Atmospheric control of radon emissions from a waste rock dump

At the former Schlema-Alberoda mining site, located in the south-east of Germany, waste rock dump 38neu was built along a natural slope on the edge of a valley. The dump is 750 m wide, with a maximum thickness of 30 m and a total height of 100 m. Dump remediation involved surface re-sloping and placement of a 1 m thick low-permeability soil cover. A characterisation aimed to verify the soil cover efficiency: its air permeability, radon concentrations under the cover and radon fluxes, as well as differential air pressures across the soil cover. In summer, relatively high radon concentrations and fluxes were found to occur across the cover in the lower slope of the dump. Differential pressures indicate that gas flow is upwards in the dump below the 9.5 degrees C mean atmospheric temperature and downwards otherwise. A numerical model was developed to explain the relation between dump airflow and atmospheric conditions. The results indicate that airflow is controlled by dump gas buoyancy relative to atmospheric air. Preferential gas entry and exit occur across the dump's lower slope, where differential pressures are highest, explaining the higher radon fluxes observed near the base of the dump under high atmospheric temperatures.