Leakage problems in dams built on evaporites. The case of La Loteta Dam (NE Spain), a reservoir in a large karstic depression generated by interstratal salt dissolution

La Loteta Reservoir, with a storage capacity of 105 hm3, is located in a large karst depression around 6 km long on the southern margin of the Ebro River valley, NE Spain. Geomorphological mapping and borehole data indicate that the development of the basin is related to subsidence due to interstratal dissolution of the underlying halite- and glauberite-bearing evaporites. The dam site corresponds to the water gap carved by the small drainage that captured the formerly internally drained depression. Here, the foundation and abutments of the dam include a horizontal and laterally extensive gypsum unit 11 m thick. This sedimentary package showed considerable evidence of karstification in the excavation carried out during the construction of the dam, especially in the left abutment, where it was largely removed. The watertightness system of the ca. 1.5 km long earth dam includes a vertical clay core, a horizontal clay blanket, a cut-off wall, and grout curtains 675 m and 255 m long on the left and right abutments, respectively. Multiple data including leakage discharge measured in the different zones of the drainage system, seepage points mapped downstream of the dam, borehole and piezometric data, and an equipotential map, reveal that leakage essentially occurs through the gypsum unit. The main leakage occurs beneath and next to the left edge of the dam body. This water loss and the associated enlargement of karst conduits are also supported by settlement measured on the dam crest and the occurrence of sinkholes within the reservoir, next to the left abutment. Additional seepage across the grout curtain in both abutments is also identified. Although unlikely, there is also the potential for the water to escape towards an adjacent watershed, where the base level is located below the maximum water level of the reservoir. An additional cut-off wall has been projected to block the main leakage path.