Stable water isotopes in pore water of Jurassic argillaceous rocks as tracers for solute transport over large spatial and temporal scales

Abstract

[1] In order to characterize the large-scale transport properties of the Opalinus Clay formation, the pore water isotope composition (d 18 O and d 2 H) was determined on samples from the deep borehole Benken (northeastern Switzerland) across Jurassic argillaceous rocks. The sequence of claystones and marls, delimited by two aquifers, is located at depth from about 400 to 700 m and exhibits very low hydraulic conductivities (below 10 À13 m s À1). The isotope data of the pore water were obtained from core samples by diffusive vapor equilibration, vacuum distillation, and squeezing. Compared with the other methods, vacuum distillation led to too low values. To evaluate the large-scale transport properties of the formation, we performed a series of advective-dispersive model calculations and compared them with the experimental data. In accordance with the hydrogeological history, we varied initial and boundary conditions as well as model parameters. The main results can be summarized as follows: (1) Molecular diffusion to the underlying aquifer can explain the general features of the isotope profiles, (2) no signatures of advective flow could be detected, (3) the evolution time is of the order of 0.5-1 Ma (relying on laboratory diffusion coefficients) with a possible range of about 0.2-2 Ma, which is geologically plausible, and (4) parameters measured on small scales (centimeters or meters and months) are also plausible at the formation scale (tens of meters and millions of years) for the sediments investigated.Citation: Gimmi, T., H. N. Waber, A. Gautschi, and A. Rübel (2007), Stable water isotopes in pore water of Jurassic argillaceous rocks as tracers for solute transport over large spatial and temporal scales, Water Resour. Res., 43, W04410,

Determining the stable isotope composition of pore water from saturated and unsaturated zone core: improvements to the direct vapour equilibration laser spectrometry method

“…Clark and Fritz, 1997;Gimmi et al, 2007;Mueller et al, 2014). The method of core sampling and subsequent isotope analysis (δ 2 H and δ 18 O) is best applied when the objective of a study is to characterize vertical and spatial variability of the stable isotopes of water rather than temporal variability.…”

Connecting the Molecular Scale to the Continuum Scale for Diffusion Processes in Smectite-Rich Porous Media

“…In the present paper, we address this abiding question for the particular case of the "apparent" diffusion coefficient D a [defined by the continuum-scale relation N i ≡ D a,i ∂C * /∂x i , if N i is the mass flux density of the species of interest along the x i direction and C * is its total concentration per volume of porous medium] of water tracers and dissolved solutes in smectiterich porous media. These diffusion coefficients are widely used in reconstructing the geochemistry of sedimentary formations (1,2) and in predicting the performance of natural or engineered contaminant barrier systems (3,4), radioactive waste repositories (5,6), and CO 2 repositories (7,8).…”

Safety-relevant hydrogeological properties of the claystone barrier of a Swiss radioactive waste repository: An evaluation using multiple lines of evidence

“…Diffusion has been shown to be the dominant mechanism of solute transport in the Opalinus Clay and in other clayrich formations (Gimmi et al 2007;Mazurek et al 2011). Driven by concentration gradients, the porewater of the Opalinus Clay adjusts slowly to changing conditions in the adjacent aquifers.…”

“…From the spatial distribution of porewater tracers (e. g. δ 18 O, δ 2 H, Cl -, He) in the Opalinus Clay and in the clay-rich confining units in the Benken borehole, in the Mont Terri URL and in the Mont Russelin tunnel in Switzerland, Gimmi et al (2007), Mazurek et al (2009Mazurek et al ( , 2011 and Koroleva et al (2011) estimated the diffusion times needed for the build-up of the present-day tracer profiles using model calculations. In all cases, the shapes of the profiles can be explained by diffusion acting as the dominant transport process over periods of several hundreds of thousands to several millions of years and at the length scales of the profiles.…”