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As carriers of dissolved and particulate loads that connect continental surfaces to oceans, river systems play a major role in the global carbon cycle. Indeed, riverine particulate organic carbon (POC) is a melange of various origins characterized by their own 14C labeling. In addition, civil nuclear activities have brought new 14C source that remains poorly documented. We propose to unravel the Δ14C value of POC stored in a sedimentary archive collected downstream the most nuclearized European rivers (the Loire River). We postulate that riverine POC is a mixture of aquatic POC (which could be impacted by the liquid discharge from nuclear industry), terrestrial and petrogenic POC. With a combination of radiocarbon measurements, POC analyses and the palynofacies method, we assessed the respective Δ14C value of the POC origins. The gaps between the Δ14C values of the sedimentary POC and those of the atmosphere were the result of the dilution from dead-C, the freshwater reservoir effect imprinting the Δ14C of aquatic POC and the age and transit time of terrestrial POC within the catchment. Importantly, we consider that the unravelling of radiocarbon composition of riverine POC could be useful to determine either the transit time of material from source to sink, some past industrial or natural events, the resilience of the river system and milestones of the social and economic trajectory of a catchment. For the last three decades, riverine sediments could also act as a source of radiocarbon for the atmosphere.

International audience; 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 mu g.L-1 in the low flow period to 7.5 mu g.L-1 in the high flow period. Upstream, ground- and wetland waters had As-d concentrations up to 215 and 169 mu g.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 mu g.L-1 in the 0-5 cm superficial soil horizon to 14.4 mu g.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.

Spatial analysis (1994–2001) and temporal trends (1980–2000) for particulate-associated metals at key stations in the Seine River Basin have been determined using a new metal pollution index (MPI). The MPI is based on the concentrations of Cd, Cu, Hg, Pb and Zn, normalized to calculated background levels estimated for each particulate matter samples for four fractions (clays and other aluminosilicates, carbonates, organic matter, and quartz). Background levels ascribed to each fraction were determined from a specific set of samples collected from relatively pristine areas in the upper Seine basin and validated on prehistoric samples. The unitless MPI is designed to vary between 0 for pristine samples to 100 for the ones extremely impacted by human activities and to assess the trends of general metal contamination and its mapping. Throughout the Seine basin, MPI currently range from 1 to 40, but values exceeding 100 have been found in periurban streams and the Eure tributary. Based on the MPI spatial distribution, the Seine River Basin displays a wide range of anthropogenic impacts linked to variations in population density, stream order, wastewater discharges and industrial activities. Correlations between the MPI and other trace elements indicate that anthropogenic impacts also strongly affect the concentrations of Ag, Sb, and P, marginally affect the concentrations of Ba, Ni, and Cr, and appear to have little effect on the concentrations of Li, Be, V, Co, and the major elements. Temporal MPI trends can also be reconstituted from past regulatory surveys. In the early 1980s, MPI were 2–5 times higher than nowadays at most locations, particularly downstream of Greater Paris where it reached levels as high as 250 (now 40), a value characteristic of present Paris urban sewage. The exceptional contamination of the Seine basin is gradually improving over the last 20 years but remains very high.