Cécile Grosbois

An Overview of Dissolved and Suspended Matter Fluxes in the Loire River Basin: Natural and Anthropogenic Inputs

The spatial and temporal distributions of major elements were investigated in the surface waters and in associated suspended matter at two sites of the upper Loire basin (Orleans and Brehemont) between 1995 and 1998.According to geochemical and isotopic patterns, the dissolved load appears to result from a process of mixing rainwater inputs, weathering processes of carbonate and silicate bedrock, and agricultural and urban inputs. Natural inputs influence 60% of water chemical composition at both sites. Annual dissolved fluxes were estimated to be 1300 103 t/y at Orleans and 1620 103 t/y at Brehemont. Major elements are transported mainly in the dissolved fraction. After correcting for atmospheric and anthropogenic inputs, the silicate specific export rate was calculated to be 11 t/y/km2 throughout the basin and the carbonate specific export rate to be from 47 t/y/km2 at Orleans to 23 t/y/km2 at Brehemont.The suspended load appears to result from at least two particle reservoirs: a silicate reservoir and a carbonate reservoir. The silicate reservoir has a detrital origin, mainly during periods of high flow, while the carbonate reservoir has a detrital origin during periods of high flow and an authigenic origin during periods of low flow. Of the total annual flow of suspended matter, this authigenic material represents 16% at Orleans, 25% at Brehemont and 37% in the fluvial part of the estuary. After correcting authigenic inputs, the specific export rate due to mechanical weathering was estimated to be 8 t/y/km2 throughout the Loire basin.

Severe and contrasted polymetallic contamination patterns (1900–2009) in the Loire River sediments (France)

The Loire River basin (117,800 km(2), France) has been exposed to multiple sources of metals during the last 150 years, originating from major mining districts (coal and non-ferrous metals) and their associated industrial activities. Geochemical archives are established here from the analysis of a 4m sediment core in the downstream floodplain and then compared to stream bed sediments from pristine monolithological sub-basins and from bed and bank sediments in impacted tributaries. The contamination is assessed for 55 major and trace elements through their enrichment factors to Al (EF), normalized to the pre-anthropogenic background. Archives from 1900 to 2009 show enrichment (EF<1.3) not only for Ba, Be, Cs, Ga, Rb, REE, Sr, V, and Zr but also for U and Th, despite U mining activities until the 1990s. From 1900 to 1950, the level of contamination is severe for Hg, Au, Ag (10<EF<30), important for Sb and Sn (3<EF<7) and moderate for Cu, Pb and Zn (1.5<EF<3). This state was mostly attributed to coal uses and metal mining. During the period 1950-1980, severe polymetallic contamination is noted for Hg (EF up to 53), Cd (23), Ag (18), Zn (6.2), Cu (6.0), Sn (5.6), Pb(4.8), Sb(4.4) and for new impacted elements as Bi (23.8), As (3.7), Cr (3.4), W (3.1), Mo (2.6), Ni (2.8), Co (1.65) due to mines, smelters, industries and from urban sewers, collected mostly after 1950 (total population of 8.4 million people). The limited dilution by detrital material (Loire sediment load about 1.5 Mt/year) is an additional cause of such severe contamination. After 1950, river eutrophication is well marked by the general increase of endogenic calcite (EF (Ca)=4), diluting all other elements by 20%. From 1980 to 2009, all contaminants, except Au (EF=100), decrease steadily.

CHANGES IN CHEMICAL AND 87SR/86SR SIGNATURE DISTRIBUTION PATTERNS OF SUSPENDED MATTER AND BED SEDIMENTS IN THE UPPER LOIRE RIVER BASIN (FRANCE)

Abstract The mineralogy of the suspended particulate matter (SPM) transported by the upper Loire river consists mainly of quartz and K-feldspar during periods of high river flow, with an increase in calcite concentration during periods of low flow. Concurrently, large fluctuations are observed in the levels of CaO and Fe2O3, the main oxides present in the SPM along with SiO2. The analysed trace elements also fluctuate significantly. The bed sediments (BS) have a similar oxide content to the SPM, but lower Zn and Pb levels and higher Zr levels. Fluctuations in the chemical-element concentrations with river discharge are related to fluctuation in the mineral assemblages present in the SPM. Thus K, Ti and Rb levels increase with increasing in K-feldspar abundance during high flow, and Ca and Sr levels decrease with increasing discharge due to a decrease in calcite abundance. These mineralogical and chemical variations relate to different sources of sediments under different flow conditions—sources that can be distinguished by Sr isotopic study. The Sr isotopic composition fluctuates according to the rate of river discharge; i.e., the 87 Sr / 86 Sr ratio increases with increasing discharge and reaches a maximum with peak flow, and vice versa. The 87 Sr / 86 Sr ratio also increases with the combined increase in K-feldspar abundance and decrease in calcite abundance. These similarities suggest the existence of at least two particulate-matter reservoirs, one with detrital silicates and the other with carbonates. Finally, the relationship between the 87 Sr / 86 Sr ratios in the suspended and dissolved loads suggests the coexistence of authigenic calcite in the carbonate reservoir. The SPM flow is related to a specific mechanical erosion rate over the whole Loire watershed ranging from 9 to 23 t km−2 yr−1. During the survey at Orleans, the SPM flow of the Loire river was 37·104 t yr−1, providing an estimate of the specific erosion rate of around 9.8 t km−2 yr−1. From the shift in the SPM flux between the sampling point and the mouth of the Loire river and the divergence in the mean annual discharge, a calculation was made of the input of solid matter by the tributaries and by supplementary erosion processes along the river.

The labile fraction of suspended matter in the Loire River (France); multi-element chemistry and isotopic (Rb-Sr and C-O) systematics

Suspended particulate matter transported by the Loire River (France) comprises mainly quartz and K-feldspar during periods of high river flow, with an increase in calcite contents during periods of low flow. We studied the labile fraction (leached by HCl 0.2 N) associated with the suspended matter transported by the upper Loire River. The acid extractable matter (AEM) concentrations in the Loire exhibit a wide range from 8% during high-flow conditions in the river to 47% during the low-flow conditions. The dispersion of AEM data differentiates two fields; one related to a domain where CaCO3 precipitation is possible, and the other related to a domain where erosion is dominant. The concentrations of trace elements and REE fluctuate widely and generally decrease with increasing AEM concentrations. A middle REE (MREE) enrichment over the LREE and HREE is observed which can be related to Fe–Mn oxide coatings developed on the clastic particles. The 87Sr/86Sr ratio varies from 0.710582 to 0.711472 and displays two different trends when plotted against AEM abundance, again reflecting varying contributions from two main end-members. The hydrous Fe–Mn oxides have the lowest 87Sr/86Sr ratio (≈0.7105) and the carbonates have the highest 87Sr/86Sr ratio (≈0.7115). The highest 87Sr/86Sr ratios of AEM, obtained during the period of authigenic calcite precipitation, agree with values measured for the dissolved Sr over the same period. During the period where erosion dominates, the 87Sr/86Sr ratio of the hydrous Fe–Mn oxide end-member diverges from the mean value of the dissolved load of the Loire River. This implies that the oxides were formed in water with a lower 87Sr/86Sr ratio, suggesting that (i) the location of oxide precipitation is upland, and (ii) the oxides conserve their Sr isotope signature during transport by the river and do not re-equilibrate with local waters during transport. Stable carbon and oxygen isotope compositions of the carbonate fraction of the AEM during low-flow conditions are consistent with authigenic calcite formation in isotopic equilibrium with Loire River water. At high-flow conditions we observe an isotopic enrichment that may reflect a higher proportion of atmospheric CO2 within the river waters at this time, or a contribution of detrital carbonate material from marine sediments exposed in the watershed, or secondary calcite derived from eroded soils.

Deconvolution of trace element (As, Cr, Mo, Th, U) sources and pathways to surface waters of a gold mining-influenced watershed

The Upper Isle River (SW France) drains the second most productive gold-mining district of France. A high resolution survey during one hydrological year of As, Cl(-), Cr, Fe, Mn, Mo, SO(4)(2-), Th and U dissolved concentrations in surface water aimed to better understand pathways of trace element export to the river system downstream from the mining district. Dissolved concentrations of As (up to 35000 ng/L) and Mo (up to 292 ng/L) were about 3-fold higher than the regional dissolved background and showed a negative logarithmic relation with discharge. Dissolved concentrations of Cr (up to 483 ng/L), Th (up to 48 ng/L) and U (up to 184 ng/L) increased with discharge. Geochemical relationships between molar ratios in surface water, geochemical background as well as rain- and groundwater data were combined. The contrasting behavior of distinct element groups was explained by a scenario involving three seasonal components: (i) The high flow component is poorly concentrated in As and Mo but highly concentrated in Cr, Th, U. This has been attributed to diffuse sources such as water-soil interactions, atmospheric inputs, bedrock and bed sediment weathering. Although this component probably also includes a contribution by weathering of sulfide veins, this signal is masked by dilution. (ii) One low flow component presents high SO(4)(2-), Fe, As and Mo and moderate Cr, Th and U concentrations. This component has been attributed to point sources such as mine gallery effluents, mining waste weathering and groundwater inputs from natural and/or mining-induced sulfide oxidation in the ore deposit. (iii) A second low flow component showing high As plus Mo concentrations associated with very low SO(4)(2-), Fe, Cr, Th and U concentrations, probably reflects trace element scavenging by ferric oxyhydroxide formation in the adjacent aquifer. This is supported by the decrease of Fe, Cr, Th and U in surface waters. Flux estimates suggest contrasting element-specific impacts on annual dissolved fluxes. Runoff may account for the major part of annual dissolved As, Mo, Th and U fluxes in the Upper Isle River. Inputs related to sulfide oxidation respectively contributed approximately 30% and approximately 24% to annual As and Mo fluxes. The formation of ferric oxyhydroxides strongly retained Cr, Th and U during the low flow, limiting their dissolved concentrations in surface waters. If this process may eventually decrease As mobility, its impact on dissolved As concentrations in surface water may be limited or/and counterbalanced by As release during sulfide oxidation.

Transformation of natural As-associated ferrihydrite downstream of a remediated mining site

Natural ferrihydrite (Fh), a poorly crystalline iron oxy-hydroxide with high concentrations of As, was investigated with respect to its crystallinity in a remediated mining environment. The Fh crystallinity increases from proto-Fh to a better crystallized 2-line Fh (called better 2-line Fh), while the associated As contents decrease from 7.8 to 1.9 wt %, respectively. This evolution of crystallinity correlated with decreasing As suggests that As is more likely coprecipitated in the Fh structure than adsorbed onto the surface. The evolution of Fh crystallinity is related to (i) aqueous transformations for samples continuously submerged and controlled by water composition (pH, ionic strength and redox potential) and (ii) dry thermal transformations for samples in a seasonally humid area with variations of temperature, humidity, agglomeration and compaction. The evolution of Fh crystallinity is more pronounced during dry thermal transformation than during aqueous transformations. Although Fh evolution is observed on the field, no stable form (goethite, hematite) is detected as usually noted during dry thermal transformation. This may be explained by incorporated cations such as Si and Al that can inhibit the transformation of Fh to a more stable form. To understand this transformation in a natural environment, the discussion in this study focuses on two main points: (i) the relationship between spatial mineralogical distribution and As content and (ii) the importance of Fh evolution as a function of transformation processes observed in the field.

Influence of fluvial environments on sediment archiving processes and temporal pollutant dynamics (Upper Loire River, France)

Floodplains are often cored to build long-term pollutant trends at the basin scale. To highlight the influences of depositional environments on archiving processes, aggradation rates, archived trace element signals and vertical redistribution processes, two floodplain cores were sampled near in two different environments of the Upper Loire River (France): (i) a river bank ridge and (ii) a paleochannel connected by its downstream end. The base of the river bank core is composed of sandy sediments from the end of the Little Ice Age (late 18th century). This composition corresponds to a proximal floodplain aggradation (<50 m from the river channel) and delimits successive depositional steps related to progressive disconnection degree dynamism. This temporal evolution of depositional environments is associated with mineralogical sorting and variable natural trace element signals, even in the <63-μm fraction. The paleochannel core and upper part of the river bank core are composed of fine-grained sediments that settled in the distal floodplain. In this distal floodplain environment, the aggradation rate depends on the topography and connection degree to the river channel. The temporal dynamics of anthropogenic trace element enrichments recorded in the distal floodplain are initially synchronous and present similar levels. Although the river bank core shows general temporal trends, the paleochannel core has a better resolution for short-time variations of trace element signals. After local water depth regulation began in the early 1930s, differences of connection degree were enhanced between the two cores. Therefore, large trace element signal divergences are recorded across the floodplain. The paleochannel core shows important temporal variations of enrichment levels from the 1930s to the coring date. However, the river bank core has no significant temporal variations of trace element enrichments and lower contamination levels because of a lower deposition of contaminated sediments and a pedogenetic trace elements redistribution.

Fate of arsenic-bearing phases during the suspended transport in a gold mining district (Isle river Basin, France).

Arsenic-rich (~140-1520 mg x kg(-1)) suspended particulate matter (SPM) was collected daily with an automatic sampler in the Upper Isle River (France) draining a former gold mining district in order to better understand the fate of arsenic during the suspended transport (particles smaller than 50 μm). Various techniques at a micrometric scale (EPMA, quantitative SEM-EDS with an automated particle counting including classification system and μXRD) were used to directly characterize As-bearing phases. The most frequent ones were aggregates of fine clay particles. Their mineralogy varied with particle sources involved. These aggregates were formed by chlorite-phlogopite-kaolinite assemblages during the high flow and chlorite-illite-montmorillonite during the low flow. Among all the observed As-carriers in SPM, these clay assemblages were the least As-rich (0.10 up to 1.58 wt.% As) and their median As concentrations suggested that they were less concentrated during the high flow than during the low flow. Iron oxyhydroxides were evidenced by μXRD in these clay aggregates, either as micro- to nano-sized particles and/or as coating. (Mn, Fe)oxyhydroxides were also present as discrete particles. Manganese oxides (0.14-1.26 wt.% As) transport significantly more arsenic during the low flow than during the high flow (0.16-0.79 wt.% As). The occurrence of Fe oxyhydroxide particles appeared more complex. During the low flow, observations on banks and in wetlands of freshly precipitated Fe hydroxides (ferrihydrite-type) presented the highest As concentrations (up to 6.5 wt.% As) but they were barely detected in SPM at a microscale. During the high flow, As-rich Fe-oxyhydroxides (0.10-2.80 wt.% As) were more frequent, reflecting mechanical erosion and transport when the surface water level increased. Arsenic transfers from SPM to corresponding aqueous fraction mostly depend on As-carrier stability. This study shows the temporal occurrence of each type of As-bearing phases in SPM, their As concentrations at a particle scale and abundance according to hydrological periods.

Dynamics of metallic contaminants at a basin scale--Spatial and temporal reconstruction from four sediment cores (Loire fluvial system, France).

From the 19th century, the Loire basin (France) presents potentially pollutant activities such as mining and heavy industries. This paper shows spatio-temporal distribution of trace elements in sediments at a basin-scale, based on a comparison of archived temporal signals recorded in four sedimentary cores. Anthropogenic sources contributing to sediment contamination are also characterized, using geochemical signatures recorded in river bank sediments of the most industrialized tributaries. This study highlights upstream-downstream differences concerning recorded contamination phases in terms of spatial influence and temporality of archiving processes. Such differences were related to (i) various spatial influences of contamination sources and (ii) polluted sediments dispersion controlled by transport capacity of metal-carrier phases and hydrosedimentary dynamics.

Contributions of natural arsenic sources to surface waters on a high grade arsenic-geochemical anomaly (French Massif Central)

The subwatershed studied drains a non-exploited area of the St-Yrieix-la-Perche gold mining district (French Massif Central) and it is located on an arsenic (As) geochemical anomaly. In this context, it is important to know the geochemical processes involved in the transfer of As from solid environmental compartments to the aquatic system. The stream showed a temporal variation of dissolved As (As(d)) content from 69.4 μg.L(-1) in the low flow period to 7.5 μg.L(-1) in the high flow period. Upstream, ground- and wetland waters had As(d) concentrations up to 215 and 169 μg.L(-1), respectively. The main representative As sources were determined at the subwatershed scale with in-situ monitoring of major and trace element contents in different waters and single extraction experiments. The As sources to stream water could be regrouped into two components: (i) one As-rich group (mainly in the low flow period) with groundwater, gallery exploration outlet waters and wetland waters, and (ii) one As-poor group (mainly in the high flow period) with rainwaters and soil solutions. In the soil profile, As(d) showed a significant decrease from 52.4 μg.L(-1) in the 0-5 cm superficial soil horizon to 14.4 μg.L(-1) in the 135-165 cm deep soil horizon. This decrease may be related to pedogenic processes and suggests an evolution of As-bearing phase stability through the soil profile. Quantification of As(d) fluxes at the subwatershed scale showed that groundwater was the major input (>80%) of As(d) to surface water. Moreover, natural weathering of the As-rich solid phases showed an impact on the As release, mainly from superficial soil horizons with runoff contributing about 5% to As input in surface water.