Groundwater monitoring impacts of deep excavations: hydrogeology in the Venda Nova repowering schemes (NW Portugal)

The Venda Nova hydroelectric repowering schemes consist of two fully underground hydraulic circuits, more than 4 km long, situated on the left bank of the Rabagão River (NW Portugal). The first one, Venda Nova II (VNII) started operating in 2005 with two reversible units located in caverns at depths higher than 320 m. The rapid growth of wind power capacity in the last years has increased the importance of the reversibility role and of water storage in existing reservoirs and led to the interest of Venda Nova III (VNIII), which was designed for two reversible power units and an underground hydraulic circuit (5.6 km long) almost parallel to the existing repowering scheme of VNII. Besides the direct contribution of these repowerings to the electric system, the favourable impact on the reduction of greenhouse gases emissions and of imported fossil fuels use was also considered. One important feature of this region located in granitic and schistose environment is that infiltration and recharge are quite important, enabling extensive use in water supply to small rural communities and agriculture. The eventual disturbance on groundwater regime, with impacts at the surface, has been one of the key analyses that had to be taken within this large amount of underground excavations. The impacts were monitored by controlling the surrounding springs, wells and piezometers, before, during and after the construction works. Flow and water level measurements are done to detect eventual surface disturbances over an area of almost 500 ha, monitoring 270 water points. The groundwater behaviour during VNII excavations allowed the prediction of the impacts on water levels during VNIII excavations, now ongoing. The task described in this paper essentially addresses the issues regarding impacts on groundwater levels before and during design construction and at operation.