Impact of intermittent rainwater and wastewater flow on coated and uncoated fractures in chalk

Two coated and two uncoated slices from the fracture surface of an unsatura ted chalk were exposed to short flow events (24, 8, and 9 hours) of industr ial wastewater and/or synthetic rainwater, followed by long drying periods (weeks). The topography of the fracture surface was shown to be unstable du e to the detachment of colloidal and large-sized particles during the first 3-7 hours of flow. Following rainwater flow, erosion was more pronounced o n the coated than on the uncoated surface (mean erosion of 0.313 and 0.134 mm, respectively). Interaction with industrial wastewater generated a skin of organic matter and gypsum that collapsed following contact with rainwate r, leading to a deeper erosion of the uncoated surface (1.238 mm) than of t he coated one (0.549 mm). Erosion was measured using a laser-scanning syste m and was calculated from high-resolution topographical maps (elevation z l ess than or equal to +/- 0.01 mm) generated by Geographic Information Syste m (GIS, ARCInfo) prior to and following the flow experiments. The mean thic kness of the erosion was found to be strongly correlated with the thickness of a layer calculated from the total accumulated mass of particles and sol uble salts released from the fracture surface. This relationship can be use d to evaluate fracture surface erosion in large field and laboratory experi ments.