Environ. Sci. Technol. 2008, 42, 6494–6501
Zinc Isotopes in the Seine River Waters, France: A Probe of Anthropogenic Contamination ´ RÔME GAILLARDET, JIUBIN CHEN,* JE AND PASCALE LOUVAT ´ Equipe Geochimie et Cosmochimie, Institut de Physique du Globe de Paris (IPGP), Universite´ Paris-Diderot, UMR CNRS 7154, 4 place Jussieu, 75252 Paris, France.
Received March 14, 2008. Revised manuscript received June 09, 2008. Accepted June 09, 2008.
The use of zinc (Zn) isotope ratios as a tracer of anthropogenic contamination has been assessed using an extensive collection of river water samples from the Seine River basin (France) collected between 2004 and 2007. The 66Zn/64Zn ratios (expressed as δ66Zn) of dissolved Zn have been measured by MC-ICP-MS after chemical separation of Zn using an improved technique adapted to large volumes of water. Significant isotopic variations (0.07-0.58‰) occur along a transect from pristine areas of the Seine basin to the estuary and with time in Paris, and are coherent with the Zn enrichment factor. Dissolved Zn in the Seine River displays conservative behavior, making Zn isotopes a good tracer of the different sources of contamination. Dissolved Zn in the Seine River is essentially of anthropogenic origin (>90%) compared to natural sources (5 × 105) are also much higher than that of the Chalk sample (0.8 × 105), showing that Zn is less mobile than Ca (and Cu) during chemical weathering. In other words, given the dissolved Ca concentration in the Seine and the natural Zn/Ca ratios, much more natural dissolved Zn should be found. Again, this suggests Zn retention in soils or in the karst network. Future work will be necessary to fully understand the behavior of Zn isotopes during chemical weathering and soil formation, but our data clearly show that riverine Zn in rural areas of the Seine basin is essentially derived from the dissolution of carbonate bedrock. Figure 7 shows that, in order to explain the isotopic composition of the Seine River upstream Paris, an endmember enriched in Zn and having a δ66Zn value close to 0.30-0.40‰ is needed. Based on the data for samples from the Paris area, we have predicted above that the domestic wastewater component should have δ66Zn > 0.30‰, once the Zn having lower δ66Zn values derived from Paris roof leaching is subtracted. This conclusion, therefore, suggests that the required end-member is likely a domestic input, most probably released in villages and small or middle-sized cities. The contrasted δ66Zn values of rural domestic wastewater (>0.30‰) compared to the Parisian PTWW (0.03‰) can be explained by the use of different roof cover materials, with preferential use of Zn in the Paris mega-city and terracotta tiles for rural villages and cities. The possibility of explaining this required end-member by a desorbed comVOL. 42, NO. 17, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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ponent from suspended particles exported from the soil (where Zn is essentially adsorbed) to the river (where it is expected to be desorbed) can be ruled out based on the results reported by Chen et al. (23), who demonstrated that the particulate load exported from soils is insufficient to constitute a significant pool of Zn. In Figure 7, the contribution of a Parisian PTWW-like source is needed to explain the dispersion of data points for rural samples (i.e., S34, S55). Since metal Zn from roof covers in Paris controls the isotopic composition of the PTWW end-member, we can speculate that these river waters are contaminated by metallic Zn of the same source (i.e., pipes) or by Zn associated with industrial inputs in which metallic Zn is used. Further systematic work is clearly needed to better characterize Zn isotope compositions of the different contamination sources in the Seine basin, but our results show the great potential of Zn isotopes to trace anthropogenic activities. We therefore explain the Zn isotopic composition of the Seine basin rivers by the mixing of three main sources: natural Zn derived from limestone weathering and two sources of anthropogenic contamination, domestic inputs and metalderived Zn. This latter source is similar to the major source of Zn contamination encountered in the Paris conurbation and dominated by Zn derived from roof covers. Our results show that Zn derived from urban regions is relatively mobile and discharged into the rivers, whereas Zn derived from rural areas is mainly scavenged into soils due to its adsorption properties (30) and the large reactive surface availability in soils. Zn from limestone, mainly derived from karstic aquifers, is then the dominant source in rural regions. Contribution of the Different Sources. Using X/Zn molar ratios (X being Cu, Cl, Cr, Sr, K, etc.) and δ66Zn values, mixing equations can be written to calculate the proportions of Zn derived from the end-members. For a case of three endmembers A, B, and C, we can write that δ66Znriv ) δ66ZnAxA + δ66ZnBxB + δ66ZnCxC
(1)
( ZnX ) ) ( ZnX ) x + ( ZnX ) x + ( ZnX ) x
(2)
and
riv
A
A
B
B
C
C
with xA xB, and xC are the proportion of Zn in one liter of water derived from the end-member A, B, and C, respectively. The following condition must be satisfied: 1 ) x A + xB + xC
(3)
In the Paris conurbation, Zn concentrations and isotope compositions for samples in Paris result from a mixture between the waters upstream from Paris (S35 and S40) and the WWTP wastewaters (Figure 6). Based on Zn isotopic compositions and different element ratios (Cu/Zn, Sr/Zn, SO4/Zn, K/Zn, etc.), we have calculated the contributions of both sources using simplified versions of the above equations. Results demonstrate that the WWTP wastewater contribution varies from 35% (S43) to 95% (S24), with an average value of 70%. As expected, the WWTP input increases when the river water discharge decreases. At higher water stage this contribution is diluted by the input of Zn derived from rural areas. For rivers in rural areas, the calculation is less evident, mainly because the end-members are less well constrained. We approximated the metal-derived Zn end-member using measured values of metallic Zn in the Parisian roof covers. We chose the δ66Zn value of 0.88‰ (Cretaceous Chalk) for the weathering end-member, which implies Cu/Zn of 9 (Cr/ Zn of 7, Cl/Zn of 300 000, etc.). For the domestic end-member, we assume a reasonable δ66Zn value of 0.35 ( 0.05‰ and Cu/Zn of 0.5 (Cr/Zn of 0.2, Cl/Zn of 400, etc.). Using these 6500
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first order values, results indicate that the proportion of natural Zn does not exceed 20%, even in the samples with highest δ66Zn values (headwaters of the Seine and Aube Rivers). The natural contribution is only about 7% for river waters S35 and S40 collected just before Paris mega-city.
Acknowledgments We thank J. Bouchez, J.L. Birck, M. Benedetti, C. Gorge, D. Calmels, B. Chetelat, B. Dupre´, J.Viers and D. Thevenot for constructive discussion and analytical assistance. T. Bullen, E. Tipper, and A. Galy are thanks for thorough reviews and English correction. SIAAP and in particular J. P. Bouvet, J. L. Almayrac, and R. Nedelec are greatly thanked for sample supplies. This work was financially supported by the Region Ile-de-France. This is IPGP contribution number 2392.
Supporting Information Available Table 1 and the appendix. This material is available free of charge via the Internet at http://pubs.acs.org.
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