Quantitative evaluation of hydrogeological impact produced by tunnel construction using water balance models

Drawdown produced by excavations can cause geotechnical, hydrologic, environmental and social impacts. The impact and drying up of private wells and springs closer the axis tunnel may become relevant in densely populated rural areas without municipal water supply. A procedure for environmental monitoring of a tunnel excavation and for quantitative evaluation of the hydrogeological impact by means of water balance models in the affected river basins is presented. Once these models have been calibrated against undisturbed conditions, comparison of measured and predicted water table evolution can be used to distinguish actual tunnel impacts from natural oscillations. This procedure has been applied successfully to a case study of tunnel construction in fractured granitic bedrock in the NW of Spain. A hydrogeological impact has been detected and quantified in 5 of 17 monitored points. The small amounts of groundwater drained by the tunnel (less than 5 l/s) produced significant drawdown (5–86 m), on account of the low storage coefficient of the rock massif, causing important damage to private groundwater users. An increase on groundwater recharge rate after the tunnel impact has been identified, that will speed up the process of recovery of the aquifers. The detection and quantification of the main impacts of the excavation could help to take decisions about rectification measures or compensations to the affected groundwater users and about the advisability of the total or partial waterproofing of the tunnel.