Significance of floods in metal dynamics and export in a small agricultural catchment

High-resolution monitoring of water discharge and water sampling were performed between early October 2006 and late September 2007 in the Montoussé River, a permanent stream draining an experimental agricultural catchment in Gascogne region (SW France). Dissolved and particulate concentrations of major elements and trace metals (i.e. Al, Fe, Mn, As, Cd, Cr, Cu, Ni, Pb, Sc and Zn) were examined. Our results showed that contamination levels were deficient to moderate, as a result of sustainable agricultural practices. Regarding dynamics, metal partitioning between particulate and dissolved phases was altered during flood conditions: the particulate phase was diluted by coarser and less contaminated particles from river bottom and banks, whereas the liquid phase was rapidly enriched owing to desorption mechanisms. Soluble/reactive elements were washed-off from soils at the beginning of the rain episode. The contribution of the flood event of May 2007 (by far the most significant episode over the study period) to the annual metal export was considerable for particulate forms (72–82%) and moderate for dissolved elements (0–20%). The hydrological functioning of the Montoussé stream poses dual threat on ecosystems, the consequences of which differ from both temporal and spatial scales: (i) desorption processes at the beginning of floods induce locally a rapid enrichment (up to 3.4-fold the pre-flood signatures on average for the event of May 2007) of waters in bioavailable metals, and (ii) labile metals – enriched by anthropogenic sources – associated to particles (mainly via carbonates and Fe/Mn oxides), were predominantly transferred during floods into downstream-connected rivers

Background levels of heavy metals in surficial sediments of the Gulf of Lions (NW Mediterranean): An approach based on 133Cs normalization and lead isotope measurements

This paper presents an attempt to reach natural background levels of heavy metals in surficial sediments of the Gulf of Lions(NW Mediterranean). To correct for the grain-size effect, normalization procedures based on a clay mineral indicator element are commonly used, after a first grain size separation by sieving. In our study, we tested the applicability of this method with respect to commonly used normalizer elements, and found that stable Cs shows the best ability to reflect the fine sediment fraction. Background levels were successfully reached for Co, Cr, Cu, Ni and Pb, compared to various literature references. Nevertheless, in the case of lead, the normalized data depicted a general enrichment in all samples, and the natural levels could only be reached when concentrations were corrected for the atmospheric contribution by analysing lead isotope ratios. Also for Zn, a general enrichment was found in our samples, although less important

Input of particulate heavy metals from rivers and associated sedimentary deposits on the Gulf of Lion continental shelf

Fluxes of the heavy metals chromium (Cr), cobalt (Co), nickel (Ni), copper (Cu), lead (Pb), cadmium (Cd) and zinc (Zn) delivered by rivers to the Gulf of Lion (NW Mediterranean Sea) were estimated over a three year study of the River Rhone and its smaller tributaries. Most of the particulate metal fluxes (80e90%) delivered by these rivers occurred within a very short period of time (less than 12%), a typical trend for the Mediterranean environment, where highly contrasting hydrological regimes were observed over the year. Temporal and spatial variations in the fluxes of these particulate metals were driven by the fluxes in both water discharge and suspended particulate matter load. On the shelf, these particulate metal fluxes, largely arising from the Rhone watershed, were two to ten times more important than those resulting from atmospheric deposition. Co, Cr and Ni in the rivers and on the shelf surface sediments were mainly natural and associated with the finest particles. Cd and Phosphorus appeared to be associated with the silt fraction and to be enriched in the prodelta areas. Pb, Zn and Cu were more closely associated with the organic matter content and also showed enrichment in the organic rich prodeltaic sediments. Anthropogenic influences diminished offshore, except for Pb and Zn which could be supplied from the atmosphere by man-made aerosols. Although most of the metals tended to be enriched in the prodelta areas, these did not constitute a permanent sink due to resuspension processes affecting these shallow depths. A resuspension experiment conducted on sediment cores from the Rhone prodelta demonstrated that metal deposited on the surface layer, especially those associated with the organic matter, may be resuspended; this should be taken into account for a complete understanding of the biogeochemical cycle of these metals

Fate of metals in coastal sediments of a Mediterranean flood-dominated system: An approach based on total and labile fractions

The dynamics of sediment-bound metals (Cs, Cu, Ni, Pb, Ti and Zn) were studied off the Têt River (western Gulf of Lion), a typical Mediterranean coastal river punctuated by short and violent flash-floods. Spatial and temporal sampling strategies were combined to elucidate the fate of these elements in response to both the riverine sediment input and the offshore transport of these sediments through hydrodynamics. Our results show the temporal entrapment of riverborne particles and associated metals, consecutively to a major flood event, in the nearshore sedimentary unit called prodelta. Here, deposition and resuspension mechanisms define a sedimentological cycle that could be followed completely in this study. In terms of speciation between reactive (labile) and residual fractions along the fluvio-deltaic continuum, our results show that Cu, Pb and Zn are the most labile (potentially mobile) metals in the river, in accordance with their contributions from anthropogenic sources. But in the marine surficial sediments, two main behaviours can be discriminated when compared to the riverine suspended particulate matter. While Pb and Zn depict rather a constant labile fraction, Cu is characterized by decreasing levels (up to 50% difference). In terms of environmental impact, these contrasting trends have direct repercussions for the contaminant dispersal in the coastal area. Whereas Pb and Zn conserve their enhanced levels because of their stronger affinity with fine sediments, Cu is marked by the entire loss of its anthropogenic component that is progressively transferred to the dissolved phase, likely mediated by organic ligands. We ascribe these behaviours to different post-depositional partition mechanisms with respect to oxidation of the particulate organic phase at the bottom sediment/water interface. Also, analysis of one sediment core from the prodelta indicates that these early diagenetic processes govern the chemical forms of land-derived contaminants sequestrated in the nearshore sedimentary archives

Sources and sinks of sediment-bound contaminants in the Gulf of Lions (NW Mediterranean Sea): A multi-tracer approach

Surficial sediments collected in 2002 throughout the Gulf of Lions continental shelf (NW Mediterranean) were analysed for trace metals (Cd, Co, Cr, Cs, Cu, Ni, Pb, Sr, Zn and Zr), major elements (Al, Ca, Fe, P and Ti) and the sewage marker coprostanol. In addition, particle size distribution, organic carbon (OC) and carbonates were also determined. Results showed that the metal contamination (Cd, P, Cu, Pb and Zn) is mainly introduced by the local rivers and accumulates—via a regulation by OC and silt fraction (2–63 mm)—in the direct vicinity of the mouths, in high shear stress environments. Here also the signal of sewage contamination is the best preserved, especially off the eastern point sources where local sedimentation rates save the faecal marker from biodegradation processes. It is demonstrated that the shallow prodeltas are the first repository areas for land-derived particles, exposing local ecosystems to both inorganic and organic contaminations. When going seaward, however, sediment dilution, particle sorting and biodegradation processes make that most riverborne contaminants rapidly reach natural levels. Only some metals (i.e. Pb and Zn)—closely associated with the clay fraction—still depict anthropogenic enrichment, which seems to be inherited from man-made aerosols

Impact of oceanic floods on particulate metal inputs to coastal and deep-sea environments: A case study in the NW Mediterranean Sea

An exceptional flood event, accompanying a marine storm, was investigated simultaneously at the entrance and the exit of the Gulf of Lion's hydrosystem (NW Mediterranean) in December 2003. Cs, Cr, Co, Ni, Cu, Zn, Cd and Pb signatures of both riverine and shelf-exported particles indicate that continental inputs and resuspended prodeltaic sediments were intensively mixed with resuspended sediments from middle/outer shelf areas during advective transport. As a result, particles leaving the Gulf of Lion inherited the mean signature of shelf bottom sediments, exporting anthropogenic Pb and Zn out into the open sea. When assessing the particulate metal budget in relation with the event, it appears that the output fluxes accounted for between 15% and 60% of the input fluxes, depending on the element and the period of reference. This trend is also observed for annual budgets, which were drawn up by compiling the data from this study and the literature. Results evidenced that, except some element fluxes during extreme output scenario, outputs never counter-balance the inputs. In its current functioning, the Gulf of Lion's shelf seems to act as a retention/sink zone for particulate metals. Regarding anthropogenic fluxes, the contribution of the oceanic flood of December 2003 to the mean annual scenario is considerable. Environmental impacts onto coastal and deep-sea ecosystems should therefore tightly depend on both the intensity and the frequency of event-dominated sediment transport

Modelling trace metal background to evaluate anthropogenic contamination in arable soils of south-western France

The trace metal (TM) content in arable soils has been monitored across a region of France characterised by a large proportion of calcareous soils. Within this particular geological context, the objectives were to first determine the natural levels of trace metals in the soils and secondly, to assess which sites were significantly contaminated. Because no universal contamination assessment method is currently available, four different methods were applied and compared in order to facilitate the best diagnosis of contamination. First, the TM geochemical background was determined by using basic descriptive statistics and linear regression models calculated with semi-conservative major elements as predictors. The natural concentrations of trace metals varied greatly due to the high soil heterogeneity encountered on the regional scale and were more-or-less accurately modelled according to the considered TM. Second, the basic descriptive statistics and the linear regression methods were then compared with the enrichment factor (EF) method and multivariate analysis (PCA), in order to evaluate whether the concentrations measured in soils were abnormally high or not. The advantages and disadvantages of each method were discussed and their results used to identify the most probable contamination cases, the influence of the soils characteristics, as well as the agricultural land cover. The basic descriptive method was good as a first and easy approach to describe the TM ambient concentrations, but may misinterpret the natural anomalies as contaminations. Based on geochemical associations, the linear regression method provided more realistic results even if the relationships between major and trace metals were not significant for the most mobile TM. The EF method was useful to identify high point source contaminations, but it was not suitable when considering a large dataset of low TM concentrations. Finally, the PCA method was a good preliminary tool for the description of the global TM concentrations in a studied area, but it could only give indication on the highest contaminated points. By comparing the results of the different methods in the studied region, we estimated that 24% of the arable soils were contaminated by at least one trace metal, mainly Cu in vineyards/orchards and Cd, Pb and/or Zn in grazing lands. In addition, the calcareous soils exhibited globally higher natural and anthropogenic TM concentrations than non-calcareous soils, probably because of the lower TM mobility at alkaline pH

Modelling trace metal extractability and solubility in French forest soils by using soil properties

Soil/solution partitioning of trace metals (TM: Cd, Co, Cr, Cu, Ni, Sb, Pb and Zn) has been investigated in six French forest sites that have been subjected to TM atmospheric inputs. Soil profiles have been sampled and analysed for major soil properties, and CaCl2-extractable and total metal content. Metal concentrations (expressed on a molar basis) in soil (total), in CaCl2 extracts and soil solution collected monthly from fresh soil by centrifugation, were in the order: Cr > Zn > Ni > Cu > Pb > Co > Sb > Cd , Zn > Cu > Pb = Ni > Co > Cd > Cr and Zn > Ni > Cu > Pb > Co > Cr > Cd > Sb , respectively. Metal extractability and solubility were predicted by using soil properties. Soil pH was the most significant property in predicting metal partitioning, but TM behaviour differed between acid and non-acid soils. TM extractability was predicted significantly by soil pH for pH < 6, and by soil pH and Fe content for all soil conditions. Total metal concentration in soil solution was predicted well by soil pH and organic carbon content for Cd, Co, Cr, Ni and Zn, by Fe content for Cu, Cr, Ni, Pb and Sb and total soil metal content for Cu, Cr, Ni, Pb and Sb, with a better prediction for acidic conditions (pH < 6). At more alkaline pH conditions, solute concentrations of Cu, Cr, Sb and Pb were larger than predicted by the pH relationship, as a consequence of association with Fe colloids and complexing with dissolved organic carbon. Metal speciation in soil solutions determined by WHAM-VI indicated that free metal ion (FMI) concentration was significantly related to soil pH for all pH conditions. The FMI concentrations of Cu and Zn were well predicted by pH alone, Pb by pH and Fe content and Cd, Co and Ni by soil pH and organic carbon content. Differences between soluble total metal and FMI concentrations were particularly large for pH < 6. This should be taken into account for risk and critical load assessment in the case of terrestrial ecosystems

Leséléments métalliques (TRaceurs de la pression anthropique et du fonctionnement hydro-sédimentaire du glofe du Lion)

La zone côtière du golfe du Lion est l une des marges continentales les plus vastes de la Méditerranée. Les fleuves constituent sa source principale de matériel particulaire, à laquelle s ajoute la contribution atmosphérique. Une fois en mer, ces particules - naturelles et anthropiques - intègrent une dynamique sédimentaire complexe, elle-même régulée par les forçages hydro-météo-climatiques. Une meilleure compréhension de cette dynamique est donc nécessaire pour mieux entrevoir le devenir et l impact de la contamination dans le système côtier. Pour y parvenir, cette étude propose d utiliser les métaux associés à la phase solide comme traceurs de la dynamique des particules. Le résultat est double. Tout d abord, les grandes lignes du fonctionnement hydro-sédimentaire du golfe du Lion ont été décrites. Ensuite, l établissement des facteurs d enrichissements a permis pour la première fois de dresser un bilan détaillé de l état de la contamination dans cette région. Il indique que l impact anthropique dans la zone côtière est faible à modéré. La propagation de la contamination est contrôlée par les prodeltas. Sous influence directe des apports continentaux, ces unités accumulent les contaminants, dont une partie peut ensuite être exportée avec les fractions fines. Vers le large, la persistance de la contamination tient principalement aux comportements élémentaires vis-à-vis des phénomènes de la diagenèse précoce et des perturbations hydrodynamiques. Enfin, un premier bilan de métaux particulaires a pu être établi, faisant état d une fonction puits . Environ 50% des métaux introduits en zone côtière quitteraient la plateforme continentale en direction de la mer ouverte.The coastal zone of the Gulf of Lions is one of the most important Mediterranean shelves. It is exposed to numerous particulate sources, in particular the rivers draining the surrounding catchments and the direct atmospheric fallout. These solid discharges bring natural particles but also contaminated ones to the marine system where they follow a complex sedimentary dynamics highly dependent on hydro-meteorological conditions. Improving our knowledge on this dynamics will help to better understand the fate, and hence, the impact of contaminated particles in the coastal environment. To do so, we proposed to use particulate metals as tracers of the particle dynamics. First, the sedimentary functioning of the Gulf of Lions has been highlighted in global terms. Second, the environmental quality of the study area could be evaluated. It is concluded, on the basis of regional background levels, that the coastal zone is poorly to moderately impacted by human activities. Contaminants accumulate within the prodeltas after floods, and a part of them can be further flushed in association with fine materials. Off-shore, the persistence of the contamination strongly depends on elemental behaviours with respect to diagenetic processes and hydrodynamic disturbances. In terms of budget, it is shown that the Gulf of Lions is a sink for particulate metals - on the basis of mean input and output particulate fluxes. About 50 % of the metals introduced in the coastal waters (by fluviatile and atmospheric sources) may leave the shelf, towards the open sea and deep ecosystems.PERPIGNAN-BU Sciences (661362101) / SudocSudocFranceF