Transport of solutes in fractured media, based on a channel network model : B. Gylling, L. Moreno & I. Neretnieks, in: Groundwater quality: remediation and protection. proc. conference, Prague, 1995, ed K. Kovar & J. Krasny, (IAHS; Publication, 225), 1995, pp 107–113

Abstract

Abstract Fluid flow and solute transport in fractured media take place mainly through preferential paths in the fractures. Solutes are transported through these paths or channels and they may diffuse into the stagnant water in the porous matrix. This is a very important interaction mechanism for long times. They also may be sorbed within the rock matrix. A new type of model concept for describing flow and transport in fractured rock has been devised which specifically emphasizes the matrix interaction. The flowing water in the rock is envisaged to take place in a three-dimensional network of channels with stochastic properties. The important transport parameters are summarized and how they may be obtained in the field is described. A long term pumping test with tracers at the Swedish Âspo rock laboratory has been predicted using this model. The main tracer test was predicted reasonably well without using any adjustable parameters. INTRODUCTION AND BACKGROUND Fluid flow and solute transport in fractured media occur mainly in the fractures. Due to the variable aperture and heterogeneities on the fracture surfaces the fluid flow will seek out preferential paths. Solutes are transported through these paths. The solutes may migrate from the water in the fracture into the stagnant water in the porous matrix. They also may be sorbed on the micro surfaces within the rock matrix. These two mechanisms strongly influence the residence time distribution, RTD, in fractured media. We call the flow in the preferred paths channelling. The channels need not be physical entities, the water may seek out other paths if the gradient is changed. For a given gradient one can, however, visualize the flow paths as more or less distinct channels. This is often seen in drifts and tunnels in fractured rocks. Intuitively we can conceive that in a narrow channel with large flow there is little surface area (Flow Wetted Surface, FWS) over which the solute can interact with the porous matrix. In a channel with a large FWS and low flow, the solute may have time to diffuse into the porous matrix and thus access more water volume where it can reside. The RTD can be considerably influenced by this process (Neretnieks, 1980). A new type of model concept for describing flow and transport in fractured rock has been devised. The flowing water in the rock is envisaged to take place in a three-dimensional network of channels with stochastic properties. The channels are characterized by lengths, widths, apertures and transmissivities. The rock matrix is assumed to be porous and the solute can diffuse in and out of the matrix. For long term