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

- Niebla-Posadas aquifer is an important water source in southwest Spain.
- First groundwater dating for Niebla-Posadas aquifer.
- Complete assessment of hydrogeochemical processes that affects groundwater dating.
- Contribution to scientific-based aquifer management.
- Distinction between renewable and non-renewable groundwater resources.

SummaryThe 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 3H, 14C and 36Cl. 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 δ13CDIC in the groundwater is isotopically enriched. Further modifications take place along the flow path in deeper zones, where depleted δ13CDIC values are overprinted due to SO42− and iron oxide reduction, triggered by the presence of organic matter. Dating with 3H, 14C and 36Cl 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.