THE MICROSCALE HYDROGEOLOGICAL PROPERTIES OF THE LINCOLNSHIRE LIMESTONE, UK

As parr of an integrated study of the 'dual porosity' hypothesis, a de tailed micro-scale hydrogeological study was conducted of the rock mat erial from a quarry excavated in the Lincolnshire Limestone. Permeabil ity and porosity determinations allow comparisons to be made with the lithostratigraphy and show that the matrix hydraulic conductivities ar e inconsequential in terms of groundwater flow. The relatively high po rosities typical of these limestones, combined with large surface area s associated with the major bedding plane fissures, provide a mechanis m in which diffusive transport can become a significant Factor in cont aminant migration. The diffusion coefficients (D) of three tracers us ed in field investigations (bromide, fluorescein and amino g acid) wer e 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 m ethod. Alteration of the matrix due to calcite dissolution of the fiss ure faces creates a zone of enhanced permeability and porosity. The pr esence of a fissure fill results in a 'multiporosity' interface betwee n 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.