Long-term temporal variability of the radon-222 exhalation flux from a landform covered by low uranium grade waste rock

Radon-222 exhalation flux densities from two different substrates of several metres thickness, waste rock and waste rock mixed with approximately 30% lateritic material, were measured over a period of five years in the wet-dry tropics of Northern Australia. Fourteen measurement campaigns using activated charcoal canisters (n > 1000) covered both dry and wet seasons and showed differences in seasonal and long term trends of the 222Rn exhalation flux densities normalised to the 226Ra activity concentrations of the substrate. Dry season 222Rn exhalation was generally higher for the mixed substrate, due to the larger fraction of fines. Seasonality established within the first year of landform construction on the mixed substrate, due to the higher water holding capacity of the lateritic material. In contrast, waste rock only shows no seasonality until years four and five after construction, when average normalised dry season 222Rn exhalation flux densities from waste rock increase to values (0.47 ± 0.06 mBq m−2 s−1 per Bq kg−1) similar to the mixed substrate (0.64 ± 0.08 mBq m−2 s−1 per Bq kg−1), likely due to an increase in fines from rapid weathering of the schistose waste rock. Volumetric water content has been used to parametrize relative 222Rn exhalation and we determined that wet season 222Rn exhalation is about 40% of the dry season exhalation.