Environ. Sci. Technol. 2005, 39, 2486-2493
Boron Isotopes in the Seine River, France: A Probe of Anthropogenic Contamination BENJAMIN CHETELAT* AND J EÄ R O ˆ ME GAILLARDET Laboratoire de Ge´ochimie et Cosmochimie, Institut de Physique du Globe de Paris, Universite´ Paris 7, UMR 7579, Tour 14/24 3e`me e´tage, 4 Place Jussieu, 75252 Paris Ce´dex 05, France
Boron concentrations and isotopic compositions have been measured in the dissolved load of the Seine Basin rivers, France. Hydrology and chemistry of the Seine River and its tributaries are strongly influenced by human activities, as the anthropogenic pressure on the Seine catchment is one of the highest in Europe. The samples were collected between 1994 and 1996 during various stages of flow, complemented by a time-series of the Seine River in Paris for 1 yr. In particular, the decennial flood event of winter 1994 was sampled. Boron appears to be conservative in rivers and not influenced by adsorption onto suspended matter and/or consumption by microorganisms. Despite the complexity of the Seine River system, dissolved boron and its isotopes are found to be suitable tracers of contamination. The total dissolved boron of the Seine River at Paris is explained by the contribution from three distinct components: Urban effluents constitute 65% of the boron discharge measured in the Seine River whereas agriculture-affected waters contribute less than 10% with a more marked influence during high water discharges. Rainwater contribution is important (25% mean), reaching 30% of dissolved boron during high flood events.
Introduction Anthropogenic boron is introduced into the environment from several sources. Glass manufacturing uses boron to increase resistance to thermal shocks and acid. Boron is also used as a bleaching agent in detergents and soaps as well as widespread use in fertilizers, ceramics, and cosmetics. The concentrations and isotopic compositions of boron have been used over the last several decades to trace municipal, agricultural, industrial, or slag contamination; landfill leachate; and irrigation contamination (1-8). Most studies have investigated groundwaters while rivers and rainwater have received less attention. The highly soluble character of boron, in the form of boric acid and conjugate borate ion, favors its release into the environment. Boron has two stable isotopes, 10B and 11B (relative abundance 80.2% and 19.8%, respectively). The low atomic weight of boron induces a large relative mass difference between the isotopes, which leads to significant isotopic fractionation in nature. For steric reasons, threecoordinated boron (such as boric acid in aqueous solution) * Corresponding author. phone: +33-1-44-27-48-16; fax: +331-44-27-37-52; e-mail:
[email protected]. 2486
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is enriched in the heavy isotope (by about 20‰), while fourcoordinated (such as tetrahedral borate) is enriched in the light isotope (9). Any physical or chemical process such crystallization, precipitation, complexation, and sorption of boron that preferentially involves four-coordinated boron or implies a change of coordination induces a large isotopic fractionation between solids and solution (10-12). The measured range of boron isotopic variations in nature is about 100‰ (using the classical δ notation), making boron a very good hydrologic and geochemical tracer. In particular, anthropogenic boron is derived from marine or continental borates deposits where it was deposited and crystallized by evaporation. Anthropogenic sources of boron in France and Europe are mostly derived from borate deposits of California and Turkey. Boron isotopic compositions reported for sodium perborates used in detergent (from -5‰ to 3‰; 5) are overlapped by the range of nonmarine evaporites (from -30‰ to 10‰; 13). Although the isotopic composition of anthropogenic boron can be modified by secondary processes, rain and river waters have usually positive values (mean river δ11B is 10‰; 14, 15), making boron a potentially interesting tracer of anthropogenic inputs to the atmosphere or river systems. To our knowledge, the present study is the first one assessing the utility of boron isotopes to trace the anthropogenic impact on river chemistry at a regional scale. To test the applicability of boron isotopes as a proxy of pollution, we have chosen one of the most environmentally impacted river in Europe, the Seine River in northern France. The Seine River drains a surface area of 79 000 km2 within the sedimentary basin of London-Paris. The lithology is dominated by carbonates, which are boron-poor rocks. The mean annual precipitation is relatively low (700 mm). A strong gradient of land occupation occurs in the Paris Basin. Upstream from Paris, the Seine River Basin contains few relatively small cities (
