The Tordera Desalination Plant located in Blanes (NE Spain) has seawater intake through 10 beach wells located a few meters inland on the shoreline at the Tordera River Delta. Between October 2002 and October 2003, the extracted groundwater showed a decrease in conductivity, especially in the wells located in the northern area, prompting the present study. A multi-isotopic approach (δD, δ18OH2O, 3H, δ34SSO4, 87Sr/86Sr and 228Ra/226Ra) coupled with chemical data was applied in order to assess the origin of the water collected for the desalination plant and to quantify the extent of freshwater collection from the Tordera aquifer, when applicable. Three multi-piezometers located in the Tordera aquifer were also sampled in order to characterize the freshwater end-member. A seasonal survey was performed in order to assess the evolution of mixed freshwater-seawater intake. Tritium isotopes showed values ranging from 0.6 to 2.5TU indicating recent origin of the collected waters. This was further confirmed using radium isotopes (226Ra and 228Ra), as the 228Ra/226Ra activity ratio (AR) indicated a continuous input of seawater on a yearly time scale. The water extracted from the beach wells was at least 95% seawater, except for wells 8-10. The latter two were extracting up to 15% of freshwater from the Tordera aquifer system. From a methodological point of view, while δ34S of dissolved sulphate and the ratio 87Sr/86Sr are good tracers of seawater mixing with freshwaters, the isotopic composition of water (δD and δ18OH2O) and the Cl-/Br- ratio are conservative tracers that allow for quantifying the contribution of freshwater to the extracted water. Although slight variations linked to seasonality were observed in all wells during the 3-year study period (November 2003 to December 2006), wells 1 and 7 showed an increase in freshwater contribution from 4% to 11% and well 10 a decrease from 15% to 10% over this period ; This study was funded by the Catalan Water Agency (Agència Catalana de l’Aigua), CICYT projects CGL-2008-06373-CO3-01/03-BTE from the Spanish Government, project 2009 SGR 103 from the Catalan Government, and the I3P Programme funded by the EU