Experimental and numerical investigations on the effect of fracture geometry and fracture aperture distribution on flow and solute transport in natural fractures.

The impact of fracture geometry and aperture distribution on fluid movement and on non-reactive solute transport was investigated experimentally and numerically in single fractures. For this purpose a hydrothermally altered and an unaltered granite drill core with axial fractures were investigated. Using three injection and three extraction locations at top and bottom of the fractured cores, different dipole flow fields were examined. The conservative tracer (Amino-G) breakthrough curves were measured using fluorescence spectroscopy. Based on 3-D digital data obtained by micro-computed tomography 2.5-D numerical models were generated for both fractures by mapping the measured aperture distributions to the 2-D fracture geometries (x-y plane). Fluid flow and tracer transport were simulated using COMSOL Multiphysics®. By means of numerical simulations and tomographic imaging experimentally observed breakthrough curves can be understood and qualitatively reproduced. The experiments and simulations suggest that fluid flow in the altered fracture is governed by the 2-D fracture geometry in the x-y plane, while fluid flow in the unaltered fracture seems to be controlled by the aperture distribution. Moreover, we demonstrate that in our case simplified parallel-plate models fail to describe the experimental findings and that pronounced tailings can be attributed to complex internal heterogeneities. The results presented, implicate the necessity to incorporate complex domain geometries governing fluid flow and mass transport into transport modeling.

Natural micro-scale heterogeneity induced solute and nanoparticle retardation in fractured crystalline rock.

We studied tracer (Tritiated Water (HTO); Tritium replaces one of the stable hydrogen atoms in the H(2)O molecule) and nanoparticle (quantum dots (QD)) transport by means of column migration experiments and comparison to 3D CFD modeling. Concerning the modeling approach, a natural single fracture was scanned using micro computed tomography (µCT) serving as direct input for the model generation. The 3D simulation does not incorporate any chemical processes besides the molecular diffusion coefficient solely reflecting the impact of fracture heterogeneity on mass (solute and nanoparticles) transport. Complex fluid velocity distributions (flow channeling and flowpath heterogeneity) evolve as direct function of fracture geometry. Both experimental and simulated solute and colloidal breakthrough curves show heavy tailing (non-Fickian transport behavior), respectively. Regarding the type of quantum dots and geochemical conditions prevailing (Grimsel ground water chemistry, QD and diorite surface charge, respectively and porosity of the Äspö diorite drill core) experimental breakthrough of the quantum dots always arrives faster than the solute tracer in line with the modeling results. Besides retardation processes like sorption, filtration, straining or matrix diffusion, the results show that natural 3D fracture heterogeneity represents an important additional retardation mechanism for solutes and colloidal phases. This is clearly verified by the numerical simulations, where the 3D real natural fracture geometry and the resulting complex flow velocity distribution is the only possible process causing solute/nanoparticle retardation. Differences between the experimental results and the simulations are discussed with respect to uncertainties in the µCT measurements and experimental and simulation boundary conditions, respectively.