Effects of high variance of fracture transmissivity on transport and sorption at different scales in a discrete model for fractured rocks

A three-dimensional (3-D) variable-aperture fracture network model for flow and transport in fractured crystalline rocks has been applied to study the effects of large variability in fracture transmissivity on non-sorbing and sorbing tracer transport, and scale effects in transport distance. The variable-aperture character of the fractures is introduced into a 3-D network model through a library of single-fracture permeabilities and associated particle transport residence time spectra. Sorption onto the fracture walls is added by a mathematical model for linear sorption. The resulting variable-aperture fracture network model, VAPFRAC, can handle flow and transport from single-fracture scale to the multiple-fracture scale. The model produces multi-peak transport breakthrough curves even for relatively moderate values of the fracture transmissivity variance. These breakthrough curves display dispersion on two different scales in the same way as has been observed in several field experiments conducted in crystalline rocks. The multi-peak structure is due to so-called channeling. For high values of the fracture transmissivity variance the solute transport is unevenly distributed and the channeling effects are more prominent. The effect of linear sorption is not just a simple translation in mean residence time as in a homogeneous medium. The dispersion characteristics of the breakthrough curves also change when linear sorption is included. The degree of the change depends strongly on the fracture transmissivity variance, as does the translation, In particular, with a high fracture transmissivity variance the translation in mean residence time due to sorption is significantly smaller compared to the cases with a low fracture transmissivity variance. Finally, the high variability in the model output data suggests that extrapolation of results from a particular tracer experiment will be highly uncertain.