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Estuaries and coastal zones are areas with complex biogeochemical and hydrological cycles and are generally facing intense pollution due to anthropogenic activities. An emblematic example is the Scheldt Estuary which ends up in the North Sea and has been historically heavily contaminated by multiple pollutants, including mercury (Hg). We report here Hg species and their levels in surface waters of the Scheldt Estuary and the Belgian Part of North Sea (BPNS) from different sampling campaigns in February–April 2020 and 2021. Along the estuary, Hg concentration on suspended particles ([HgSPM]) progressively decreased with increasing salinity and was strongly correlated with organic matter content (%Corg) and origin (identified with δ13Corg). While [HgSPM] drives total Hg concentration in the estuary (total dissolved Hg, HgTD is only 7 ± 6 %), annual and daily variations of total Hg levels were mostly attributed to changes in SPM loads depending on river discharge and tidal regime. In the BPNS, a significant fraction of totalHg occurs as HgTD (40±21 %) and the majority of this HgTD was reducible (i.e. labile Hg), meaning potentially available for microorganisms. Compared to the ‘90s, a significant decrease of [HgSPM] was observed in the estuary, but this was not the case for [HgTD], which can be due to (1) still significant discrete discharges from Antwerp industrial area, and (2) higher Hg partitioning towards the dissolved phase in thewater column relative to the ‘90s.Our results highlight the important contribution of the Scheldt estuary for theHg budget in North Sea coastal waters, as well as the need for seasonal monitoring of all Hg species.

for the poster presentation at CRF-ChemCYS 2022 conference.

Poster presentation

Oral presentation at the 15th International Symposium on the Interactions Between Sediments and Water (IASWS) conference.

Synthetic solutions of Rh and Pt at near neutral pH, as well as spiked natural waters are subjected to an ageing effect, which means that over time the speciation of the PGEs will be changed from their original composition due to aquation and hydrolysis, forming various hydroxy and aqua complexes. Due to slow ligand exchange reactions equilibrium speciation may not be rapidly attained. Ageing may affect the reactivity of the PGE solutions towards chelating resins used in the analysis of PGEs by Diffusive Gradients in Thin films (DGT). The aim of the study was therefore to investigate the performance of the chelating resins (Purolite S914 and S920 with thiourea and isothiourea functional groups, and IONQUEST® MPX-317 with a phosphine oxide thiourea functional group) for their applicability as a binding resin for DGT applications for Pt and Rh in natural waters using 17-day aged spiked solutions. Uptake kinetics experiments revealed that the direct uptake of Pt on the resins from aged solutions was fast and quantitative, whereas only 30% of Rh was directly bound on the resin. Likewise, much lower apparent diffusion coefficients were obtained for Rh in aged spiked river water (70% decrease) which can be explained by the formation of Rh(OH)3(s) which is not taken up by the resin gel. The addition of dissolved organic matter results in an increased solubilisation of Rh(OH)3(s) and an increased DGT labile fraction. The formation of Rh(OH)3(s) is less pronounced in seawater. For Pt, no species are formed which are not DGT labile. Thus, DGT accurately measures labile metal species in solution, discriminating labile from inert metal species.

The population resident in areas around the East African Rift System (EARS) is facing serious problems associated with the availability and quality of drinking water. Groundwater wells, boreholes and springs are the main sources of drinking water in these areas. These sources are, however, characterised by elevated fluoride concentration (> 1.5 mg/L), resulting from the interactions with the surrounding volcanic rocks. The high fluoride levels found in the groundwater are often associated with the presence of other naturally occurring potentially toxic elements (PTEs), such as As, Mo, U and V, which are known to cause adverse effects on human health. The occurrence of such PTEs in the groundwater on the populated flanks of Mt. Meru, an active volcano situated in the EARS, has not been previously reported. The objective of this study is to determine the concentrations and distribution of PTEs in groundwater on the flanks of Mt. Meru to assess the potential health hazards that extend beyond the known fluoride issues in the study area. A large set of PTEs (Al, As, Ba, Cd, Cr, Cu, Fe, Mn, Mo, Ni, Se, Sr, Pb, U, V, W and Zn), physicochemical parameters and major elements were analysed in 48 groundwater samples sourced from wells, springs and tap water systems. To estimate the potential health risk associated with consumption of water with elevated concentrations of PTEs, chronic daily intake (CDI) and hazard quotient (HQ) were calculated. Preliminary results show that some of the water samples from the NE and SW flanks exceed the WHO tolerance limit for drinking water for some elements such as U (> 30 µg/L) and Mo (> 70 µg/L) as well as Na (> 200 mg/L) and K (> 50 mg/L). Estimated HQs indicate that such concentrations of Mo and U are not safe for consumption. Therefore, the continued drinking of water from these sources may result in serious health issues. Furthermore, the results show that some samples are characterised by total dissolved solids (TDS) values exceeding the permissible limits of WHO making the groundwater in the study area unsafe for drinking. The results presented in this paper are a part of a larger study on the groundwater quality in the Mt. Meru area and will serve to inform and direct exploitation of safe drinking water.

International audience

Sediment capping with activated carbon (AC) is an effective technique used in remediation of contaminated sediments, but the ecological effects on benthic microbial activity and meiofauna communities have been largely neglected. This study presents results from a 4-week experiment investigating the influence of two powdered AC materials (bituminous coal-based and coconut shell-derived) and one control material (clay) on biogeochemical processes and meiofauna in contaminated sediments. Capping with AC induced a 62-63% decrease in denitrification and a 66-87% decrease in dissimilatory nitrate reduction to ammonium (DNRA). Sediment porewater pH increased from 7.1 to 9.0 and 9.7 after addition of bituminous AC and biomass-derived AC, respectively. High pH (>8) persisted for at least two weeks in the bituminous AC and for at least 24 days in the coconut based AC, while capping with clay had no effect on pH. We observed a strong impact (nitrate fluxes being halved in presence of AC) on nitrification activity as nitrifiers are sensitive to high pH. This partly explains the significant decrease in nitrate reduction rates since denitrification was almost entirely coupled to nitrification. Total benthic metabolism estimated by sediment oxygen uptake was reduced by 30 and 43% in presence of bituminous coal-based AC and coconut shell-derived AC, respectively. Meiofauna abundances decreased by 60-62% in the AC treatments. Taken together, these observations suggest that AC amendments deplete natural organic carbon, intended as food, to heterotrophic benthic communities. Phosphate efflux was 91% lower in presence of bituminous AC compared to untreated sediment probably due to its content of aluminum (Al) oxides, which have high affinity for phosphate. This study demonstrates that capping with powdered AC produces significant effects on benthic biogeochemical fluxes, microbial processes and meiofauna abundances, which are likely due to an increase in porewater pH and to the sequestration of natural, sedimentary organic matter by AC particles.

International audience

The international GEOVIDE expedition (GEOTRACES GA01, May-June 2014, spring period) had notably investigated the link between trace element cycling and the production, export and remineralization of particulate organic matter in the North Atlantic Ocean. Sampling was undertaken within different biogeochemical provinces including the Iberian Margin, the West European Basin, Reykjanes Ridge, Irminger Sea, Greenland Margin and the Labrador Sea showing contrasted physical, biological and chemical characteristics. Some of these areas are known to present an important spring phytoplankton bloom leading to the export of particles. Here, we report Particulate Organic Carbon (POC) export fluxes estimated using the 234Th-based approach at 12 stations as part of the GEOVIDE expedition. POC export fluxes were deduced by combining export fluxes of 234Thtotal with the POC/234Th ratio of sinking particles at the depth of export. Profiles of POC to 234Th ratio of particles were obtained for two size classes (>53µm and 1-53 µm) using in-situ pumps. Considering the large (>53 µm) particle fraction as representative of sinking material, POC export fluxes revealed latitudinal variations between provinces ranging from 25 mgC.m-2.d-1 in oligotrophic waters of the Iberian Plain to 62 mgC.m-2.d-1 in the Labrador Sea. Comparison between export and production, defined as ThE ratio, will be addressed in order to study POC export efficiencies. Preliminary data for ThE ratio show a range from 1% (Irminger Sea) to 12% (Labrador Sea) and suggest a relatively low-efficiency biological carbon pump in the North-Atlantic during late spring conditions.

Background and aims: One of the goals of human biomonitoring by the Flemish Centre of Environment and Health was to investigate exposure-response associations for various biomarkers of exposure and effect. The aim of this study is to describe the results of the exposure-effect associations involving exposure to metals and metalloids. Methods: Participants donated blood and/or urine for determination of concentrations of metals and metalloids (by ICP-MS) and biomarkers of effect (by various analytical methods). All subjects filled in a questionnaire to provide information on health, life-style and personal factors. The age groups sampled were mother-newborn pairs (maternal and cord blood), 14-15 year old adolescents (blood and urine) and 20-40 year old adults (urine). Regression models were fitted to determine associations between effect- and exposure markers, adjusted for relevant confounders and covariates (e.g. smoking, age, gender). Results: Many significant associations were in line with previously reported findings, while some were not published before. The size of newborns was negatively correlated with cord blood As, and positively with Cu. For adolescent girls, blood and urinary Tl were negatively correlated with development of pubic hair and breasts. In boys negative correlations were found between urinary Tl and pubic hair development, and between both urinary Tl and blood Pb and genital development. In adolescents, urinary Cd showed a negative correlation with the T4 thyroid hormone and a positive correlation with an indicator of DNA damage, while urinary Cd and urinary Cr showed positive correlation with indicators of kidney damage. Various correlations were found between metals and sex hormones (aromatase, SHBG, testosterone, estradiol) in boys. In adults, there was only a significant negative association between urinary As and blood platelet aggregation. Conclusions: Many exposure-effect associations were supported by the literature, some were reported earlier in a previous cycle of the Flemish human biomonitoring.

The techniques of diffusive equilibrium in thin films (DET) and diffusive gradients in thin films (DGT) were used for the first time in the Outer Section of Songkhla Lake in order to obtain high resolution profiles of total dissolved and labile trace metals in the sediment porewater. These tools provide useful information on sediment biogeochemistry and benthic fluxes. DGT measures labile metal species in situ by immobilizing them on a resin gel after diffusion through a diffusive gel whereas for DET equilibrium is established between the DET gel and the porewater. The expedition was carried in July, 2011 under the collaborative research program between the Laboratory of Analytical and Environmental Chemistry, Vrije Universiteit Brussel and the Marine and Coastal Resources Institute, Prince of Songkla University. Six DET probes and six DGT probes were deployed at the mouths of the Phawong, Samrong and U-Taphao canals.

International audience; PAHs, PCDD/Fs and non-ortho PCBs have been assessed in Yser and Upper-Scheldt river sediments. Higher contamination levels were observed in the Upper-Scheldt sediments: maximum concentrations for the 16 US-EPA PAHs, PCDD/Fs and non-ortho PCBs respectively amount to 8.9mgkg -1, 12ng TEQkg -1 and 5.1ng TEQkg -1. Diagnostic PAH ratios in sediments and atmospheric samples suggest that the PAH compounds are from pyrolytic origin, more specifically combustion processes. The huge consumption of coal in cokes-ovens and smelters and its use for house-heating in Northern France, although decreasing during the last decades, are in support of that suggestion. PCDD/F fingerprints in sediments and deposition material indicate that OCDD is the dominant congener. In addition use of pentachlorophenol (PCP) in the past led to a minor contribution of PCDD/Fs in our sediment samples. Non-ortho PCBs form a substantial fraction of the total TEQ concentrations observed in the sediments. Since the 1980s and 1990s a substantial reduction of the PCDD/F sediment concentrations is observed, but this is not the case for the PAHs.

The Southern Bight of the North Sea is strongly influenced by nutrients input giving rise to eutrophication effects. The river and atmosphere inputs are important sources of nutrients and organic matter for the coastal marine waters and directly influence the productivity/respiration of the system and the exchange of CO2 with the atmosphere. We studied the global cycling of inorganic nutrients (ammonium, nitrate, silicates, and phosphates) and dissolved inorganic carbon (DIC) using a budget approach. Nutrient, salinity and DIC distribution data were collected during 7 cruises in the framework of the Belgian funded project CANOPY. Budgets were computed for each of the nutrients and DIC and for different periods of the year by using the general Land-Ocean Interactions in the Coastal Zone (LOICZ) recommendations. Major inflows, outflows and transformations are identified and discussed to understand the interactions between nutrients cycling and DIC dynamics in a shallow coastal sea.

Dissolved and particulate mercury and methylmercury concentrations were determined in the Southern Bight of the North Sea and the Scheldt estuary in the period 1991–1999. Mercury and methylmercury concentrations are higher before 1995 than after 1995, especially in the fluvial part.The North Sea: In the offshore stations, dissolved Hg concentrations are generally higher in winter than in summer while the reverse is true for particulate Hg KD values (KD=the concentration of particulate Hg (HgP in pmol kg-1) divided by the concentration of dissolved Hg (HgD in pmol l-1)) range from 100,000 to 1000,000 l kg-1. Dissolved methylmercury concentrations vary from 0.05 to 0.25 pmol l-1 in summer and from d.l. to 0.23 pmol l-1 in winter and particulate methylmercury concentrations from 1.8 to 36 pmol g-1 in summer and from 0.9 to 21 pmol g-1 in winter. The KD ranges from 9,000 to 219,000 l kg-1.The Scheldt estuary: In winter, dissolved Hg concentrations are elevated in the upper estuary, decrease exponentially in the low salinity range followed by a very slow decrease towards the mouth. In summer, they are low in the fluvial part, increase in the low salinity range or in the mid-estuary and sometimes show an increase in the lower estuary. Particulate Hg concentrations do not show any seasonal trend.Dissolved MMHg concentrations are much lower in winter, when maximum concentrations are found in the upper estuary, than in summer. In summer, the MMHg concentrations are low at low salinity, they show a first increase in the salinity range from 3 to 12, a decrease in the mid-estuary and a second increase in the lower estuary.The highest particulate MMHg concentrations are found in the upper estuary, while in the lower estuary generally lower and more constant values are observed. The ratio of dissolved MMHg to dissolved Hg (cruise averages between 1.3% and 20%), is higher than the ratio of particulate MMHg to particulate Hg (cruise averages of 0.27–0.90%). The KD values for MMHg are lower in the summer (30,000–65,000) than in autumn and winter (77,000–114,000).The Scheldt river: In the fluvial part of the Scheldt, dissolved increases in the most upstream stations, while particulate Hg shows no particular pattern. Dissolved MMHg ranges from 0.1 to 0.39 pmol l-1 and particulate MMHg from 3.1 to 43.5 pmol g-1. The MMHg concentrations are comparable to those found in the estuary and no seasonal variations could be observed.

The northwestern (NW) continental shelf of the Black Sea undergoes seasonal hypoxia. The benthic environment, the exchanges at the water-sediment interface and the diagenetic reactions are influenced by this phenomenon. In the framework of the BENTHOX project, two field cruises were conducted on the shelf in spring 2016 and in summer 2017.The first part of this investigation concerned the study of the impact of low oxygen levels in bottom waters on the diagenetic reactions. The microprofilings of geochemical parameters and the flux measurements showed both spatial and temporal variabilities in the benthic compartment of the NW continental shelf for the two seasons studied. The areas closest to the coasts exhibited the most important fluxes of oxygen consumption and of sulfate. These regions were strongly influenced by riverine inputs inducing a higher productivity and in turn resulting in an increase in the fluxes of organic matter deposited on the seabed.The diagenetic reactions were impacted by seasonal deoxygenation in bottom waters. The oxic respiration was less important in the summer as reflected by the shallower penetration depth of dissolved oxygen. Since 1995, the diffusive oxygen uptake (DOU) reported during hypoxic period indicated that the concentration of dissolved oxygen played an important role in the benthic exchange fluxes. Furthermore, a shallower reduction of sulphate and of Mn- and Fe-oxides observed in the sediments evidenced the impact of low oxygen levels on the diagenetic cascade. As a consequence, the benthic mineralization of organic matter was affected. During summer 2017, the oxic mineralization of organic carbon was less important and the contribution of the reducing species to oxygen consumption increased. The anaerobic mineralization of organic matter became thus the dominant process during the period of deoxygenation.The second part concerned the study of the sulfur and iron cycling in the shelf sediments during the low oxygen event of summer 2017. The sediments from the station close to the mouth of the Danube and that near the Dnieper exhibited a signature of detrital material different from that of the upper crust. The inputs of organic matter, in particular transported by the rivers, could influence the signal. The sediments of the NW shelf investigated were however of marine origin.Following the sulfate reduction already observed and mentioned above, the hydrogen sulfides produced were rapidly consumed as suggested by the low concentrations of acid volatile sulfide (AVS) and of the dissolved sulfide. The presence of pyrite in the upper layers of the sediments close to the water-sediment interface was plausible because the formation of pyrite in the water column had been reported due to the higher resistance of pyrite to oxidation compared to AVS (Wijsman et al. 2001). A non steady-state deposition was observed as suggested by Wijsman et al. (1999). It was characterized by an abrupt drop in the pyrite content caused possibly by fluctuations in salinity, dissolved O2 concentrations and organic matter fluxes.The non-reactive iron constituted the principal fraction of the total iron present in the sediments. Its contribution fell in the range reported for the marine sediments of the continental margins. A spatial variation of the concentration of highly reactive iron was nevertheless observed. Near the Dnieper mouth, the contents of reactive iron were lower and could be attributed to the less important fluvial inputs, to the sorption and/or precipitation processes or yet to the reallocation of the reactive iron of the shelf deposits towards the sediments in deep and euxinic waters.The last part of this thesis concerned the biogeochemistry of metals (Fe, Mn, Zn, Cu, Ni, Hg, Co and Cd) during the early diagenesis and the assessment of enrichment and pollution of these metals in the sediments of the shelf. The results showed that trace metals are probably linked to iron and manganese oxides as well as to sulphides. Ni and Zn could be involved in adsorption onto and co-precipitation with Fe- and Mn-oxides. Cu seemed to be associated preferentially to sulphides but no clear correlation was found. The enrichment and pollution of these metals in the shelf sediments showed spatial variability. The Danube delta area was enriched in Ni, Cu and Zn while the Odessa region was enriched in Co and Cd. The pollution of shelf sediments ranged from “unpolluted” to “very highly polluted”. Since the late 20th century, the heavy metal contents in shelf sediments remained stable in the Odessa region but increased in the Danube delta area., Le plateau continental du Nord-Ouest de la Mer Noire subit une hypoxie saisonnière. L'environnement benthique, l'échange à l'interface eau-sédiment et les réactions diagénétiques sont influencés par ce phénomène. Dans le cadre du projet BENTHOX, deux campagnes de prélèvement ont été menées sur le plateau au printemps 2016 et à l'été 2017 dans le but de comprendre ces changements.La première partie de cette recherche concerne l’impact des faibles concentrations en oxygène dans les eaux de fond sur les réactions diagénétiques. Les micro-profils de paramètres géochimiques et les mesures de flux ont montré des variabilités spatiales et temporelles dans le compartiment benthique pour les deux saisons étudiées. Les zones les plus proches des côtes ont présenté les flux les plus importants de consommation d'oxygène et de sulfate. Ces régions ont été fortement influencées par les apports fluviaux induisant une productivité plus élevée et entraînant à son tour une augmentation des flux de matière organique déposée sur le fond marin. Les réactions diagénétiques sont affectées par la désoxygénation saisonnière des eaux de fond. La respiration oxique etait moins importante en été comme en témoigne la profondeur de pénétration moins profonde de l’oxygène. Depuis 1995, l’absorption d’oxygène dissous (DOU) rapportée pendant la période limitée en oxygène a indiqué que la concentration d'oxygène dissous jouait un rôle important dans les flux d'échanges benthiques. De plus, une diminution de la profondeur à laquelle la réduction des sulfates et la réduction des oxydes métalliques témoigne de l'impact d'une faible concentration en oxygène sur la cascade diagénétique. En conséquence, la minéralisation benthique de la matière organique a été affectée. Au cours de l'été 2017, la minéralisation oxique du carbone a été moins importante et la contribution des espèces réductrices à la consommation d’oxygène a augmenté. La respiration anaérobie de la matière organique est ainsi devenue le processus dominant pendant la période d'hypoxie.La deuxième partie portait sur l'étude du cycle du soufre et du fer dans les sédiments du plateau lors de l'événement de faible teneur en oxygène de l'été 2017. Les sédiments de la station proche de l'embouchure du Danube et de celle du Dniepr présentaient une signature de matière détritique différente de celle de la croûte supérieure. Les apports de matière organique, notamment transportés par les rivières, pourraient influencer le signal. Les sédiments du plateau nord-ouest étudiés étaient cependant d'origine marine. Suite à la réduction du sulfate déjà observée et mentionnée ci-dessus, les sulfures d'hydrogène produits ont été rapidement consommés comme le suggèrent les faibles concentrations en sulfure acide volatil (AVS) et en sulfure dissous. La présence de pyrite dans les couches supérieures des sédiments près de l'interface eau-sédiment était plausible car la formation de pyrite dans la colonne d'eau avait été signalée en raison de la résistance plus élevée de la pyrite à l'oxydation par rapport à l'AVS (Wijsman et al. 2001). Un dépôt à l'état non stationnaire a été observé comme le suggèrent Wijsman et al. (1999). Elle a été caractérisée par une chute brutale de la teneur en pyrite causée peut-être par des fluctuations de salinité, des concentrations d'O2 dissous et des flux de matière organiqueLe fer non réactif constituait la fraction principale du fer total présent dans les sédiments. Sa contribution est tombée dans la gamme signalée pour les sédiments marins des marges continentales. Une variation spatiale de la concentration en fer hautement réactif a néanmoins été observée. Près de l'embouchure du Dniepr, les teneurs en fer réactif étaient plus faibles et pouvaient être attribuées aux apports fluviaux moins importants, aux processus de sorption et / ou de précipitation ou encore à la réallocation du fer réactif des dépôts du plateau vers les sédiments en profondeur et eaux euxiniques.La dernière partie de cette thèse concerne la biogéochimie des métaux (Fe, Mn, Zn, Cu, Ni, Hg, Co et Cd) lors de la diagenèse précoce et l'évaluation de l'enrichissement et de la pollution de ces métaux dans les sédiments du plateau. Les résultats ont montré que les métaux traces sont probablement liés aux oxydes de fer et de manganèse ainsi qu'aux sulfures. Ni et Zn pourraient être impliqués dans l'adsorption et la coprécipitation avec les oxydes de Fe et de Mn. Le Cu semble être associé préférentiellement aux sulfures mais aucune corrélation claire n'a été trouvée. L'enrichissement et la pollution de ces métaux dans les sédiments du plateau ont montré une variabilité spatiale. La zone du delta du Danube a été enrichie en Ni, Cu et Zn tandis que la région d'Odessa s'est enrichie en Co et Cd. La pollution des sédiments du plateau allait de « non polluée » à « très fortement polluée ». Depuis la fin du 20e siècle, la teneur en métaux lourds des sédiments du plateau est restée stable dans la région d'Odessa mais a augmenté dans la région du delta du Danube., Doctorat en Sciences, info:eu-repo/semantics/nonPublished