Interaction between Different Groundwaters in Brittany Catchments (France): Characterizing Multiple Sources through Strontium- and Sulphur Isotope Tracing

International audienceStrontium- and sulphur-isotope data as well as major ions, from shallow and deep groundwater in three granite and Brioverian `schist' areas of the Armorican Massif (northwest France) are discussed. The investigated areas, e.g. the Pleine-Fougères, Coët Dan and Arguenon catchments, are located in Brittany with intensive agriculture covering large parts.The stable-isotope signatures of the waters plot close to the general meteoric-water line, reflecting a meteoric origin and the lack of significant evaporation or water-rock interaction. The water chemistry from the different catchments shows large variation in the major-element contents. Plotting Na vs. Cl contents and Mg, NO3, K, SO4, Sr concentrations reflect agricultural input from hog and livestock farming and fertilizer applications, with local sewage-effluent influence, although some water samples are clearly unpolluted.The δ34S(SO4) is controlled by several potential sources (atmospheric sulphate, pyrite-derived sulphates, fertilizer sulphates). Some δ18O and δ34S values are expected to increase through sulphate reduction, with higher effect on δ34S for the dissimilatory processes and on δ18O for assimilatory processes. The range in Sr contents of groundwater samples from different catchments agrees with previous work on groundwaters sampled from granitic rocks in France. The Sr content is well correlated with Mg and therefore both are related to agricultural practises. The 87Sr/86Sr ratios range from 0.71265 and 0.72009 also agreeing with Sr isotopic ratios found on granite-gneiss watersheds in France. The relationship between the 87Sr/86Sr and the Mg/Sr ratios allows defining the different end-members (e.g. rain, agricultural practises, water-rock interaction)