Recent and old groundwater in the Niebla-Posadas regional aquifer (southern Spain): Implications for its management

The Niebla-Posadas (NP) aquifer in southern Spain is one of the main groundwater sources for the lower Guadalquivir Valley, a semiarid region supporting an important population, agriculture and industry. To contribute to the understanding of this aquifer the assessment of sustainable use of groundwater, the residence time of groundwater in the NP aquifer has been estimated using H-3, C-14 and Cl-36. Along the flow paths, recharged groundwater mixes with NaCl-type waters and undergoes calcite dissolution and is further modified by cation exchange (Ca-Na). Consequently, the water loses most of its calcium and the residual delta C-13(DIC) in the groundwater is isotopically enriched. Further modifications take place along the flow path in deeper zones, where depleted delta C-13(DIC) values are overprinted due to SC42- and iron oxide reduction, triggered by the presence of organic matter. Dating with H-3, C-14 and Cl-36 has allowed the differentiation of several zones: recharge zone (<0.06 ky), intermediate zone (0.06-20 ky), deep zone 1(20-30 ky), and deep zone 2 (>30 ky). An apparent link between the tectonic structure and the groundwater residence time zonation can be established. Regional faults clearly separates deep zone 1 from the distinctly older age (>30 ky) deep zone 2. From the estimated residence times, two groundwater areas of different behavior can be differentiated within the aquifer. (C) 2015 Elsevier B.V. All rights reserved.