USING DISCRETE FRACTURE NETWORK MODELS TO COMPUTE EQUIVALENT POROUS MEDIA PARAMETERS FOR FRACTURED TILL

Due to the computational challenge of discretely modeling fracture flow across large domains, the effect of fractures on nutrient transport in till at the watershed scale currently unknown. To address this problem, we used a linkage of the software codes HydroGeoSphere and Fracman to develop an equivalent porous medium (EPM) approach that upscales fracture flow with a 3-D hydraulic conductivity (K) tensor. FracMan was used to model a 64-m³ block of discretely-fractured Des Moines Lobe till with a fracture intensity of 24.4 m²/m³, an average fracture spacing of 0.043 m, a total porosity of 28.6%, and a fracture porosity of 0.038%. Ten smaller blocks, ranging in size from 0.0041 m³ (0.16 x 0.16 x 0.16 m) to 8 m³ (2 x 2 x 2 m), were subsampled from the center of the 64 m³ block and converted to a finite-element mesh. HydroGeoSphere was then used to model groundwater flow and transport through each of the subsamples. The representative elementary volume (REV) of fractured till was estimated at the point where the fluid flux and K along each of the component directions (x, y, and z) remained constant with increasing block size. The 3-D K tensor of the REV was computed by linearly regressing K values against results of 78 different model runs, each with a different orientation of the hydraulic gradient. The estimated REV of Des Moines Lobe till was approximately 6 m³ and the determinant of the K tensor of the REV was 4.3x10^-8 m/s. To evaluate solute transport behavior, breakthrough curves of a conservative solute were simulated using the discretely-fractured grids, a single-continuum, EPM model based on the computed K tensor, and a dual-continuum EPM model including K tensors for both the porous media and fracture domains. Preliminary results showed an average velocity in the fractures of 9.7x10^-6 m/s (0.86 m/d), an average linear (porous media) velocity of 1.4x10^-7 m/s (0.012 m/d), a time of first arrival of 15 minutes, and breakthrough (C/Co=0.5) occurring after approximately 18 days. The results are consistent with laboratory column tests from till core of similar size and fracture dimensions.