TIMS measurements of Ra-226 and Ra-228 in the Gulf of Lion, an attempt to quantify submarine groundwater discharge
Résumé
Submarine groundwater discharge (SGD) is now recognized as an important pathway for water and chemical species fluxes to the coastal ocean. In order to determinate SGD to the Gulf of Lion (France), we measured the activities of Ra-226 and Ra-228 by thermal ionization mass spectrometry (TIMS) in coastal waters and in the deep aquifer waters of the Rhone deltaic plain after preconcentration of radium by MnO2. Compared to conventional counting techniques, TIMS requires lower quantities of water for the analyses, and leads to higher analytical precision. Radium isotopes were thus measured on 0.25-2 L water samples containing as little as 20 fg of Ra-226 and 0.2-0.4 fg of Ra-228 with precision equal to 2%. We demonstrate that coastal surface waters samples are enriched in Ra-226 and Ra-228 compared to the samples further offshore. The high precision radium measurements display a small but significant Ra-226 and Ra-228 enrichment within a strip of circa 30 km from the coast. Radium activities decrease beyond this region, entrained in the northern current along the shelf break or controlled by eddy diffusion. The radium excess in the first 30 km cannot be accounted for by the river nor by the early diagenesis. The primary source of the radium enrichment must therefore be ascribed to the discharge of submarine groundwater. Using a mass-balance model, we estimated the advective fluxes of Ra-226 and Ra-228 through SGD to be 5.2 x 10(10) and 21 x 10(10) dpm/d respectively. The Ra-226 activities measured in the groundwater from the Rhone deltaic plain aquifer are comparable to those from other coastal groundwater studies throughout the world. By contrast, Ra-228 activities are higher by up to one order of magnitude. Taking those groundwater radium activities as typical of the submarine groundwater end-member, a minimum volume of 0.24-4.5 x 10(10) l/d is required to support the excess radium isotopes on the inner helf This has to be compared with the average rivers water runoff of 15.4 x 10(10) l/d during the study period (1.6 to 29% of the river flow).