Isotope tracing of submarine groundwater discharge offshore Ubatuba, Brazil: results of the IAEA-UNESCO SGD project

Results of groundwater and seawater analyses for radioactive (H-3, Rn-222, Ra-223, Ra-224, Ra-226, and Ra-228) and stable (D and 180) isotopes are presented together with in situ spatial mapping and time series Rn-222 measurements in seawater, direct seepage measurements using manual and automated seepage meters, Pore water investigations using different tracers and piezometric techniques, and geoelectric surveys probing the coast. This study represents first time that such a new complex arsenal of radioactive and non-radioactive tracer techniques and geophysical methods have been used for simultaneous submarine groundwater discharge (SGD) investigations. Large fluctuations of SGD fluxes were observed at sites situated only a few meters apart (from 0 cm d-1 to 360 cm d(-1); the unit represents cm(3)/cm(2)/day), as well as during a few hours (from 0 cm d-1 to 110 cm d(-1)), strongly depending on the tidal fluctuations. The average SGD flux estimated from continuous Rn-222 measurements is 17 10 cm d(-1). Integrated coastal SGD flux estimated for the Ubatuba coast using radium isotopes is about 7 x 1 03 M3 d-1 per km of the coast. The isotopic composition (delta D and delta O-18) of submarine waters was characterised by significant variability and heavy isotope enrichment, indicating that the contribution of groundwater in submarine waters varied from a small percentage to 20%. However, this contribution with increasing offshore distance became negligible. Automated seepage meters and time series measurements of Rn-222 activity concentration showed a negative correlation between the SGD rates and tidal stage. This is likely caused by sea level changes as tidal effects induce variations of hydraulic gradients. The geoelectric probing and piezometric measurements contributed to better understanding of the spatial distribution of different water masses present along the coast. The radium isotope data showed scattered distributions with offshore distance, which imply that seawater in a complex coast with many small bays and islands was influenced by local Currents and groundwater/seawater mixing. This has also been confirmed by a relatively short residence time of 1-2 weeks for water within 25 km offshore, as obtained by short-lived radium isotopes. The irregular distribution of SGD seen at Ubatuba is a characteristic of fractured rock aquifers, fed by coastal groundwater and recirculated seawater with small admixtures of groundwater, which is of potential environmental concern and has implications on the management of freshwater resources in the region. (c) 2008 Elsevier Ltd. All rights reserved.