Daniel Cossa

Trace metal distribution, partition and fluxes in the Seine estuary (France) in low discharge regime

Samples collected in surface waters of the Seine estuary (France) during a low discharge period were analyzed for dissolved and particulate trace metals (Cd, Co, Cu, Mn, Ni, Pb and Zn). The studies showed that these trace elements are reactive in the estuary: an addition to the dissolved phase is observed for Cd, Co, Cu, Mn, Ni and Zn and a removal for Pb. Cd, Cu, Ni and Zn are released from seaward fluxing riverine particles or estuarine sediments directly in the water column, while the dissolved Co-salinity distribution, similar to Mn distribution suggests a preliminary release in the sediment pore water. Rapid adsorption onto the suspended load plays a major role in Pb removal from the dissolved phase. Partition coefficients for the dissolved and particulate phase (KD) decrease in the order: Pb > Co > Cu, Zn > Ni > Cd, i.e. a sequence similar to those reported from others European areas. Gross riverine inputs to the estuary and net efflux of dissolved trace metals to the English Channel are estimated. Values show that the Seine river and its estuary constitute a significant source of trace metals for the North Sea.

The distribution and cycling of mercury species in the western Mediterranean

Total mercury concentrations were determined in water samples from ten vertical profiles in the western Mediterranean. Most Hg concentrations ranged from 0.5 to 4 pM, with a geometric mean of 2.26 pM. Such concentrations are in the same range as those measured in the North Atlantic and equatorial Pacific waters. There is no indication that the presence of geothermal activity or cinnabar deposits around the Mediterranean basin induces higher mercury concentrations in waters. Vertical concentration profiles were characterized by a maximum just below the thermocline, which is mainly developed in regions with relatively high primary production in the overlying waters. These observations support the remobilization model according to which, the mercury, associated with degradable organic matter, is solubilized from the particles accumulated in the thermocline layer. Additional measurements of certain mercury species, including reactive mercury (HgR) and gaseous species [elemental mercury (Hg°) and dimethylmercury (DMHg)], were performed on three profiles in the Alboran Sea and the Strait of Gibraltar. While 50% of the total mercury consisted of unidentified organic association, a maximum of the three determined species were observed below the thermocline: up to 0.71 pM, 0.43 pM and 0.30 pM for HgR, Hg° and DMHg, respectively. DMHg and Hg° appear to be formed in the low oxygen zone. A specific methylation rate of 3 × 10−9 s−1 can be estimated, which is six times higher than the values proposed for the North Atlantic waters (Mason et al., 1995a, Water, Air and Soil Pollution, 80, 665–677). The mass balance budget in the western Mediterranean shows that, while the total mercury exchanges at the straits are not unbalanced, mercury enters the Mediterranean as inorganic mercury and is exported to the Atlantic Ocean partially as methylated species. Riverine input is mainly composed by particulate mercury which is readily accumulated in coastal sediments. The dissolved mercury flux from non-marine sources is largely dominated by atmospheric deposition, which is one order of magnitude higher than riverine input. The mercury accumulation rate in the sediments is quantitatively equivalent to the atmospheric inputs, but mainly constituted by terrestrial material deposited nearshore. On the basis of ☐-core analyses and the global model of Mason et al. (1994a, Geochimica et Cosmochimica Acta, 58, 3191–3198) and present results the pre-industrial Hg riverine, atmospheric and sedimentation fluxes are estimated to have been about one-third the current one.

Cadmium diagenesis in Laurentian Trough sediments

The depth distributions of Cd, Mn and Fe in pore water and sediment were determined on three replicate box cores collected at a 325 m deep station in the Laurentian Trough. The results reveal a surface layer in which the content of solid-phase Cd first decreases but then increases sharply with depth. In contrast, Mn decreases regularly over the same depth interval. In the oxygenated zone of this layer, Cd, probably associated with organic matter, is released to the pore water, resulting in concentrations that are more than an order of magnitude higher than in the overlying water column. Some of the dissolved Cd is returned to the water column and some migrates downward and is precipitated at depth. This part of the Cd cycle is virtually complete within the surface layer, the base of which corresponds to the base of the zone of Mn enrichment. In the subsurface layer, solid-phase Cd and Mn show little or no concentration change. However, dissolved Cd, which reaches often non-detectable concentrations (<0.05 nM) in this layer, increases once again deep within the anaerobic zone and attains concentrations even higher than in the surface layer. The presence of dissolved Cd complexes (revealed by 1.5 to 8 fold increases in electrochemically active Cd in pore water following UV-treatment) as well as inconsistent distributions of dissolved and solid-phase Cd, indicate complexities in diagenesis that merit further investigation.

Speciation and sorption of mercury in two macro-tidal estuaries

Mercury speciation and partition were determined in two macro-tidal estuaries, the Loire and the Seine estuaries (France) during several cruises covering various hydrological conditions. Additional measurements were performed in freshwaters in order to assess temporal variations of mercury inputs to the estuaries during an annual cycle. In freshwater, average total ‘dissolved’ (< 0.8 μm) mercury concentration [(HgT)D] was 4.1 ± 2.1 pM in the Loire and 11.5 ± 9.7 pM in the Seine. Mean particulate mercury concentration [(HgT)p] was 0.95 ± 0.63 nmol g−1 in the Loire and 5.4 ± 2.6 nmol g−1 in the Seine. The seasonal variation in the mercury concentrations in the two rivers are interpreted in terms of changes in the various water sources during flood events and variable contribution of colloidal materials. In the estuarine parts, (HgT)D concentrations generally varied from 2 to 6 pM in the Seine and from 1 to 3 pM in the Loire, while (HgT)P concentrations varied between 0.5 and 5 nmol g−1 in the Loire and between 2.5 and 10 nmol g−1 in the Seine. Because of its high distribution coefficient (log Kd = 5–6) and the high turbidity of these environments, mercury is mainly transported in particulate form in freshwaters as well as in mixing zones. Relationships between Kd and concentrations of suspended particles, dissolved and particulate organic carbon allow to describe the partition of mercury between solid and dissolved phases in terms of colloids, organic complexation in solution and sorption processes. Chlorocomplex formation does not play the major role in the speciation of Hg during the estuarine mixing. Organic associations, through sulphydryl groups, seems to be the dominant processes governing mercury distribution and partition in the two macro-tidal estuaries studied. Primary production appears to play an important role in the mercury dynamic. (HgT)P is correlated with the chlorophyll content of the particles in high flow regime of the Loire river, while elemental mercury (Hg0) was found to be related to the oxygen saturation rate in both estuaries. The production of Hg0 was sufficient to sustain a significant evasion flux to the atmosphere. Dimethylmercury was detected at fentomolar level during one cruise in the Baie de la Seine during an hypoxia episode. Highest monomethylmercury concentrations (up to 4% of the HgT) were associated with warmer and organic rich freshwater. Candidate sources for methylated mercury species are discussed. Both estuaries appear to be affected by local anthropogenic inputs. However, the Seine river is more deeply impacted by human activities. Watershed yields of the total Hg were calculated for the two drainage basins. The Seine yield (41 mmol km−2 a−1) is in the range of what is found in other urban areas, while for the Loire (9 mmol km−2 a−1) it is close to values found for agricultural watersheds. In spite of the anthropogenic inputs within the two estuaries, the riverine dissolved mercury inputs to the adjacent coastal waters (la Baie de la Seine and le Grand estuaire de la Loire) have been estimated to be half of the corresponding atmospheric depositions.

Metal biogeochemistry in the Tinto-Odiel rivers (Southern Spain) and in the Gulf of Cadiz: A synthesis of the results of TOROS project

TOROS (Tinto-Odiel-River-Ocean Study) has been studying the biogeochemical processes which control metals and nutrients cycling in the mixing zone of the Tinto and Odiel rivers (SW Spain) and has established the fate of metals in the Gulf of Cadiz in relation to hydrodynamics and biological activity. The Tinto and Odiel rivers are small, with a combined mean discharge of 18m 3/s. They drain the largest sulphide mineralisation in the world. Predominantly, Zn-Cu-Pb mineralisation has been worked since 2500yr BC. The estuarine zone includes both an extensive area of salt marsh and an intensively industrialised urban area. As a consequence of pyrite oxidation, the Tinto and Odiel rivers are strongly acidic (pH<3) with extremely high and variable metal concentrations. Transition metals are poorly removed from the water column in the mixing zone. Moreover, drainage from large phosphogypsum waste deposits contributes to As, Hg, U and phosphate contamination of the estuary. The collapse of the tailing reservoir at los Frailes in 1998 had not impacted the chemistry of the coastal waters up to 6 months later. A large plume of metal-rich waters due to the Tinto and Odiel discharges occurs along the coast of the Gulf of Cadiz. This plume affects seasonally the Atlantic inflow through the Strait of Gibraltar. The dispersion of the metal discharges has been simulated by injection of a tracer in the 3-D hydrodynamical model. Both model and field study clearly show the inflow of metal contaminated Spanish Shelf Water through the Strait of Gibraltar.

Mercury Fluxes at the Ocean Margins

A mass balance budget for mercury at Ocean margins is proposed on the basis of recent data on exchange fluxes with terrestrial, atmospheric and marine reservoirs. The largest single mercury flux consists of particulate species from rivers (~5 Mmol a-1), which is, primarily, unreactive and quickly buried in nearshore sediments. The most important source of mobile mercury for ocean margins is the atmosphere directly via deposition and indirectly via upwellings (total ≈ 5 Mmol a-1). The direct atmospheric input to coastal zones rivers (~2 Mmol a-1), exceeds considerably the dissolved inputs from rivers (~0.13 Mmol a-1). The evasion of elemental mercury from coastal surface waters is balanced by the mercury deposition from the atmosphere, but geographical differences exist suggesting a net mercury transfert to the higher latitudes. The overall residence time for mercury on the continental shelves is about 4 months. This is less than the mean residence time of water on the shelves (~1.3 year) and confirms the known reactivity of mercury in aquatic environments. However, two fractions of mercury are actively recycled (through the atmosphere and through the organic carbon recycling). The highly productive zones associated with frontal stuctures near the shelf edges appear to be very active in redistributing mercury species between ocean and coastal areas. One consequence is that the main source of methylmercury for coastal waters is upwelled oceanic waters.

The distribution of dissolved and particulate mercury in three Siberian estuaries and adjacent arctic coastal waters

Dissolved and particulate mercury distributions were determined in the three largest Siberian rivers and in adjacent Arctic coastal waters during two cruises. Water samples were collected in the Lena River and its mixing zone in the Laptev Sea in September 1991, and in the Ob and Yenisei Rivers and the adjacent Kara Sea in September 1993. Average total dissolved Hg concentration was 5.0 pM in the Lena River, 2.8 pM in the Ob River and 1.5 pM in the Yenisei River. Mercury content of suspended particulate matter was low, averaging 0.17 mg kg -1 in the Lena and 0.05 mg kg -I in the Ob and Yenisei Rivers. These concentrations are lower than those observed in other world rivers affected by local input of man-made origin. In the estuarine mixing zones, higher concentrations of dissolved and particulate Hg which may originate from the spring flood were found. The carbon cycle is apparently a driving mechanism for Hg distribution in Arctic coastal waters. Particulate Hg content was positively correlated with the content of organic matter of the particles. In the Kara Sea, uptake by phytoplankton is suspected to be responsible for the increase in particulate Hg levels. Mercury fluxes from the three rivers to the Arctic Shelf are estimated and compared to direct atmospheric inputs.

Mercury in 16 demersal sharks from southeast Australia: Biotic and abiotic sources of variation and consumer health implications

Total mercury (THg) and monomethylmercury (MMHg) concentrations were determined in the tissues of demersal shark (Order Squaliformes and the Families: Scyliorhinidae, Hexanchidae) and chimaera species (Families: Chimaeridae and Rhinochimaeridae) from continental shelf and slope waters off southeast Australia, including embryos, juveniles and adults. The distribution of THg in various tissues (muscle, liver, kidney and skin), examined in ten species, shows higher levels in the muscle tissue (1.49+/-0.47mgkg(-1), ww), which accounted for between 59% and 82% of the total body burden of mercury and in the kidney (0.93+/-0.14mgkg(-1), ww) and liver (0.61+/-0.25mgkg(-1), ww) with lower levels observed in the skin (0.12+/-0.06mgkg(-1), ww). Additional THg determinations were performed in the muscle tissue of five other species allowing geographical and inter-specific comparisons. Speciation analysis demonstrated that more than 90% mercury was bound in muscle tissue as MMHg with higher percentages (>95%) observed in sharks species occupying deeper environments. Species differences were observed. Highest THg levels in the muscle tissue (up to 6.64mgkg(-1) wet weight, ww) were recorded in Proscymnodon plunketi and Centrophorus zeehaani (mean values; 4.47+/-1.20 and 3.52+/-0.07mgkg(-1), ww, respectively). Consistent with the ongoing paradigm on mercury bioaccumulation, we systematically observed THg concentrations increasing with animal size from the embryos to the larger sharks. Embryos of Etmopterus baxteri and Centroselachus crepidater had average levels 0.28 and 0.06mgkg(-1) (ww), while adult specimens reached 3.3 and 2.3mgkg(-1) (ww), respectively. THg concentrations in Australian sharks were compared with the same genus collected in other world regions. Levels were closer to data reported for East Atlantic than for the epicontinental Mediterranean margins. At a smaller geographical scale, the habitat effect on mercury concentration in sharks seems less clear. Squalid sharks occupying shelf waters showed higher mean mercury levels relative to their size (body weight, bw) than mid-slope species (0.4-6.7mgkg(-1) bw and 0.3-2.2mgkg(-1) bw, respectively). However, local regional differences (East and South Tasmania vs. Victoria) in Hg levels were not detected for the majority of taxa examined. All species, with the exception of Figaro boardmani showed values greater than 0.5mgkg(-1) (ww) and all but four were above many international regulatory thresholds (1.0mgkg(-1), ww).

Mercury transformations and exchanges in a high turbidity estuary:: The role of organic matter and amorphous oxyhydroxides

The speciation and partition of mercury in a macrotidal estuary (Seine estuary, France) was studied in order to explore the role of the high turbidity zone (HTZ) in mercury transfer to the adjacent coastal waters. Water and particles were analyzed to determine the concentrations of various mercury species, including monomethylmercury and the inorganic fraction. The exchangeable particulate mercury, which varies with salinity, and the mercury fraction associated with the amorphous oxyhydroxides were evaluated. The distribution of dissolved mercury species during early mixing suggests non-conservative behavior of organically bound mercury at the head of the estuary. Mercury in the particles covaried positively with suspended particulate matter concentrations up to a threshold, which constitutes the typical mercury load of particles and deposited sediments of the HTZ. This distribution pattern is well explained by a dilution model: a slowly settling, low metal population of particle, characterized by relatively invariant turbidity, becomes admixed with a variable amount of higher metal content particles derived from the resuspension in the HTZ. In addition, in the HTZ, which acts as a degradation reactor for particulate organic matter, particulate mercury concentrations increase with increasing C:N ratios and amorphous oxyhydroxides particles. Mercury reaches the estuarine HTZ mainly associated with the riverine and marine particles, including organic matter and oxyhydroxides, which are temporarily trapped in the HTZ and mixed with autochthonous HTZ particles. The largest particles periodically settle and undergo diagenetic reactions and resuspensions, which lead to their mercury enrichment. Depending upon hydrodynamic conditions, mercury escapes seaward as fine particulate within the plume, partially associated with the oxyhydroxides. A surface complexation model reproduces most of the partitioning observed. In the dissolved phase the model simulation suggests that a very strong ligand must be present to explain the field observations.

A 3500-Year Record of Hg and Pb Contamination in a Mediterranean Sedimentary Archive (The Pierre Blanche Lagoon, France)

A sediment core encompassing 3500 years of continuous sedimentation has been collected from a coastal lagoon located on the southwestern French Mediterranean coast. Lead concentrations and stable isotopes show that the sediments have recorded the three major periods of Pb pollution: the Etruscan-Greek-Roman period (650 BC to AD 50), the medieval period (AD 650 to AD 1450), and the modern period (from around AD 1850 to the present). These periods were separated by low pollution periods during the Dark Ages (between AD 50 and 650) and during the 16th century. From the end of the 19th century to the 1960s, Pb pollution increased exponentially. Coal combustion was the major source of Pb in the lagoon in the second half of the 20th century. Both the decrease in coal consumption and the ban on leaded gasoline resulted in a decrease in Pb pollution by a factor of 1.5 between 1973 and 1995. From 1991, sewage treatment plants and incinerators could be the major source of Pb. The average baseline Hg concentration from 1525 BC to AD 900 was 0.017 ± 0.003 μg g⁻¹ (n = 54). The Hg concentrations profile shows three major peaks: in AD 1150, AD 1660, and AD 1969, with the concentrations being respectively 8, 5, and 34 times higher than the baseline levels. The medieval peak (AD 1150) is attributed the medical use of Hg in the town of Montpellier and/or the burning of soil and vegetation. Noticeable Hg pollution was also detected during the 17th century in relation to gold and silver amalgamation in Europe. From the end of the 19th century, Hg concentrations increased exponentially until 1969. This modern pollution is attributed to the burning of coal.