Geochemical and boron, oxygen and hydrogen isotopic constraints on the origin of salinity in groundwaters from the crystalline basement of the Alpine Foreland

The B isotopic composition, in combination with O and H isotopes and hydrochemical tracers, is utilized to constrain the evolution of basement-hosted groundwaters via water-rock interactions and fluid infiltration from external (sedimentary) reservoirs. Two distinct groundwater types have been identified in the Central European crystalline basement (N Switzerland-SW Germany): (1) fresh groundwaters characterized by low;values of delta(11)B (-3.5 to -0.6 parts per thousand), delta(18)O (-12.0 to -10.0 parts per thousand), and delta D (-86.8 to -71.9 parts per thousand), and (2) brackish groundwaters with distinctly heavier B, O, and H isotopic compositions (delta(11)B = + 6.4 to + 17.6 parts per thousand, delta(18)O = -9.4 to -5.6 parts per thousand, delta D = -67.6 to -60.8 parts per thousand). Fresh groundwaters show a systematic decrease in delta(11)B, related to an increase in B concentrations (and degree of total mineralization), along the pathway of groundwater migration which can only be interpreted in terms of leaching of crystalline host rocks. A delta(11)B value of -3.3 parts per thousand is inferred for the crustal B source (mainly Hercynian granites) involved in the leaching process, in agreement with the known delta(11)B range of granitic rocks. The evolution of brackish groundwaters, derived from crystalline basement reservoirs with little water circulation, is more complex. As indicated by B-O-H stable isotope and hydrochemical (e.g. B/Cl, Na/Cl, and Br/Cl) constraints, brackish groundwaters from the study area are influenced by admixture of sediment-derived fluids which infiltrated from Late Paleozoic (Permo-Carboniferous) and Early Mesozoic (Lower Triassic) sedimentary strata. The data presented show that B isotopes are sensitive to mixing processes of fluids derived from different crustal reservoirs and, hence, may be utilized as a tracer for constraining the internal (autochthonous) vs external (allochthonous) origin of salinity in basement-hosted groundwaters. (C) 2000 Elsevier Science Ltd. All rights reserved.