Characterization of transport properties in granitic rock fractures with skins
Hydraulic properties of fracture skins in granitic rocks from three climatically different field sites show that fracture skins can increase permeability, compared to unaltered granite, through microfractures or weathering rinds, or decrease it through the effects of mineral nucleation or precipitation and growth of biological organisms. In comparable granitic crystalline rocks, fracture skins formed in more arid climates have a higher porosity than fracture skins formed in more humid climates. Granite fractures have been collected from three field sites Elberton, Georgia, Fredericksburg, Texas, and Eyre Peninsula, South Australia. Fracture skins in Elberton are predominantly surface coatings of dust films, clay infillings, organic growths, and infillings of iron precipitates. Town Mountain Granite skins are dominated by iron banding, weathering rinds, and surface coatings of pyrolusite. The Calca Granites from the Eyre Peninsula have similar fracture skins to the Town Mountain Granite, but skin thicknesses are greater. Modeling studies with fracture skins demonstrate that the most important hydraulic properties are skin porosity, diffusion coefficients, and retardation. Through combined laboratory analyses and field investigations, the variations in transport properties of rock matrix and fracture alteration zones are documented. Transport properties of fracture skins in granitic rocks with surface coatings enhance fracture transport and alteration zones of more porous skins attenuate transport. A new laboratory method is employed to measure diffusion coefficients using laser ablation ICP-MS. Diffusive transport is shown to be preferential to grain boundaries and zones of mineral cleavage planes. Tracer tests show channeling to be a significant factor in breakthrough results on the field scale. Channels on the field scale are imaged using high frequency GPR providing an image of preferential flow paths in a fracture.