Determination of naturally occurring Ra isotopes in Ubatuba-SP, Brazil to study coastal dynamics and groundwater input

Abstract

The four naturally occurring Ra isotopes were measured in seawater, river and groundwater samples collected in a series of small embayments of Ubatuba coastal region, covering latitudes between 23°26’S and 23°46’S and longitudes between 45°02’W and 45°11’W, in order to estimate coastal mixing rates and groundwater discharge fluxes. During the period of this investigation, the activity concentrations of 223Ra in surface seawater varied from 2.7 to 41 mBq 100 L- 1, 224Ra in excess from 8.5 to 624 mBq 100 L- 1, 226Ra from 131 to 187 mBq 100 L- 1 and 228Ra from 109 to 409 mBq 100 L- 1. The activities of 223Ra, 224Ra, 226Ra and 228Ra in a surface river water sample (Escuro River), which reaches the coast on the local area, were 46 mBq 100 L- 1, 954 mBq 100 L- 1, 229 mBq 100 L- 1 and 745 mBq 100 L- 1, respectively. Groundwater samples from monitoring wells presented activity concentrations up to 2,033 mBq 100 L for 223Ra, 72,540 mBq 100 L- 1 for 224Ra in excess, 2,722 mBq 100 L- 1 for 226Ra and 35,688 mBq 100 L- 1 for 228Ra. The 223Ra/224Ra activity ratios observed in seawater samples ranged from 0.7 × 10- 1 to 0.46, whereas 228Ra/226Ra AR varied in the interval from 0.6 to 1.9. These results seems to indicate that Ra isotopes from 232Th series prevail in a major number of samples, when compared with Ra isotopes from 238U and 235U series. Based on this data, shore-perpendicular profiles of 223Ra and 224Ra in surface waters along the coast were modeled to yield eddy diffusion coefficients. These coefficients allow an evaluation of cross-shelf transport and provide further insight on the importance of groundwater to coastal regions.