Quantification of radon migration from a uranium mine through the soil into buildings by the use of tracer techniques.

This paper describes the results of a series of tracer gas tests performed in the mining community of Schlema in eastern Germany. The purpose of these tests was to determine the influence of various mechanisms and subterranean features on the radon levels in the ambient air and in the buildings of the community. Under the former Democratic Republic of Germany (the DDR regime), the mines in and near Schlema and in the ore mountains in Sachsen were an importance source of uranium. These mines have now been closed down and the area is currently under remediation. The remedial measures being applied are varied and consist of, for example: flooding the lower levels of the mine with water, filling the upper tunnels under the community with rock aggregate and blocking their entrances and applying impermeable layers and vegetation over the tilling dump piles near the community. The subterranean complex under the community is extensive due to the many centuries of mining activity. It is currently under depressurization due to the operation of huge mine exhaust fans whose primary function is to provide adequate ventilation air and environmental control for the miners working underground. The mining company WISMUT has undertaken several projects to study the effects of the remediation and the closing down of the mine on the nearby communities. The project described here is one of these projects which was designed to determine the importance of air currents emulating from the mine complex on the radon levels in the community and to determine the influence of the flooding of the mine and the importance of the depressurization caused by the mine exhaust fans on the radon levels in the community. The purpose of this project was to quantify the component of the radon flux into the buildings from the uranium mine caused by the flow of air currents from the mine both when the mine was depressurized by the operation of the mine exhaust fans and when the mine exhaust fans were shut off. Tracer gas measurements were carried out in two phases: with the main exhaust fan of the mine turned off and then with the exhaust fans on. By seeding the tunnels of the mine with a tracer gas, SF,, the transport of air from tunnels in the mine through the soil above and then through the foundation of the buildings into the cellar was determined. Simultaneously the air change rate in the cellar was measured by the use of PDCB (Perfluorodicyclobutane) as a tracer gas to allow a complete mass balance of tracer for the cellar. A simple mass balance using the air flows calculated from the tracer gas measurements and the measured radon concentrations in ambient air and the mine was used to predict the radon level in the building due to the air flows from the mine into the buildings. These predicted radon levels were compared to measured radon levels in the buildings. Fifteen ( 15) buildings in different areas of the community were examined. Some buildings exhibited almost immediate classical exponential tracer build-up response curves which indicated a strong communication with the mine tunnel complex. The calculated radon concentrations in the buildings based on the tracer measurements were in good agreement with the measured radon concentrations in the buildings, i.e. the buildings' radon concentration could be well predicted using the air flows from the mine into the buildings and the radon concentrations in the mine tunnels. Additional preliminary measurements of tracer migration from mine complex into radon dumps and more distant subterranean parts of the complex for which there was no direct flow paths indicated that the tracer technique used would be very well suited for also studying underground movement of contaminants including the determination of contaminant egress from radon dump piles in the area.