Impact of coating and weathering on the properties of chalk fracture surfaces

The surface topography of a natural fracture intersecting a chalk formation was mapped using a high-resolution laser-scanning device, and its roughness was evaluated using five different criteria. Two coated and two uncoated slices of the natural fracture surface were encased in flow cells and exposed to short (24 and 8 hours) interactions with percolating synthetic rainwater and industrial wastewater, followed by long drying periods (weeks). These flow experiments simulated the intermittent infiltration of surface runoff and industrial effluents typical of the study area and resulted in erosion of the fracture surfaces and modification of their roughness. The temporal variations in surface roughness were evaluated using three commonly used criteria (standard deviation from mean heights, differences between maximum and minimum heights, and the root-mean-square roughness) and two newly developed measures, namely, the slopes on the surfaces and a calculated roughness-related ratio (RRR). The mean slope maps and RRR calculations were found to be good criteria for evaluating surface topography and its uniformity across the fracture surface. The coated surfaces were significantly rougher, had greater relief, more heterogeneous topography, and a larger surface area than the uncoated surfaces. The results suggest that coating and fast weathering (caused by intermittent flow events) significantly change roughness, surface area, relief, and the heterogeneity of a fracture surface. These, in turn, may affect the spatial distribution of flow paths across that fracture surface. Consequently, when exploring the properties of fracture surfaces and fracture flow, the impact of surface skin and variations of the surface properties over short timescales should be taken into account.