Zn isotopes in the suspended load of the Seine River, France: Isotopic variations and source determination

We report Zn isotopic ratios (delta Zn-66) of river suspended particulate matter (SPM) and floodplain deposits (FD) from the Seine basin, France, with a precision <= 0.057 parts per thousand. A decrease in delta Zn-66 from 0.30 parts per thousand to 0.08 parts per thousand, is observed in SPM from the upstream to downstream parts of the fluvial system, associated with an increase in Zn concentration from 100 ppm to 400 ppm. The Zn/Al of SPM at the river mouth is up to five times greater than the Zn/Al of the natural background, and by normalizing to the later value we define a Zn enrichment factor. Suspended sediments from a temporal series of samples collected in Paris display a similar variation in delta Zn-66 of between 0.08 parts per thousand and 0.26 parts per thousand, while showing an inverse relationship between the Zn enrichment factor and delta Zn-66. The amount of Zn transported as suspended load varies from 10% to 90%, as a function of increasing discharge. The delta Zn-66 of SPM and the dissolved load are correlated, suggesting that adsorption processes are probably not the dominant process by which the Zn enrichment of SPM takes place. Instead, we interpret our data as reflecting the mixture of two main populations of suspended particles with distinct delta Zn-66. The first is characteristic of natural silicate particles transported by erosion processes to the river, while the second likely represents anthropogenic particles derived from wastewater treatment plants or combined sewer overflows. Based on isotopic ratios, we calculate that 70% of Zn in SPM of the Seine River in Paris is of anthropogenic origin. (c) 2009 Elsevier Ltd. All rights reserved.