Oxidation Phenomena and Processes in Opalinus Clay: Evidence From the Excavation-Disturbed Zones in Hauenstein and Mt. Terri Tunnels and Siblingen Open Clay Pit

Abstract

ABSTRACTConstraints on oxidation phenomena and processes in Opalinus Clay are derived from studies at three localities: the 140-year old Hauenstein tunnel, the six-year old Mt. Terri tunnel, and an open clay pit at Siblingen that documents processes active over thousands of years. The excavationdisturbed zone (EDZ) in both tunnels is characterized by an extensive fracture network that extends to at least 1.6 m. The network consists of saturated fractures as well as some air-filled fractures with apertures up to several mm that contain effervescent gypsum. At Hauenstein, the fracture network displays regularly developed brownish oxidation zones extending 3-15 mm into matrix rock. Such oxidation zones are absent at Mt. Terri. Siblingen displays a remarkable decrease of both fracturing and oxidation phenomena with depth below surface.Microscopic observations, mass balance constraints, and estimated diffusion coefficients of oxygen indicate a minor extent of pyrite oxidation at Hauenstein possibly controlled by diffusionlimited oxygen transport into water saturated rock. Oxidation of siderite is quantitatively dominating over pyrite oxidation at Siblingen. There is evidence at Hauenstein and Siblingen that rock alteration penetrated deeper than the visible alteration zone.During the operational phase (20-100 years) of a deep repository at least the major EDZ fractures are expected to become unsaturated, and gypsum to be precipitated. Fracture apertures reach locally several millimeters within a few years, which enhances hydraulic and gas conductivities. Oxidation rims bordering fractures in the EDZ develop over timescales of 10-100 years, but mass transfers are quantitatively minor.