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 unsaturated chalk were exposed to short flow events (24, 8, and 9 hours) of industrial wastewater and/or synthetic rainwater, followed by long drying periods (weeks). The topography of the fracture surface was shown to be unstable due to the detachment of colloidal and large-sized particles during the first 3-7 hours of flow. Following rainwater flow, erosion was more pronounced on 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 rainwater, leading to a deeper erosion of the uncoated surface (1.238 mm) than of the coated one (0.549 mm). Erosion was measured using a laser-scanning system and was calculated from high-resolution topographical maps (elevation z ≤ ± 0.01 mm) generated by Geographic Information System (GIS, ARCInfo) prior to and following the flow experiments. The mean thickness of the erosion was found to be strongly correlated with the thickness of a layer calculated from the total accumulated mass of particles and soluble salts released from the fracture surface. This relationship can be used to evaluate fracture surface erosion in large field and laboratory experiments.