Effects of junction transfer characteristics on transport in fracture networks

Abstract

The influence of fracture junction, solute transfer characteristics on transport patterns in discrete fracture networks is analyzed. Regular fracture networks with either constant or variable aperture distributions are considered in conjunction with particle tracking methods. Solute transfer probabilities at fracture junctions are determined from analytical considerations. The second spatial moment and the dilution index are used as measures of the spreading and the degree of channelized transport, respectively; these measures also account for varying mixing ratios at fracture junctions under different flow conditions. For fracture networks with variable aperture distributions, mixing conditions at fracture junctions, determined by the local flow conditions and by the junction geometry, are always dominated by complete mixing and streamline routing "end-member" cases. Moreover, the frequency distribution of the low-velocity regime that arises from variable aperture distributions strongly affects channelized transport and local flow conditions in fracture networks. Simulations suggest that simplified particle tracking models of solute transport in discrete fracture networks will best represent large-scale transport by assuming streamline routing, rather than complete mixing, at fracture junctions. Finally, analysis of both the constant and variable aperture distributions indicates that solute spreading may typically be underestimated in forced hydraulic gradient tracer tests owing to changes in local flow conditions at fracture junctions.