Environmental impact of underground construction from a hydrological and hydrochemical disturbance perspective

In most projects involving underground construction, impact to hydrogeological conditions is monitored through measurements of groundwater levels. In contrast, hydrological and hydrochemical changes are rarely monitored throughout a project and the assessment of environmental resilience is often limited to an evaluation that broadly links the behavior of groundwater levels with a large number of engineering geological and ecological parameters. An ongoing field research project in Sweden is based on monitoring the recovery of hydrogeological, hydrological and hydrochemical conditions after a long-term groundwater extraction. The groundwater discharge was intended to simulate effects of underground construction. Groundwater was extracted during a period of five years from fractured igneous rock in a previously glaciated area with thin soil cover. During this time the hydrogeological and hydrological conditions were altered with lowered groundwater levels and the surface runoff from the area decreased by nearly 50%. Meanwhile significant hydrochemical changes were observed with spikes in sulfate and lowered pH in groundwater as well as in surface water. Preliminary results show that hydrological recovery after the groundwater extraction took more than three years, despite apparent quick recovery of groundwater levels with runoff volumes remaining lower than in a nearby reference area. Meanwhile, hydrochemical conditions did not return to those that existed prior to groundwater extraction. During the first three years of recovery, sulfate remained higher in shallow groundwater bodies, whereas base cations were lower compared to before the extraction started. After ten years of recovery sulfate concentrations have decreased by 30% to 60%, in both surface water and groundwater bodies, whereas chloride concentrations have increased compared to before the extraction. By restricting monitoring to only groundwater levels, some environmental impacts due to underground constructions may be missed