COMPARISON OF FAST FLOW PATH ROUGHNESS TO SLOW FLOW PATH AND OVERALL SURFACE ROUGHNESSES MODELED IN A DISCRETE FRACTURE OF WELDED TUFF

Two major challenges in predicting the migration of solutes in fractured media are: 1) describing the physical characteristics of a representative surface that can be scaled to aid modeling efforts and 2) the effects of channeling. This study considers surface roughness in both these areas. To address these issues, we quantify fracture surfaces of various granites and tuffs, which are imaged using computed tomography and processed to yield regularly gridded elevations of each surface. It has been shown that some fracture surfaces can be described as self-affine, and that a corresponding fractal dimension can be ascribed to create representative models. Surface roughness also can be described in terms of a joint roughness coefficient, which incorporates wall compressive strength and peak shear strength in its calculation. Describing roughness as a ratio of surface area to footprint, equivalently sized square samples are plotted against sample size to determine if a representative surface exists. For specimens of fractures measuring up to 25cm x 10cm, a 3.2cm x 3.2cm sample yields a reasonable expression of the roughness of the entire surface. To evaluate channeling in rough fractures, a discrete fracture in welded Santana Tuff from Trans-Pecos Texas is imaged to achieve 0.28mm x 0.25mm grids of the relative elevations of top and bottom surfaces. The result is used to populate a representative 2D MODFLOW model based on cubic law derived transmissivities. Flow paths constructed from MODPATH particle track data show channeling on the scale of 10s of cm. The roughnesses of dominant flow channels are compared to those defining relatively slow particle tracks and the entire surface roughness, which found the dominant channels to be less rough than the surface as a whole. The channeling effect seen on this scale further questions the utility of predictive modeling of solute transport in fractured media based on upscaling.