weaker relationships (r² = 0.11-0.50). This study adds to the evidence for the dominance of dietary uptake of metals in aquatic animals and identifies a key sedimentary fraction of metals that can account for bioavailability of sediment-bound metals.     

Sedimentary records of sewage pollution using faecal markers in contrasting peri-urban shallow lakes

Sewage contamination in shallow lake sediments is of concern because the pathogens, organic matter and nutrients contribute to the deterioration of the water-bodies' health and ecology. Sediment cores from three shallow lakes (Coneries, Church and Clifton Ponds) within Attenborough nature reserve located downstream of sewage treatment works were analysed for TOC, C/N, δ¹³C, δ¹⁵N, bacterial coliforms and faecal sterols. ²¹⁰Pb and ¹³⁷Cs activities were used to date the sediments. Elemental analysis suggests that the source of organic matter was algal and down profile changes in δ¹³C indicate a possible decrease in productivity with time which could be due to improvements in sewage treatment. δ¹⁵N for Coneries Pond are slightly higher than those observed in Church or Clifton and are consistent with a sewage-derived nitrate source which has been diluted by non-sewage sources of N. The similarity in δ¹⁵N values (+ 12‰ to + 10‰) indicates that the three ponds were not entirely hydrologically isolated. Analysis by gas chromatography/mass spectrometry (GC/MS) reveals that Coneries Pond had sterol concentrations in the range 20 to 30 μg/g (dry wt.), whereas, those from Clifton and Church Ponds were lower. The highest concentrations of the human-sourced sewage marker 5β-coprostanol were observed in the top 40 cm of Coneries Pond with values up to 2.2 μg/g. In contrast, Church and Clifton Pond sediments contain only trace amounts throughout. Down-profile comparison of 5β-coprostanol/cholesterol, 5β-coprostanol/(5β-coprostanol + 5α-cholestanol) and 5β-epicoprostanol/coprostanol as well as 5α-cholestanol/cholesterol suggests that Coneries Pond has received appreciable amounts of faecal contamination. Examination of 5β-stigmastanol (marker for herbivorous/ruminant animals) down core concentrations suggests a recent decrease in manure slurry input to Coneries Pond. The greater concentration of β-sitosterol in sediments from Church and Clifton Ponds as compared to Coneries is attributed in part to their greater diversity and extent of aquatic plants and avian faeces.     

Occurrence of arsenic in core sediments and groundwater in the Chapai-Nawabganj District, northwestern Bangladesh

Groundwater and core sediments of two boreholes (to a depth of 50 m) from the Chapai-Nawabganj area in northwestern Bangladesh were collected for arsenic concentration and geochemical analysis. Groundwater arsenic concentrations in the uppermost aquifer (10-40 m of depth) range from 2.8 μg L⁻¹ to 462.3 μg L⁻¹. Groundwater geochemical conditions change from oxidized to successively more reduced, higher As concentration with depth. Higher sediment arsenic levels (55 mg kg⁻¹) were found within the upper 40 m of the drilled core samples. X-ray absorption near-edge structure spectroscopy was employed to elucidate the arsenic speciation of sediments collected from two boreholes. Environmental scanning electron microscopy and transmission X-ray microscopy were used to investigate the characteristics of FeOOH in sediments which adsorb arsenic. In addition, a pH-Eh diagram was drawn using the Geochemist's Workbench (GWB) software to elucidate the arsenic speciation in groundwater. The dominant groundwater type is Ca-HCO₃ with high concentrations of As, Fe and Mn but low levels of NO₃⁻ and SO₄²⁻. Sequential extraction analysis reveals that Mn and Fe hydroxides and organic matter are the major leachable solids carrying As. High levels of arsenic concentration in aquifers are associated with fine-grained sediments. Fluorescent intensities of humic substances indicate that both groundwater and sediments in this arsenic hotspot area contain less organic matter compared to other parts of Bengal basin. Statistical analysis clearly shows that As is closely associated with Fe and Mn in sediments while As is better correlated with Mn in groundwater. These correlations along with results of sequential leaching experiments suggest that reductive dissolution of MnOOH and FeOOH mediated by anaerobic bacteria represents an important mechanism for releasing arsenic into the groundwater.     

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.