On inorganic tracers of wastewater treatment plant discharges along the Marque River (Northern France)

Increase of water quality in aquatic systems has become a hot button issue in recent decades. However, with the aim to implement an effective remediation strategy, the first step is to identify the sources of diffuse and point-source pollution using several tracers. In urban areas, B isotopes, Gd enrichment, Cl^- or carbamazepine concentrations can be used as wastewater treatment plant tracers. In this study, a focus was made on the quantification of a wide variety of inorganic compounds (elements, ions, isotopic ratios) all along the Marque River, a small stream located in Northern France receiving effluents coming from seven wastewater treatment plants (WWTPs). The objectives were (i) to determine the importance of the WWTPs discharge during low water events, (ii) to assess the efficiency of conventional tracers in quantifying the contribution of the WWTPs and (iii) to investigate new potential tracers less commonly used. The results have shown, through statistical analyses ANOVA (Analysis Of Variance) tests, PCA (Principal Component Analysis) and contribution calculations, that the WWTPs discharges strongly impact the water composition of all the watercourse and particularly during the first 6 km. However, due to high discharges of wastewaters not always well treated, some classical indicators (e.g. B, Rb/Sr) have shown limitations when used alone. The use of a set of relevant tracers including alkali metals could therefore be one solution for overcoming such a problem. Finally, other indicators like Rb/B or Gd/Pt ratios may also be a way to tackle this issue; they are indeed promising to discriminate the source of wastewaters.

Distribution of platinum (Pt), palladium (Pd), and rhodium (Rh) in urban tributaries of the Scheldt River assessed by diffusive gradients in thin films technique (DGT)

The performance of the newly developed DGT technique for the platinum group elements (PGEs) rhodium (Rh), platinum (Pt) and palladium (Pd) was evaluated in two tributaries of the Scheldt River, the Marque River close to the city of Lille (France), and the Zenne River which flows through the city of Brussels (Belgium). In the Marque River, an interlaboratory comparison was performed between the two laboratories where the DGT techniques dedicated to PGEs were developed (AMGC, VUB & LASIRE, U-Lille). PGEs were also analysed in an effluent of a Brussels hospital and monthly grab sampling was performed at the wastewater treatments plants (WWTPs) of Brussels. The concentrations of the 3 elements are higher in the Zenne River than in the Marque River and much higher Pt concentrations are found in the hospital effluent. Good agreement for Pt was observed between the three selected chelating resins and a relatively good agreement was observed between the two laboratories using the same chelating resin, whereas lower results were observed with the anion-exchange resin. Larger discrepancies between the two laboratories were observed for Pd and no comparison could be made for Rh due to the low natural concentrations. The results show that in small urban rivers with high impact of urbanization, WWTPs are an important source of Pt, resulting from the use of anticancer drugs in hospitals and households. The limited retention of PGEs in WWTPs results in increased concentrations in urban rivers downstream. For Pd and Rh, similar trends were found with other traffic related elements such as Cu, Zn and Pb, showing the highest concentrations in waters collecting runoff from a highway. The data show that these elements, together with Gd, can be useful to trace specific pollution sources and their dispersion.

Development and validation of DGT passive samplers for the quantification of Ir, Pd, Pt, Rh and Ru: A challenging application in waters impacted by urban activities

Platinum group elements (PGEs) are among the least abundant in the continental crust. They have become excellent tracers of anthropogenic activities, particularly due to their use in catalytic converters or in the medical industry. However, their quantification in environmental matrices is still problematic because of their low concentrations combined with the presence of interfering elements. Preconcentration methods are therefore necessary to measure accurate concentrations. In this study, the quantification of Ir, Rh, Ru, Pd and Pt was studied in depth by focusing on two resins: AG MP-1 (anion exchange) and Purolite® S-920 (chelating) with the aim of developing passive Diffusive Gradients in Thin films (DGT) samplers as in-situ pre-concentration tools. The characteristics of both resins (e.g. adsorption, elution, selectivity, etc.) were studied and the diffusion coefficients of PGEs in different matrices were determined. For the first time, carcinostatic platinum-based drugs were also studied. Better rates and percentages of adsorption were observed for S-920 while AG MP-1 was more selective with regard to spectral interferents and easier to elute. The diffusion coefficients of PGEs were resin-dependent, particularly for carcinostatic platinum-based drugs. For the first time, the applicability of these DGT samplers dedicated to PGEs was demonstrated in the field after their deployment in two wastewater treatment plants in Northern France for which concentrations were found to range from few pg L^-1 (Ir, Ru) to few ng L^-1 (Pt).

Multidisciplinary study to monitor consequences of pollution on intertidal benthic ecosystems (Hauts de France, English Channel, France): Comparison with natural areas

The intertidal areas of the Hauts-de-France (English Channel - France) stand out for the occurrence of fragile ecosystems that are exposed to natural and human-induced stress. Over the last two centuries, the northern part of this region has experienced a strong human pressure, with the settlement of numerous activities (i.e., metallurgic factories, harbors, embankments). On the contrary, the southern part includes mostly natural areas. The whole region is influenced by a macrotidal regime. A multidisciplinary approach based on sedimentological (grain-size), geochemical (trace metals, biomarkers) and biological (foraminifera) proxies was used to unravel the contrasting environmental conditions in the Hauts-de-France. Three foraminiferal-types communities, which reflect different ecological characteristics at regional scale, were identified: 1) estuarine macrotidal assemblages (Haynesina germanica associated to Elphidiidae) in low impacted estuaries; 2) industrial-perturbed assemblages (H. germanica and Cribroelphidium excavatum) in harbor areas; and 3) infaunal-dominant assemblages (Bolivina variabilis and B. pseudoplicata) in embankment areas. The outcomes of this study show that a multiproxy procedure needs to be adopted for properly characterizing intertidal ecosystems, where human impacts and natural stresses overlap and are hard to disentangle.

To what extent can the biogeochemical cycling of mercury modulate the measurement of dissolved mercury in surface freshwaters by passive sampling?

Mercury (Hg) is a pollutant of global concern owing to its great toxicity even at very low concentrations. Its toxicity depends on its chemical forms evidencing the importance to study its speciation. Dissolved Hg (Hg_(d)) and methylmercury (MeHg_(d)) monitoring in surface freshwaters represents a great challenge because of their very low concentrations and substantial temporal variability at different timescales. The Hg_(d) temporal variability depends on the environmental conditions such as the hydrology, water temperature, redox potential (Eh), and solar photo cycle. Passive samplers represent an alternative to improve the assessment of Hg_(d) and MeHg_(d) concentrations in surface freshwaters by integrating their temporal variability. An original sampling strategy was designed to assess the relevance of 3-mercaptopropyl DGT (Diffusive Gradient in Thin films) to integrate in situ the temporal variations of labile Hg (Hg_(DGT)) and MeHg (MeHg_(DGT)) concentrations. This strategy was implemented on two rivers to study the dynamics of Hg_(d), Hg_(DGT), MeHg_(d) and MeHg_(DGT) at diurnal and annual timescales. We evidenced that Hg_(DGT) and MeHg_(DGT) concentrations were generally consistent with discrete sampling measurements of Hg_(d) and MeHg_(d) in dynamic surface freshwaters. However, Hg_(DGT) concentrations were overestimated (2-16 times higher) in case of low flow or low water depth, low suspended particulate matter (SPM) concentrations and elevated daily photoperiod. The most probable hypothesis is that such conditions promoted Hg⁰ production, and resulted in Hg⁰ uptake by DGT. Thus, attention should be paid when interpreting Hg_(DGT) concentrations in surface freshwaters in environmental conditions that could promote Hg⁰ production.

Assessment of the treatment efficiency of an urban stormwater pond and its impact on the natural downstream watercourse

During the last few decades, stormwater ponds have become an alternative management practice in order to avoid flooding and to contain rainwater and runoff in urban areas where impervious land cover has increased. A second purpose of stormwater ponds is to improve the quality of runoff water that is usually contaminated with nitrogen, phosphorus, metals and organic micropollutants. Processes used are based on natural methods such as settlement and contribute to minimize the impact of these inputs to the natural aquatic system. This study aims to better understand the behavior of a wet stormwater pond, Heron Lake (33 ha) located in the city of Villeneuve d'Ascq in northern France through various indicators [trace metals, PAHs, PCBs, caffeine (CAF), carbamazepine (CBZ), nutrients and pathogens]. For that purpose, water quality was monitored for 1 year, mainly at the entrance and at the outlet of the lake. Sampling have also been done in the downstream aquatic environment, the Marque River. Sediments were sampled in the lake to evaluate the pollution trapped during sedimentation. Our results of both water and sediment sampling highlight: (i) the wastewater input into the Heron Lake is estimated to be equivalent to that of roughly 3800 inhabitants; (ii) the removal rates observed at the outlet, relative to concentrations at the entrance channel, vary as follows for these dissolved species: 24% for NO₃^- and PO₄^3-, 28% for CBZ, 35% for Cu, 63% for Pb, 78% for CAF, 84% for Zn and up to 93% for NH₄⁺; (iii) there are high levels of sediment contamination with metals, PAHs and PCBs at the entrance channel; (iv) the eutrophication of this pond is attributed to persistent high nutrient concentrations in both water and sediment, and has contributed to the development of an invasive macrophyte, the Elodea nuttallii; and (v) there appears to be only a negligible impact of the discharge from the lake to the natural watercourse, contributing annual loads of <2 up to 6% of the total amount of Cu, Pb, Zn, CAF, CBZ and nutrients measured in the Marque River, and having a slight diluting effect on concentrations in the Marque River.

Active biomonitoring for assessing effects of metal polluted sediment resuspension on gammarid amphipods during fluvial traffic

The resuspension of polluted sediments by boat traffic could release substantial amounts of metals to the water column, affecting at the same time their bioavailability. In order to characterize the impact of sediment resuspensions on biota, caged amphipods have been deployed on three different channelized watercourses in Northern France. Firstly, the biological responses of transplanted freshwater gammarid amphipods, Gammarus fossarum, described by trace metal accumulation, feeding and reproduction activities were quite similar for the three water courses despite the differences of metal contamination and navigability. Secondly, the concentrations of metals accumulated in gammarids never exceeded the contamination thresholds previously defined for Co, Cu, Cr and Zn. Values were in the same order of magnitude whatever the studied site despite: (i) large differences noticed in the sediment quality and (ii) some concentrations in the overlying waters exceeding the Environmental Quality Standards (EQS) defined by the Water Framework Directive. Conversely, Pb was highly bioaccumulated with values systematically exceeding the threshold value whatever the site. Therefore, the impact of navigation cannot be proved and the difference between the 3 monitoring periods is rather attributed to environmental variability, probably linked to the seasonality. Moreover, this study also confirms that organisms sampled from a local population in the vicinity of the three studied watercourses could be used as test organisms, leading to similar results than the ones obtained with reference gammarids initially used for developing all the biological responses. This would simplify and then promote the development of studies based on gammarid amphipod, G. fossarum, as bioindicators.

Palaeo-ecological quality status based on foraminifera of Boulogne-sur-Mer harbour (Pas-de-Calais, Northeastern France) over the last 200 years

Over the last centuries, coastal areas have experienced dramatic degradations of their environmental quality, which has led to a huge reduction of marine biodiversity. The objective of the present study was to use geochemical parameters and benthic fossil foraminifera to assess environmental changes that have occurred over the last 200 years in a harbour area (Boulogne-sur-Mer, Northern France) heavily modified by human activities. A multidisciplinary approach including major and trace metals, grain-size, total organic carbon and benthic fossil foraminifera, has been performed on a 33-cm long core. The dating was carried out using the activity of (210)Pb and (137)Cs. Embayment of the area and increase of trace metals concentrations induced a shift in benthic communities. Human activities modified a sandy nearshore bank, colonized by typical marine foraminiferal species, such as Cribroelphiudium excavatum, into a sheltered environment, dominated by brackish end-members, such as Haynesina germanica. Along the sedimentary record, the interaction between meiofaunal and geochemical elements made it possible to distinguish between a pre-impacted period and an industrial period. The upper part of the core reflects better ecological conditions, indicating an environmental recovery. Our results provide baselines for future environmental bio-monitoring in the area.

Water quality assessment of a small peri-urban river using low and high frequency monitoring

The biogeochemical behaviors of small rivers that pass through suburban areas are difficult to understand because of the multi-origin inputs that can modify their behavior. In this context, a monitoring strategy has been designed for the Marque River, located in Lille Metropolitan area of northern France, that includes both low-frequency monitoring over a one-year period (monthly sampling) and high frequency monitoring (measurements every 10 minutes) in spring and summer. Several environmental and chemical parameters are evaluated including rainfall events, river flow, temperature, dissolved oxygen, turbidity, conductivity, nutritive salts and dissolved organic matter. Our results from the Marque River show that (i) it is impacted by both urban and agricultural inputs, and as a consequence, the concentrations of phosphate and inorganic nitrogen have degraded the water quality; (ii) the classic photosynthesis/respiration processes are disrupted by the inputs of organic matter and nutritive salts; (iii) during dry periods, the urban sewage inputs (treated or not) are more important during the day, as indicated by higher river flows and maximal concentrations of ammonium; (iv) phosphate concentrations depend on oxygen contents in the river; (v) high nutrient concentrations result in eutrophication of the Marque River with lower pH and oxygen concentrations in summer. During rainfalls, additional inputs of ammonium, biodegradable organic matter as well as sediment resuspension result in anoxic events; and finally (vi) concentrations of nitrate are approximately constant over the year, except in winter when higher inputs can be recorded. Having better identified the processes responsible for the observed water quality, a more informed remediation effort can be put forward to move this suburban river to a good status of water quality.

On biogeochemistry and water quality of river canals in Northern France subject to daily sediment resuspension due to intense boating activities

In Northern France, channelized rivers facilitate greatly the waterway transport that should still increase in the coming years to replace as much as possible the road traffic, considered as a heavier source of pollution. These rivers are now subjected to the good potential status objectives required by the Water Framework Directive. The impact of the recurrent resuspension by the current boat traffic of polluted sediments (due to strong historical pollution) on the water quality is the main concern of this work. Our study reveals that the navigation seems to play a limited role on the enrichment of the water columns by dissolved metals and phosphorus, as well as on the oxygenation of surface waters. Conversely, the cycling of the phytoplankton over the year appears to partly control the physico-chemical and chemical evolutions of the overlying waters.

Monthly variation of trace metals in North Sea sediments. From experimental data to modeling calculations

Seasonal variation in trace metal contamination in surface sediments was studied through high resolution profiles assessed monthly by DGT probes in muddy sediments of the North Sea. General parameters such as pH, temperature, dissolved oxygen and sulfides were also recorded to estimate their role in the speciation of trace metals. Experimental data were included in a thermodynamic equilibrium model to calculate major (geo)chemical processes at the water-sediment interface and to predict the fate of the trace metals in case of (physico-)chemical changes. Results showed lowest Fe, Co, Ni and Cd concentrations in summer, which are most probably due to the very high sulfide concentrations according to our theoretical calculations. Cu and Pb behavior were found to be less influenced by sulfides, since they are also strongly associated to organic matter. The whole set of results clearly indicated that metal speciation in these sediments is controlled by sulfides and OM contents.

High-intensity interval exercise in chronic heart failure: protocol optimization

BACKGROUND

There are little data on the optimization of high-intensity aerobic interval exercise (HIIE) protocols in patients with chronic heart failure (CHF). Therefore, we compared acute cardiopulmonary responses to 4 different HIIE protocols to identify the optimal one.

METHODS AND RESULTS

Twenty men with stable systolic CHF performed 4 different randomly ordered single HIIE sessions with measurement of gas exchange. For all protocols (A, B, C, and D) exercise intensity was set at 100% of peak power output (PPO). Interval duration was 30 seconds (A and B) or 90 seconds (C and D), and recovery was passive (A and C) or active (50% of PPO in B and D). Time spent above 85% of VO(2peak) and time above the ventilatory threshold were similar across all 4 HIIE protocols. Total exercise time was significantly longer in protocols with passive recovery intervals (A: 1,651 ± 347 s; C: 1,574 ± 382 s) compared with protocols with active recovery intervals (B: 986 ± 542 s; D: 961 ± 556 s). All protocols appeared to be safe, with exercise tolerance being superior during protocol A.

CONCLUSION

Among the 4 HIIE protocols tested, protocol A with short intervals and passive recovery appeared to be superior.

Chemical speciation of sulfur compounds in surface sediments from three bays (Fresnaye, Seine and Authie) in northern France, and identification of some factors controlling their generation

The determination of total reduced sulfur, elemental sulfur, sulfide, thiosulfate and sulfite in porewaters--which were previously extracted from surface sediments collected in three bays, i.e. Seine, Fresnaye and Authie, located in northern France--has been undertaken using different electroanalytical techniques (linear sweep cathodic stripping voltammetry (LSCSV), square wave cathodic stripping voltammetry (SWCSV) and differential pulse cathodic stripping voltammetry (DPCSV)) at a static mercury drop electrode. Furthermore, the analyses of sulfur solids present in these sediments have been performed by sequential extraction procedures. Overall, the description of speciation of dissolved sulfur compounds and reduced sulfur solids has shown that the availability of sulfate and reactive iron, the sedimentation rate and probably even the nature and content of organic matter are important factors for controlling the sulfidisation and pyritisation processes involved in the different sedimentary systems studied. Thus, in the Seine-bay sediments, it has been found that reactive iron scavenges the most part of the generated sulfide as a result of a particularly activated sulfate reduction by bacterial activities; as a consequence, this precipitation limits the accumulation in the porewaters of reduced sulfur compounds such as elemental sulfur and polysulfides which ought to be generated in the redox boundary (where the oxydants, viz. oxygen, nitrate and metal oxides exist abundantly) through a partial oxidation of the H(2)S and HS(-) species. This process is further accentuated by a sedimentation rate measured in the Seine estuary. Conversely, in the Authie bay and, to a lesser extent, in the Fresnaye bay the contribution of the sedimentation rate and/or reactive iron scavenging to the sedimentary sulfur processes is weaker. This explains the increase of the built-up of dissolved reduced sulfur in the interstitial waters as well as the existence of elemental sulfur and polysulfides that permit the conversion of FeS into FeS(2).

Solid partitioning and solid-liquid distribution of 210Po and 210Pb in marine anoxic sediments: roads of Cherbourg at the northwestern France

A sequential extraction protocol has been used to determine the solid-phase partition of (210)Po and (210)Pb in anoxic marine sediment from the roads of Cherbourg (France) in the central English Channel. Measurements were also obtained in pore waters, in which (210)Po activities range between 1 and 20 mBq L(-1) and (210)Pb activities between 2.4 and 3.8 mBq L(-1), with highest activities in the topmost layer. These activities are higher than in seawater, suggesting that sediment act as a source of both (210)Po and (210)Pb for overlying water. The (210)Po profile in the pore waters is apparently correlated with those obtained for Fe, Mn and SO(4)(2)(-), suggesting an influence of early diagenetic processes on the (210)Po solid-liquid distribution. In the sediment, (210)Po is predominantly bound to organic matter or chromium reducible sulphides, and residuals (clay minerals and refractory oxides). Our results indicate that (210)Po is not significantly bound to AVS, i.e. acid volatile sulphides: bioturbation could play a role by the early redistribution of (210)Po bound to acid volatile sulphides in the sediment. (210)Po, (210)Pb and Pb exhibit differences in terms of distribution, probably due to a different mode of penetration in the sediment. This work provides information on solid and liquid distribution of (210)Po and (210)Pb in marine sediment. These data are very scarce in the literature.

Early diagenetic processes aspects controlling the mobility of dissolved trace metals in three riverine sediment columns

The behaviour of Mn, Fe, Co, Zn, Cd, Pb and Ni has been studied during early diagenesis in three different riverine sediments (Spierre, Lys and Sheldt). For that purpose (1) pore waters were extracted from sediment cores by centrifugation under nitrogen and further analyzed for the determination of total dissolved metal concentrations and (2) DET and DGT probes have been deployed in situ for the determination of high resolution profiles of labile and total dissolved metal concentrations. Furthermore, sulfidization processes have been examined; they revealed a production of pyrite near the water-sediment interface at Helkijn and Wervik sampling sites, probably due to a partial re-oxidation of reduced sulphur species. In Spierre sediments, where Eh values are the most negative, pyrite production should be mainly due to strict anaerobic processes. Concentrations of AVS in Spierre sediments are also very high and result in low TI values and low trace metal concentrations in the pore waters. Otherwise, in Wervik sediments, the low pH values combined to a TI value close to 0 results in the highest observed dissolved trace metal levels. DOS remains low at the three sites, since it does not exceed 0.4. In Wervik and Helkijn, the limitation is probably due to low sedimentary inputs of sulphate. In Spierre, sulphate is never exhausted in the pore water, suggesting a limitation of the DOS by a lack of bio-degradable organic matter. Values of Cd, Cu and Pb DGT concentrations remain low in pore waters whatever the site, due to their strong affinity with the reduced sulphur pool. It has also been demonstrated that the labile fractions of Pb and Cd are the lowest and do not exceed 0.5, while Co and Ni are the most available metals.

High resolution profiles of thallium, manganese and iron assessed by DET and DGT techniques in riverine sediment pore waters

High resolution profiles of Mn, Tl and Fe concentrations have been assessed in the pore waters of river Leie sediments at Warneton and Menen (at the border of Belgium and France) by DET (Diffusive Equilibrium in Thin Films) and DGT (Diffusive Gradients in Thin Films) techniques. The oxidized, solid Mn (IV), Tl (III) and Fe (III) compounds were reduced in the suboxic (+255 to -20 mV versus Standard Hydrogen Electrode (SHE)) riverine sediments and since these reduced species are much more soluble also they are released into the pore waters. The highest DET (total dissolved) concentrations of Fe (76 mg l(-1)), Mn (2 mg l(-1)) were observed at the station of Menen, while Tl maxima differed only slightly between the 3 surveys (21 to 27 microg l(-1)). The average ratios of Fe/Mn/Tl in the pore waters at the 3 sampling stations are fairly constant for both the DET and DGT samplings. However, the results indicate that compared to Fe and Tl a greater proportion of the Mn measured by DET is accumulated by DGT, reflecting the ready supply of Mn from solid phase to solution.

Voltammetric procedure for trace metal analysis in polluted natural waters using homemade bare gold-disk microelectrodes

Voltammetric procedures for trace metals analysis in polluted natural waters using homemade bare gold-disk microelectrodes of 25- and 125-microm diameters have been determined. In filtered seawater samples, square wave anodic stripping voltammetry (SWASV) with a frequency of 25 Hz is applied for analysis, whereas in unfiltered contaminated river samples, differential pulse anodic stripping voltammetry (DPASV) gave more reliable results. The peak potentials of the determined trace metals are shifted to more positive values compared to mercury drop or mercury-coated electrodes, with Zn always displaying 2 peaks, and Pb and Cd inversing their positions. For a deposition step of 120 s at -1.1 V, without stirring, the 25-microm gold-disk microelectrode has a linear response for Cd, Cu, Mn, Pb and Zn from 0.2 microg L(-1) (1 microg L(-1) for Mn) to 20 microg L(-1) (30 microg L(-1) for Zn, Pb and 80 microg L(-1) for Mn). Under the same analytical conditions, the 125-microm gold-disk microelectrode shows linear behaviour for Cd, Cu, Pb and Zn from 1 microg L(-1) (5 microg L(-1) for Cd) to 100 microg L(-1) (200 microg L(-1) for Pb). The sensitivity of the 25-microm electrode varied for different analytes from 0.23 (+/-0.5%, Mn) to 4.83 (+/-0.9%, Pb) nA L micromol(-1), and sensitivity of the 125-microm electrode varied from 1.48 (+/-0.7%, Zn) to 58.53 (+/-1.1%, Pb nA L micromol(-1). These microelectrodes have been validated for natural sample analysis by use in an on-site system to monitor Cu, Pb and Zn labile concentrations in the Deûle River (France), polluted by industrial activities. First results obtained on sediment core issued from the same location have shown the ability of this type of microelectrode for in situ measurements of Pb and Mn concentrations in anoxic sediments.

High-resolution profiles of trace metals in the pore waters of riverine sediment assessed by DET and DGT

The techniques of DET (diffusive equilibrium in thin films) and DGT (diffusive gradients in thin films) were applied to obtain high-resolution vertical profiles of trace metals in freshwater sediments. In the framework of the EU-Interreg project Stardust (http://www.vliz.be/projects/stardust/) between France and Belgium, in which the mobility of sediment bound metals is investigated, sediment samples were collected from the Upper Scheldt River (at Helkijn, Belgium) and the Leie River (at Warneton, located at the Belgian-French border). Intra- and inter-laboratory comparisons of the gel techniques were carried out between the two laboratories involved. In general, a good agreement was observed, taking sediment heterogeneity into account. At both stations, metal pore water profiles show more or less similar tendencies although the sediment at Warneton was more anoxic than at Helkijn. A strong correlation between Fe and Co was found at Helkijn as well as at Warneton. The metal gradients at the water/sediment interface were calculated from the high resolution profiles and the conventional, low resolution profiles. Significant differences were observed.