Dataset: Global SGD 226Ra and 228Ra

Samples were collected using either a peristaltic pump or a well pump and the groundwater was passed through a 1 or 5 μm prefilter to remove suspended sediment before being filtered at <1 L/min onto Mn-coated acrylic fiber. The fiber was rinsed with Ra-free MilliQ water to remove any salts or sediment, then partially dried.

Samples with low activities (<0.4 dpm) were analyzed for 228Ra via 228Th ingrowth using a delayed coincidence counter (RaDeCC) (Moore, 2008), as described in Charette et al. (2015). Briefly, when 228Ra is extracted onto the Mn fiber, 228Th is extracted in parallel. Using the initial concentration of 228Th and the concentration of 228Th after 1 to 2 years, measured via RaDeCC, along with the decay constants of 228Th and 228Ra, the initial concentration of 228Ra can be calculated. Low activity samples were analyzed for 226Ra via 222Rn emanation (Key et al. 1979), as described in Charette et al. (2015). Fibers were placed in a fiber holder that was then flushed with He for 5 minutes at 250 mL/min, sealed, and left for two weeks before analysis via 222Rn ingrowth and scintillation counting. These two methods were used for low activity samples due to better method sensitivity than gamma counting (Charette et al., 2001), which was used for samples with high activities (>0.4 dpm per sample). For this method, the fibers were ashed (880 °C, 16 h), homogenized, capped with epoxy resin, and left for >3 weeks to obtain secular equilibrium between 226Ra and its daughter radionuclides. The samples were then counted in a well-type gamma spectrometer for 228Ra (via 228Ac at 338 keV) and 226Ra (via 214Pb at 351.9 keV) (Charette et al., 2001).

When not specifically listed, errors are 10%.