• 240 laboratory experiments used to estimate Rn-222 secular equilibrium. • Lack of consistent Rn-222 secular equilibrium estimates from 4 common methods. • Comparison with 24 groundwater samples indicates reliability of one method. • Finer-grained sediments released greater amounts of Rn-222. Radon (Rn-222) is widely used as a tracer for identifying surface water-ground water interactions. To use Rn-222 as a tracer, the Rn-222 secular equilibrium concentration in groundwater is required. This is achieved by taking multiple field samples at distances from the river, which is not always achievable in practice. This study compares four commonly used laboratory techniques (diffusion experiments) for determining Rn-222 secular equilibrium to assess whether they yield consistent results. Laboratory-derived equilibrium values were compared with measured groundwater Rn-222 concentrations. The influence of sediment characteristics, including grain size, sorting, silt and clay content, and porosity on radon emanation rates was further examined. Sediment samples were collected from eight drillholes to depths of up to 30 m within a braidplain aquifer in Aotearoa New Zealand, along with groundwater samples from 24 monitoring wells completed in the same system. In total, 240 diffusion experiments and 24 groundwater samples were analysed. Results show that the four laboratory methods produced different equilibrium values, with median estimates ranging from 2.7 to 47.5 Bq/L. Method 2 (500 mL shaken samples) showed agreement with measured groundwater concentrations. However, groundwater concentrations were less than laboratory estimates of equilibrium values especially at shallow depth. Due to high groundwater velocities in braided river environments, Rn-222 in groundwater is unlikely to reach equilibrium nearby to the river and therefore diffusion experiments provide a complementary Rn-222 equilibrium estimate. Finer-grained sediments with higher silt and clay content (<63 µm) released greater amounts of Rn-222, highlighting the importance of collecting representative sediment samples.
Robb et al. (Sun,) studied this question.