Geochemical and mineralogical evaluation of mineral-water reactions and leaching potential in a black shale depot - Weathering and transport of Ni, Zn, Cd, Sr and Co from Alum- and Black shale

Abstract

Alum- and black shales are lithologies rich in sulphides and trace elements including uranium, and may pose harm to the environment during weathering. Acidification of surface- and groundwater and swelling may occur. During construction of a road tunnel through Gran, Norway, an alum shale deposit was established. Reducing conditions in the deposit groundwater are supposed to prevent weathering of alum shale, but it is uncertain if release of elements of environmental concern is primarily controlled by redox conditions. This study investigated the distribution of elements in alum shale from Gran and Jevnaker to understand the potential risk of release of these elements. This was done by examinations of the materials utilizing X-Ray Diffraction, Scanning Electron Microscopy, and by performing leaching experiments in both laboratory and natural environments together with inverse modelling in PHREEQC. Zinc, cadmium, cobalt, arsenic, copper and lead were identified in sulphides, while uranium was identified in phosphates. Weathering is suggested to release large amounts of sulphate, zinc, nickel and uranium to percolating water, though acidity is buffered through dissolution of calcite. The PHREEQC inverse model was applied to a depot water sample, and gave a similar output as an average of leaching water samples. This implies that alum shale releases elements of environmental concern under conditions with near-neutral pH, and that depot conditions may not be reducing yet.