Building an isotopic hydrogeochemical indicator of anthropogenic pressure on urban rivers

• δ34S (SO42 −) in the Orge River indicated that waters mainly came from runoff and sewage.
• 206Pb/207Pb in suspended matter showed the end of gasoline lead fluxes during the last decade.
• Lead mainly came from urban releases, and lead bearing phases showed different reactivity.
• The coupling of δ34S and 206Pb/207Pb allowed for concluding about the river contamination state.
• The isotopic indicator provided information on both river geochemical and hydrological status.

This study provides evidence on the sources and pathways of trace-metal contamination using isotopic tracers, δ34S of dissolved sulfate and 206Pb/207Pb of suspended particulate matter (SPM), in an urban French river. The trace-metal contamination of the Orge River watershed was correlated with a highly contrasted land-use pattern with mainly forests and agricultural lands upstream and a highly dense urban area (up to 8000 inhabitants/km2) downstream in the suburbs of the Parisian megacity. The increase in the sulfate concentrations (0.25–1 mmol/l) of the dissolved compartment (< 0.45 μm) monitored during a hydrological year (2010/2011) correlated with the dissolved sulfate δ34S and δ18O ratios (+ 2 to + 12 and + 6 to + 13‰, respectively). This result indicated that the runoff and sewage waters are important sources of water in the river, particularly for the highly urbanized sites. The lead enrichment factor in the SPM was observed to increase according to the urbanization gradient (1 to 6), and the associated 206Pb/207Pb ratio indicated that the signature of the lead-bearing particles transformed from a three-endmember system (2001 samples) to a two-endmember one (2010/2011 samples) over the last decade. These results suggest that gasoline-originated lead has disappeared from particulate fluxes since the lead additive prohibition in 2000. The lead-bearing species in the Orge River were also observed to originate mainly from urban activities, and the reactivity of selected urban lead-bearing species is discussed based on microscopic observations. Finally, coupling the δ34S and 206Pb/207Pb ratios provided a powerful indicator of urban influence as a function of both river hydrodynamics and physico-chemical conditions.