Direct simulation of heterogeneous diffusion and inversion procedure applied to an out-diffusion experiment. Test case of Palmottu granite

An out-diffusion laboratory experiment using a non-reactive tracer was fitted using the Time Domain Diffusion (TDD) method. This rapid particle tracking method allows simulation of the heterogeneous diffusion based on pore-scale images and local values of diffusivities. The superimposed porosity and mineral 2D maps act as computation grids to condition diffusion pathways. We focused on a Palmottu granite sample, in which the connected pore space has a composite microstructure with cracks linking microporous minerals and is above the percolation threshold. Three main results were achieved: (i) When compared to the fitting obtained with one coefficient (best mean square residual R = 1.6 x 10(-2)), diffusion is shown to be suitably characterised with two coefficients related to cracks and microporous minerals (best R = 6.5 x 10(-4)), (ii) rather than imposing a local apparent diffusion coefficient D(a) independent of the local porosity Phi, a best fit is obtained by applying Archie's relationship D(a) = D(0) x G with G = Phi(m) to each pixel of the calculation grids (G is the geometry factor, D(0) is the diffusion coefficient in free fluid, and m is Archie's exponent), and (iii) the order of magnitude of the fitted diffusion coefficient or Archie's exponents (m=0 for microcracks and m=1.82 for microporous minerals) is physically realistic.     

Interpretation of out-diffusion experiments on crystalline rocks using random walk modeling

Matrix diffusion in saturated rocks with very low permeability is one of the major mechanisms of solute transport. Laboratory out-diffusion experiments on rock samples may provide an estimate of the bulk diffusion coefficient. However, numerous results have shown that this average parameter does not really depict the complex mechanism of diffusion as a function of the internal heterogeneity of crystalline rocks. Two-dimensional images of the porosity distribution in a granite sample were obtained by impregnation with a radioactive resin and autoradiography. Some examples based on these images and synthetic images were used to perform numerical simulations of out-diffusion using two different random walk methods. The simulated shapes of the out-diffusion curves depend on the spatial distribution of the porosity and on the pore connectivity with the border of the sample. Such relations might explain the multiple nested slopes or the convex shapes often observed on real experimental curves.     

Study of porosity and migration pathways in crystalline rock by impregnation with C-polymethylmethacrylate

A new method is described, which gives quantitative information on levels and spatial distributions of porosity in the matrix of rock in the sub-mm- to cm-scale. Migration pathways can be identified and visualized. The method employs impregnation of dried rock samples by ¹⁴C-labelled methylmethacrylate (MMA), radiation polymerization, autoradiography and digital image processing. Infiltration and diffusion processes can be quenched by polymerization and the resulting profiles studied easily.     

Verbesserung des Nachweises geringster Aminosäuremengen in Papierchromatogrammen

The Science of Nature -     

Path Dependencies and Path Change in Complex Fields of Action: Climate Adaptation Policies in Germany in the Realm of Flood Risk Management

The spatial and temporal repercussions of climate change are of an extremely complex nature. Coping with climate change is, first and foremost, a challenge to political decision making and, considering the long-term effects of the climate system, to planning. However, there have never been more doubts that the political-administrative system is able to meet these requirements. Although much evidence has been put forward in favor of such skepticism, sometimes, it is dangerous to overstate the existing limits. Drawing on two case studies in the area of flood risk management in Germany, the article illustrates how and why significant path change came about. In both cases, the state proved to still being a pivotal actor, due to a number of functions that cannot be assumed by other actors. However, other actor groups—such as actors from science, the media, NGOs, and citizen groups—play a significant role as well by providing relevant expertise and influencing the public discourse, thus mobilizing significant political pressure.