Geological and land use control on δ34S and δ18O of river dissolved sulfate: The Moselle river basin, France

Abstract

The upper part of the Moselle river basin, northeastern France, shows contrasting lithology and topography: granitic formations upstream in the Vosges mountains and carbonate platform downstream on the Lorraine plateau. We measured concentration and isotopic composition of dissolved SO 4 in the Moselle river and its main tributaries in order to identify the sources of sulfate. Rainwater, lichens, S-fertilizers and natural gypsum were also analyzed as potential sulfate sources. Along the Moselle river, dissolved SO 4 concentrations and S–O isotopes display relationships which suggest two major sources of sulfates: 1) a low concentration and low delta values ( δ 34S of 6–9‰ and δ 18O of 6–9‰) component upstream and 2) a high concentration and high delta values ( δ 34S of up to 17‰ and δ 18O of up to 13‰). These are likely 1) atmospheric S input with some retention time in forest soils as carbon-bonded sulfur and 2) sulfates from the dissolution of evaporites from the Muschelkalk and Keuper formations of the Lorraine plateau. The Madon tributary drains a basin largely used for agriculture (> 35% of the land surface) and formed by carbonate and evaporite rocks. Deviation from the general δ 34S– δ 18O trend for dissolved SO 4 was found to be seasonal and related to land use and agricultural practices and are interpreted as a significant input of sulfates from fertilizers used in the area. Isotope mixing models indicate that between 1.1 ⁎ 10 7 and 1.9 ⁎ 10 7 mol of anthropogenic SO 4 are annually exported through the Madon river. These values compared well with the expected annual fertilizer spreading rates in the area and demonstrate the powerful source tracing done by using S isotopes in dissolved sulfates. This implies that combining isotopic “fingerprinting" approach to knowledge of the catchment (i.e. land use, agricultural practices, river flow) would provide key information for decision makers in the context of river basin management.