Alain J. Thomas

Sequential extraction techniques: Promises and problems

Abstract In order to gain a better understanding of biological and geochemical processes, sequential extraction techniques are widely employed to obtain information about the so-called ‘solid-speciation’ of chemical elements in soils and in suspended and deposited sediments. This present paper aims to review the different problems involved with such techniques and to specify the nature of their operation. Beyond the importance of sampling and pretreatment it is shown that the specificity and the reproducibility of the method greatly depends upon the chemical properties of the element and the chemical composition of the samples. Accordingly, these methods provide, at best, a gradient for the physicochemical association strength between trace elements and solid particles rather than their actual speciation. As previously underlined by W.F. Pickering (Crit. Rev. Anal. Chem., 2 (1981) 223–266) it is shown that, ‘a careless usage of these techniques without an appreciation of their pitfalls and limitations must lead to further generation of erroneous or misleading data’. Despite these inherent limitations, some useful information has been obtained in the fields of engineering and geoscience. However, the original idea which led to the development of the methods, i.e. the estimation of trace elements bioavailability, generally failed.

Preliminary assessment of the distributions of some trace elements (As, Cd, Cu, Fe, Ni, Pb and Zn) in a pristine aquatic environment: The Lena River estuary (Russia)

Abstract Dissolved and particulate trace elements have been determined in the Lena River and its mixing zone in the Laptev Sea during September 1989. Average dissolved concentrations (in nM) in river water were: As = 2.0, Cd = 0.05, Cu = 9.7, Ni = 5.2, Pb = 0.08, Zn = 5.3 and Fe = 410. These concentrations are comparable to and often lower than those observed in large unpolluted world rivers. In the mixing zone, the suspended matter composition in the surface plume varies from 90% detrital to 50% detrital (estimated) and the distribution of particulate trace metals reflects the relative proportions of these particles. Particulate Cu and Ni are preferentially associated to organic matter and clay minerals, respectively, whereas no distinct trend is seen for Pb and Zn. The distribution of dissolved concentrations shows little consistent evidence of elemental transfers between the dissolved and particulate phases (Cd, Ni, Pb, Zn), with the exception of the usual Fe removal, and a possible As uptake by phytoplankton. Some Cu uptake probably also occurs, but its effect does not alter significantly the apparent conservative behaviour of Cu. On the whole, terrigenous trace element supply does not appear to be significantly altered by transfer processes in the summer period, but this conclusion only relies on a limited number of preliminary measurements.

18O abundance and dissolved silicate in the Lena delta and Laptev Sea (Russia)

Water samples from the Lena River and stratified Laptev Sea (northeastern Siberia) have been analyzed to determine their stable oxygen isotope composition (18O/16O). Measurements at the Lena River reference station give a δ18O value of −18.9‰ in both surface and bottom waters. In the brackish water surface plume, a nearly perfect correlation is found between δ18O and chlorinity δO=−18.9+0.7C1−(n=15; r=0.999) A few values lie distinctly below this correlation; they all correspond to surface samples collected in the semi-enclosed Buorkhaya Gulf, and they most likely reveal the occurrence of ‘old’ water masses. Some of the δ18O values in the deep waters collected in the same zone also fall below the surface-plume correlation line. Dissolved silicate concentrations exhibit a large variability. However, when they are related to the different water masses identified using oxygen isotope data, a more coherent picture is obtained. Concentrations in the surface plume decrease more or less regularly from 50 to 72 μmol in the Lena River, to 7 μmol at the ‘marine’ end-member (Cl− = 14 g l−1). Dissolved silicate results in the Buorkhaya Gulf are quite distinct, with a clear deficiency in the surface waters, and an excess in the deep waters. These δ18O and dissolved silicate variations are discussed in relation to the hydrology and the biological productivity of the investigated area.

Atmospheric versus river inputs of metals to the Gulf of Lions: Total concentrations, partitioning and fluxes☆

Abstract This study assesses the significance of the atmospheric input of trace metals as compared to the Rhone river input into the Northern part of the Western Mediterranean directly influenced by this river. For most dissolved metals, atmospheric input is higher than dissolved riverine inputs. Atmospheric particulate input increases rapidly as one moves seaward from the river mouth. It shows the predominance of dissolved over particulate fractions in rain for all studied trace metals, except iron. Dry and wet fallout are equivalent for Co, Cu, Mn, Ni, and Pb; in the case of Al, Cd, and Fe, dry fallout is predominant. However, total atmospheric input of particulate metals (scavenged by the wet fallout and deposited as dry fallout) represents at least 70% of total atmospheric fallout.

Sources of Cd, Cu, Ni and Zn in Portuguese Coastal Waters.

Abstract In order to identify the major sources of trace metals (TM) in the Portuguese coastal waters, 58 surface water samples were collected during September 1988. The area sampled extended from the Tagus Estuary (down to a salinity of 25) to cape Ste Marie on the southern coast of Portugal. Dissolved metal concentrations in the fully marine waters ranged from 30 to 250 pM for Cd, 0.7–15 nM for Cu, 0.9–20 nM for Zn and 1.8–4.5 nM for Ni. Within the Tagus Estuary (salinity 25), concentrations increased to 3400 pM for Cd, 26 nM for Cu, 14 nM for Ni and 230 nM for Zn. The large-scale distribution of these metals is dominated by two strong continental sources, both probably linked to the exploitation of pyrite ores. In the Tagus Estuary, TM enrichments can be mostly attributed to a pyrite roasting plant located on the shore in front of Lisbon. Concerning the south Portuguese shelf waters, several hypotheses are proposed to explain their elevated metal concentrations. We particularly discussed the likely influence of the Tinto/Odiel rivers located 100 km eastward, an influence well known in the shelf waters of the Gulf of Cadiz. These rivers are extremely metal-rich because of acid mine tailings originating from their catchment. Between these two regions, upwelling of relatively metal-poor water largely contributes to the dilution of the continental inputs. Indeed, water exchanges on the shelf linked to the upwelling involve water fluxes 500 times higher than the Tagus River flow, and renew the coastal waters that are thus cleaned from terrestrial contamination. Contrary to many other upwelling systems in non-contaminated areas, the Portuguese upwelling does not act as a source of trace-metal enrichment of the continental margin waters.

Input of artificial radionuclides to the Gulf of Lions and tracing the Rhoˆne influence in marine surface sediments

Abstract The dispersal of particulate Rhoˆne River inputs to the shallow and deep Mediterranean Sea is studied using the particulate flux of artificial radionuclides originating from industrial discharges and atmospheric fallout. Among them, the long-lived 238Pu (half-life 88 years) is the most promising tracer. Studies of radionuclide fixation and mass-balance indicate that the suspended matter transport to the sea is reduced by temporal storing in the lower river course on time scales of months to a few years. An evaluation of the 137Cs and Pu isotopes inputs to the river system since 1956 defines time-integrated particulate Pu concentrations and activity ratios, which are used to partition a marine sediment into two fractions, one originating from the Rhoˆne River and the other from other sources, and an older fraction indicative of mixing processes and diffusion. A preliminary test with three sediment cores confirms the applicability of the method. It indicates a sharp decrease of the Rh^one influence between the pro-delta and the middle of the continental shelf, whereas trace amounts of238Pu in the deep-sea sediments indicate that the recent Rhoˆne sediment fraction is quite small.

Trace metal (Cd, Cu, Ni and Pb) cycling in the upper water column near the shelf edge of the European continental margin (Celtic Sea)

Abstract This study investigates the relative importance of processes that affect trace metal (TM) cycling in the upper water column at the shelf edge of the Celtic Sea on the western European continental margin. The examined processes include external inputs (by atmosphere and river), physical factors (upwelling, winter mixing and water mass advection) and biological processes (in situ uptake, regeneration and export to deep waters). The concentrations of dissolved Cd, Cu, Ni and Pb were measured with this aim in January 1994 and June 1995 at vertical stations across slope, including stations with upwelling, and in the surface waters along the Celtic Sea shelf. Additionally, deep sea (from sediment trap data) and atmospheric fluxes were estimated. The metal profiles over the slope off the Celtic Sea are quite similar to open ocean profiles already described in the northeast Atlantic, and the concentrations in surface waters are only slightly enriched compared to the nearby open ocean (1.2–1.3× for Cd and Ni). The external sources to the system appear to be of weak influence: the fluvial input is locally strong at the coast and then “diluted” along the large continental shelf; the atmospheric deposition is not significant at the annual scale in comparison to the metal content in the upper waters of the shelf edge (at least for Cd, Ni and Cu). In the upwelling zone, a significant increase in concentrations was observed in the summer surface mixed layer (×2 for nitrate and Cd and ×1.5 for Ni) in comparison to the non-upwelling zone. In winter, concentrations of bioreactive metals increased significantly in the surface waters in comparison to the low summer levels (×5 for nitrate and Cd). Our results suggest that upwelling and winter mixing act as regenerated sources that lead to the resupply of the bioreactive elements above the permanent thermocline with a low export to deeper waters. The tracing of the Mediterranean intermediate waters (MIW) from Gibraltar to the studied area shows indeed that its elemental content at the Celtic shelf edge is mainly due to the conservative mixing of the three “end-member” component waters which are thought to make up the MIW. The remineralization of organic matter within this water mass during its transport to the north would contribute only 20% of the nutrients and Cd concentrations recorded at the Celtic Sea shelf edge. According to the correlation found with nutrients in the 10–200-m layer, dissolved Pb would also be subjected to biological uptake and regeneration within the seasonal thermocline. Particulate scavenging removal of Pb would take place below the permanent thermocline throughout the water column.

Origin and fate of artificial radionuclides in the Scheldt estuary

The distribution of artificial radionuclides (60Co, 106Ru, 125Sb, 134Cs, 137Cs, 238Pu, 239+240Pu) in the suspended matter and sediments of the Scheldt estuary was investigated after the Chernobyl accident from 1986 to 1991 and was compared to the distribution of the same elements determined during a previous study in 1979-1984. The aim is to gain a better understanding of the geochemical behaviour of solids in the estuary. The longitudinal profiles indicate that 106Ru, 125Sb and 239+240Pu in the particles are of marine origin and that they are actively transported by tidal action upwards at least at 100 km from the mouth. On the other hand, 60Co and 238Pu are mainly discharged in the estuarine zone itself and a large fraction of these radionuclides are trapped in the upper zone of the estuary, in an area of intensive shoaling. The hypothesis that most of the continental suspended matter transported by the river is deposited in the area of Antwerp and does not reach the sea except during high floods is confirmed by the distribution of 134Cs and 137Cs resulting from the fall-out after the Chernobyl accident. Longitudinal distribution of dissolved Pu and Co, as well as scavenging experiments of Co and Cs, using radiotracers, have been performed over the entire salinity range of the estuary. This study shows scavenging of Pu in the anaerobic zone of the estuary. In contrast, Co is strongly and rapidly removed from solution when aerobic conditions are restored and Cs is desorbed when the salinity increases.

The global insignificance of telluric input of dissolved trace metals (Cd, Cu, Ni and Zn) to ocean margins

Abstract A tentative mass-balance of dissolved trace metals (TM) in the global ocean margin is presented. Comparison of the various TM sources demonstrates the limited contribution of the telluric source (riverine and atmospheric), as compared to the oceanic input by upwelled water: marine inputs exceed telluric inputs by a factor of 2–3.5 for Cu, 2–7 for Zn, and of about 10 for Cd and Ni. Diagenetic TM release from deposited sediments, although likely for Cu and Zn, could not be quantified without ambiguity. Following intense water exchanges, TM are exchanged between the ocean margin and the open ocean very rapidly, and their short residence time is similar to the margin water residence time, i.e. about 1.3 yr.

Time concepts in hydrodynamic systems with an application to 7Be in the Gironde estuary

Abstract The main definitions involving time concepts in dynamic systems are reviewed and extended. It is shown that a better understanding of aquatic systems can be achieved taking into consideration their mean age (either shorter or longer than the residence time). This time concept is especially useful when the well-mixed hypothesis is not applicable. Two box models are used as examples to demonstrate the relations between residence time and mean age. Then, these time concepts are extended to non-steady state. In the second part, the relationships between them and the behaviour of chemical components in dynamic systems is studied; the importance of radionuclides whose decay is perfectly known is demonstrated. Finally, as application to the Gironde estuary (France) is attempted using a cosmic ray-produced radionuclide, 7 Be, whose half-life is especially relevant to macrotidal estuarine dynamics. Particulate concentrations range from 0.2 to 11.2 pCi g −1 , while dissolved concentrations are in the range 10 to 50 fCi l −1 . The 7 Be mass balance is not equilibrated given the residence time computed from sediment discharge and the total mass of sediment in the turbidity maximum. This can be explained by an underestimation of the total mass of sediment involved in resuspension phenomena, or by a difference between the residence time and the mean age of sediment in the turbidity maximum which invalidates the usually implicit well-mixed hypothesis.