Deciphering the origin of groundwater inflow into the Talave tunnel (SE Spain).

The Talave tunnel (TT) is an infrastructure of a major water transfer from the Tajo river basin (center Spain) to the Segura river basin (SE Spain), crossing the Júcar river basin. The tunnel was drilled between 1969 and 1978. It is 32 km long, N/NW-S/SE oriented, has a maximum depth of 320 m, intersects several aquifers, and its southern stretch follows the eastern boundary of the Alcadozo aquifer. The TT drilling perturbed groundwater flow in two river basins, and the induced groundwater inlets generated social and administrative concern lasting until today. The main objectives of this paper are understanding the historical and current tunnel-massif hydrodynamic relationships, and deciphering the origin of groundwater inflow into the tunnel. The first objective was approached analyzing the discharge flow evolution since the drilling until 2016, together with old (1970s) and recent (2014-2017) piezometric data. For the second objective, hydrochemical and isotopic data were generated between 2014 and 2018 from discrete and integrated discharge to the tunnel. Attaining both goals benefited of recent studies on groundwater recharge and functioning in the Alcadozo aquifer. Discharge flows stabilized in the early 2000s. 340 hm were drained between 1969 and 2016, producing a piezometric drawdown between 15 and 120 m along the tunnel. The main inflow zones correspond to tectonic fractures concentrated in the middle and southern sections of the tunnel. The existence of a hydrogeological divide between the Júcar and the Segura river basins some 3 km to the N of the watershed divide implies that all groundwater comes from the Segura hydrogeological basin. The isotopes suggest that groundwater comes mainly from regional flow lines originated to the W of the tunnel, together with some local recharge. The effects of tunneling can be used to assess the impacts of imminent groundwater development planned by the Basin Authority.