Solute transport in formations of very low permeability: profiles of stable isotope and dissolved noble gas contents of pore water in the Opalinus Clay, Mont Terri, Switzerland

Abstract

Pore water profiles of water, stable isotope, and dissolved noble gas content have been determined across the Opalinus Clay and adjacent formations at the rock laboratory at Mont Terri. We have found enhanced helium contents (up to [4He] = 1 × 10−4 cubic centimeters at standard pressure and temperature per gram of pore water) and argon isotope ratios (40Ar/36Ar ratios up to 334) due to accumulation of 4He and 40Ar produced in situ. The helium profile was found to be in steady state with respect to in situ production and diffusive loss into the adjacent limestones where groundwater circulates. From this profile a representative mean value of the apparent diffusion coefficient for helium in the pore water of the whole formation was derived for the first time to be Da = 3.5 × 10−11 m2 · s−1, which is more than two orders of magnitude lower than the diffusion coefficient D0 in free water. The stable isotope profile, however, indicates a component of fossil marine pore water, which has not yet been replaced by molecular diffusion of meteoric water from the adjacent limestone and shale formations over the past 10 million years.