Solute Transport in Fractured Rock. Testing a New and Simple Aapproach
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6ROXWH7UDQVSRUWLQ)UDFWXUHG5RFN7HVWLQJD1HZDQG6LPSOH$DSSURDFK Luis Moreno, James Crawford, and Ivars Neretnieks Dept Chemical Engineering and Technology Royal Institute of Technology SE-100 44 Stockholm, SWEDEN Email: [email protected], [email protected], [email protected] $%675$&7 A new and simple approach for modelling solute transport in fractured rock is presented, based on our previously used Channel Network Model (CNM). In our underlying concept, solute transport in fractured rocks takes place through undulating flat channels in the rock fractures. The channels form a 3-dimensional network. For non-interacting solutes the volume of the channels determines the residence time of a stream of water. For sorbing radionuclides the residence time is mainly determined by the ratio between the Flow Wetted Surface (FWS) and the flow rate in the channel (Q), in addition to the parameters that determine the interaction between the rock and the radionuclides. The main question explored in this paper is whether it is necessary to solve the flow problem for each studied case. From previous investigations, we have found that the flow rates in the channels where the solutes travel are closely related to the transmissivities of the channels. Simulations are presented in which streams of water are allowed to randomly select channels and trace out the paths where the path volume and its FWS can be assessed. The results obtained in this manner for the RTD (Residence Time Distribution) show rather good agreement with those obtained by solving the full flow problem and tracing particles in the network. ,1752'8&7,21$1'%$&.*5281' In our concept, flow and solute transport in fractured rock takes place through channels in the fractures. On one hand, the channel volume and flow in the channel determines the residence time of the water and thus of the dissolved radionuclides that do not interact with the matrix. On the other hand, for sorbing radionuclides the residence time is mainly determined by the relationship between the Flow Wetted Surface (FWS) and the flow rate in the channel (FWS/Q) in addition to the parameter that determines the interaction between the rock and the radionuclide (Material Property Group, MPG). The main interaction mechanisms are diffusion in the porous matrix and sorption on the internal surfaces. For slightly sorbing radionuclides, both the water residence time and the retardation effects owing to matrix interaction may contribute to determining the residence time The problem of assessing the Residence Time Distribution (RTD) of a solute can be divided into two distinct steps. First, the flow rate distribution of the water from the release point to all the exit points must be determined. This subsequently makes it possible to follow a particle along the flow path. Provided the flow model contains information on the flow path cross sectional area for the water along the trajectory, $67 [ the water residence time WZ for the stream of water with flow rate T from [=0 to [ / is simply obtained by evalua
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