Origin and evolution of groundwater collected by a desalination plant (Tordera, Spain): A multi-isotopic approach

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 (delta D, delta(18)O(H2O), (3)H, delta(34)S(SO4), (87)Sr/(86)Sr and (228)Ra/(226)Ra) 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 2.5 TU indicating recent origin of the collected waters. This was further confirmed using radium isotopes ((226)Ra and (228)Ra), as the (228)Ra/(226)Ra 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 delta(34)S of dissolved sulphate and the ratio (87)Sr/(86)Sr are good tracers of seawater mixing with freshwaters, the isotopic composition of water (delta D and delta(18)O(H2O)) 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. (C) 2010 Elsevier B.V. All rights reserved.