Aquatic Geochemistry

, Volume 5, Issue 2, pp 125–165

Geochemical Study of a Granitic Area – The Margeride Mountains, France: Chemical Element Behavior and 87Sr/86Sr Constraints

  • Philippe Négrel
Article

DOI: 10.1023/A:1009625412015

Cite this article as:
Négrel, P. Aquatic Geochemistry (1999) 5: 125. doi:10.1023/A:1009625412015

Abstract

A small watershed (89 km2) underlain by granite or granite-gneiss in the Margeride mountains, southern Massif Central (France), has been studied using the chemical and isotopic composition of its dissolved load, bed sediments and soils. Dissolved concentrations of major ions (Cl, SO4, NO3, HCO3, Ca, Na, Mg, K, Al and Si), trace elements (Rb and Sr) and strontium isotopes have been determined for three different hydrologic periods in the main stream of the Desges river and its tributaries.

The aim was to characterize the chemical and isotopic signatures of each reservoir in the watershed; signature changes are interpreted as fluctuations in the different influencing components: rainwater, weathering products and anthropogenic addition. In the study area, as in other watersheds in granite environments, the only source for input of chemical species into the dissolved load at high altitude is chemical weathering and atmospheric deposition, whereas at lower altitude, human influence plays a non-negligible role.

As precipitation is a major vehicle for the addition of dissolved chemical species into the hydrosystem, a systematic rainwater study using an automatic collector was carried out over one year in order to better constrain rain elemental input. Corrections for rainwater addition, using chloride as an atmospheric-input reference, were computed for selected elements and for 87Sr/86Sr ratios. After these correction, the geochemical budget of the watershed was determined and the role of anthropogenic addition was evaluated based on strontium isotope relationships.

For particulate matter, we used the normalization of chemical species relative to parent rocks and the element ratios which reflect the depletion or enrichment in soils and sediments. Both the immobile- and mobile-element approaches have been tested, using Ti/Zr and La/Ce ratios for the former and Ca/Sr, K/Rb, and K/Fe ratios for the latter.

France Massif Central crystalline environments strontium isotopes dissolved load bed sediments soils 

Copyright information

© Kluwer Academic Publishers 1999

Authors and Affiliations

  • Philippe Négrel
    • 1
  1. 1.BRGM Research DivisionOrléans Cedex 2France

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