Using the Cl/Br ratio as a tracer to identify the origin of salinity in aquifers in Spain and Portugal

A study on the applicability of the Cl/Br molar ratio for identifying the origin of groundwater salinity is presented according to the experience gained in Spain and Portugal. Cl/Br ratios in atmospheric bulk deposition have helped to distinguish groundwater in which Cl derives only from atmospheric sources, from groundwater in which Cl is contributed by endogenous, lithological or anthropogenic sources. Considering the seawater Cl/Br ratio of 655 ± 4, Cl/Br ratios may be increased up to 1500 in groundwater by the addition of wastewater loaded with NaCl or leaching of solid waste, and decreased to 300 by the use of Br-based pesticides or leaching farm-animal or septic waste. Leaching and dissolution of natural and industrial NaCl, and of gypsum-rich formations containing some NaCl may yield Cl/Br ratios between one thousand and several thousand, although leaching of potassium halides near salt mines produces Cl/Br ratios below the seawater ratio. When Cl content in groundwater is not affected by extreme evaporation (up to halite saturation at ∼6.2 mol L−1 of NaCl) and if there are no other Cl sources, recharge by rainfall can be estimated by relating Cl in atmospheric bulk deposition to derived groundwater. The Cl/Br ratio appears to be a good tracer for discriminating non-atmospheric Cl contributions to groundwater, provided chemical analyses are accurate, since its variability is not wide. Data have been collected from different areas of Spain and Portugal and are explained and illustrated by two cases described in detail, the Doñana aquifer in southwest Spain and the Canary Islands, both showing the mixing paths.