Lithium isotopes as tracers of groundwater circulation in a peat land

Philippe Négrel (1) , Romain Millot (1) , Agnès Brenot (1) , Clotilde Bertin (1)

1 BRGM - Bureau de Recherches Géologiques et Minières (BRGM)

Water circulation in the peat bog of a maar depression in the Massif Central (France) was traced with lithium isotopes on water samples collected in the area from springs, surface- and groundwaters, as well as on solid samples taken from peat bogs. Lithium contents fluctuate significantly in the groundwaters, ranging from 0.01 μmol L−1 in springs feeding the peat bog, up to 28 μmol L−1 in the groundwater collected in the peat bog. Lithium-isotope compositions (δ7Li, ‰) are extremely variable within the site, ranging from+12‰in the stream draining the area up to a 7Li-rich value of+ 1226‰in groundwater fromthe peat bog. The δ7Li values in the streams agreewith those reported in the literature for surface waters, while those of groundwater are far beyond the range of values measured in nature. In the present study,we explain the extremely enriched 7Li signature of the groundwaters by an external input due to Ca-amendment, used in local agriculture. The relationships between Li content and Ca,Mg andHCO3 as well as the lack of a relationship between Li andNa plead in favour of 7Li-enriched carbonate amendments. This hypothesiswas tested by Li-isotope analyses on the peat and on several amendment samples (carbonates and NPK fertilizers), confirming the potential role of amendments in the control of Li isotopes in peat land groundwater and showing high δ7Li values in fertilizers. Application of δ7Li ratios to peat land waters provides a unique perspective on the hydrogeochemical dynamics at the scale of this site, as the δ7Li values for the surface water were quasi constant throughout the survey period, and the peat land groundwater does not supply the surface-water runoff and may evolve as a stagnant system. To conclude, the water within the peat land exhibits very high δ7Li values consistent with artificially enriched 7Li associated with Ca amendments. This study opens a new field for Li isotope investigations in hydro-systems and highlights the possibility of using Li isotopes as environmental tracers.

Mots clés: Peat land; Groundwater; Li isotopes; Massif Central

Domaines: Sciences de la Terre

Philippe Négrel, Romain Millot, Agnès Brenot, Clotilde Bertin. Lithium isotopes as tracers of groundwater circulation in a peat land. Chemical Geology, 2010, 276 (1-2), pp.119-127. ⟨10.1016/j.chemgeo.2010.06.008⟩. ⟨hal-00552053⟩