HR: 0800h
AN: H51A-0345    [Abstracts]
TI: The influence of ionic forces on the effective diffusion coefficient in fractured, porous chalk.
AU: * Kremer, K
EM: kkremer@uni-bonn.de
AF: Geological Institute of the University of Bonn, Nussallee 8 , Bonn, 53115 Germany
AU: Reichert, B
EM: b.reichert@uni-bonn.de
AF: Geological Institute of the University of Bonn, Nussallee 8 , Bonn, 53115 Germany
AB: Solute transport in fractured, highly porous chalk significantly depends on the diffusive mass transfer of substances between the mobile water in the fracture and the immobile water of the rock matrix. Matrix diffusion is an important transport mechanism and a central factor for the retardation of solutes. Until now, simple estimation methods for the diffusive behavior of substances such as Archie's law can only be applied to single substances. Multi-tracer experiments proved a mutual influence on the diffusion of ionic solutes thus leading to significant deviations in respect to the theoretically estimated effective diffusion coefficient D_e. An increase of ionic forces in the aqueous phase is often accompanied by a decrease of D_e for cations and an increase for anions. However, groundwater contamination usually consists of several pollutants in different mixtures. Besides ionic forces, effects of channeling and transport of colloids can result in incorrectly estimated D_e values and, hence, high inaccuracy in the modeling of contaminant transport in fractured porous media. In the context of a current DFG-project, the impact of ionic forces on D_e as well as the interaction of the diffusion of ionic ground water solutes in fractured chalk of Denmark (Cretaceous, Sigerslev) and Israel (Eocene, Negev desert) will be quantified to develop a procedure for an improved estimation of D_e in dependence of the ionic activity. Consequently, the well established Archie's law for the prediction of diffusivities on the basis of the total porosities will be modified by an extension term a. So far series of single-tracer through-diffusion experiments have been performed with potassium bromide in six different concentrations to quantify the concentration dependence on the matrix diffusion as well as to examine the influence of the ionic strength on the effective diffusion coefficients of ionic solutes. The simultaneously injected neutral deuterium serves as a reference tracer. The calculated values of D_e for bromide range from 0.91 to 2.21E-06 for Israeli chalk and from 4.41 to 5.51E-06 for Danish chalk. Until now a slight increase of D_e could be observed with higher ionic strength. By illustrating the values of the resulting diffusivities versus the total porosities according to Archie's law with the chalk specific exponent m of 2.36 the values at higher concentration are situated outside the validity of this law. In contrast the values of deuterium confirm its validity. In order to approve and quantify the observed increase of the relative diffusivities of anionic substances with increasing ionic strength additional single-tracer experiments with sodium acetate are presently carried out.
DE: 1831 Groundwater quality
DE: 1832 Groundwater transport
DE: 1847 Modeling
SC: Hydrology [H]
MN: Fall Meeting 2005