Some constraints on geochemistry and environmental isotopes for the study of low fracture flows in crystalline rocks: The Stripa case

The paper aims at discussing some aspects of the suitability of geochemistry and environmental isotopes for the study of groundwater circulations in crystalline rocks, with special reference to the Stripa case. The main problems are the origin of groundwater salinity and the origin and 'age' of the water. Use of geochemical data appears severely limited for cation contents, which are mainly controlled by water-rock interactions. Among anions F-, CO3-/CO32 are controlled by secondary precipitation of fluorite and carbonate. Sulphate is affected by redox reactions and precipitation. Only bromide and chloride may be considered as conservative tracers of origin. At Stripa, several origins may be invoked for the salinity of deep groundwaters (700 ppm of chloride): (1) marine intrusion (Present or Holocene); (2) rock derived salinity (leaching of fluid inclusions, alteration of salt bearing minerals and diffusion of saline fluid from the microporosity); and (3) intrusion of sedimentary brine. Noble gases in the deep groundwaters of Stripa indicate a somewhat cooler recharge than annual average temperature. Except for 18O, which has had long periods of contact with the rock, a distinction is made between constitutive isotopes of the water molecule which are fairly conservative, and those isotopes in the aqueous phase which are partially or totally controlled by the rock matrix or by fracture minerals. The tritium contents are significantly higher than the equilibrium value for deep production generated by the neutron flux of the rock. Some surface water reaches the deep system. Among the entirely rock controlled isotopes mention should be made of strontium isotopes and the U and Th decay series, including 222Rn and 4He. These isotopes allow study of the diagenetic reactions and of the relationship between the rock and the fluids (porosity). Among the partially rock controlled isotopes are 40Ar, 36Cl, 14C, 34S and 18O in aqueous sulphate. The presence at Stripa of large excesses of argon, even in recent (3H bearing) water, is compatible with a mixing with deep old brine. 78 refs, 11 figs