Determination of low-level Radium isotope activities in fresh waters by gamma spectrometry

A new portable sampling system was developed to extract Radium isotopes from large volumes (up to 300 L) of fresh surface-and ground-waters of low Ra-activities ( < 5 mBq/L). Ra is quantitatively adsorbed on a small amount (6.5 g) of MnO 2 -coated acrylic fibers, which are then dried and burned at 600 °C in the laboratory. The resulting Mn-oxide powder (about 2 cm 3 when compacted) is then analyzed through gamma-ray spectrometry which allows measurement of the whole Ra quartet ( 226 Ra, 228 Ra, 224 Ra and 223 Ra) in a single counting of a few days. The usual relative standard combined uncertainties (1σ) are 2-3% for 226 Ra, 228 Ra and 224 Ra; and less than 10% for 223 Ra. This method was applied to the analysis of Ra in karstic waters of the Lez aquifer, and surfaceand ground-waters of the upper and middle Vidourle watershed (South of France). The analyzed waters have relatively low 226 Ra activities (1-4 mBq/L) in both cases, regardless of the contrasted geology (Mesozoic limestone vs crystalline Variscan basement), but clearly distinct ( 228 Ra/ 226 Ra) ratios in agreement with the differences in Th/U ratios of the two drained areas. Short-lived Ra isotopes ( 224 Ra and 223 Ra) appear to be mainly influenced by near-surface desorption/recoil processes for most of the sampling sites.