Restauración del impacto hidrogeológico superficial generado por los túneles de Pajares en el arroyo de Alcedo (León, noroeste de España)

Abstract

The perforation of the Pajares Tunnels, with 24.6 km long and 700 m average depth, through the Cantabrian Range (NW Spain) caused a very important surficial hydrogeological impact on the Alcedo Valley consisting on: (i) the development of 25 sinkholes from 2007 to 2014 in Quaternary alluvial materials covering calcareous rocks, and (ii) the whole infiltration of the surficial flow of the Alcedo stream trough 7 active ponors. Previous studies identified the karstic origin of the problem and estimated the mean water volume of infiltration in 0.4 hm3/year (approx. 10 ls-1). New geotechnical and hydrogeological studies were planned to characterize in depth the study area: (i) topographic survey, (ii) borehole drilling and trench digging, (iii) geophysical prospecting, (iv) regular gauging of the surficial flow, and (v) analysis of the water flow return period. The chosen solution for stopping surficial infiltration was the sealing of the sinkholes and ponors, the construction of a concrete channel over the area affected by karst processes and its environmental restoration. After the completion of these actions, the infiltration from the Alcedo Stream to the tunnels was eliminated. The completion of these works involved a 7.2% reduction of total water volume drained by the Pajares Tunnels, from average 350 l s-1 to 325 l s-1.