Abstract

As part of an integrated study of the ‘dual porosity’ hypothesis, a detailed micro-scale hydrogeological study was conducted of the rock material from a quarry excavated in the Lincolnshire Limestone. Permeability and porosity determinations allow comparisons to be made with the lithostratigraphy and show that the matrix hydraulic conductivities are inconsequential in terms of groundwater flow. The relatively high porosities typical of these limestones, combined with large surface areas associated with the major bedding plane fissures, provide a mechanism in which diffusive transport can become a significant factor in contaminant migration. The diffusion coefficients (D*) of three tracers used in field investigations (bromide,fluorescein and amino g acid) were measured in each of the main rock materials from the site using both double reservoir and reservoir depletion techniques. In addition, an assessment was made of D* estimation using an electrical resistivity method. Alteration of the matrix due to calcite dissolution of the fissure faces creates a zone of enhanced permeability and porosity. The presence of a fissure fill results in a ‘multiporosity’ interface between the fissure and the matrix that may significantly increase the mass of contaminant entering the pore fluid by diffusion in comparison with a simplistic conceptualization of this zone.

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