Key features of the near-surface hydrogeology of the Forsmark area, Sweden

Hydrological and hydrogeological field investigations are conducted in the Forsmark area by the Swedish Nuclear Fuel and Waste Management Co. (SKB) for siting of a repository for spent nuclear fuel. The surface and near-surface parts of the investigations include monitoring of surface water levels and discharge, groundwater levels, and hydraulic characterisation of Quaternary deposits (QD). The main objectives of these investigations are to provide a detailed description of the surface and near-surface hydro(geo)logy as a basis for key aspects of the site investigation as safety and environmental impact assessment. Hydrogeological investigations of the bedrock and hydrochemical characterisation of surface water and groundwater are performed in parallel activities.
The study area is characterized by a small-scale topography and is almost entirely located below 20 m.a.s.l. Till is the dominating QD, covering 75% of the area, while granite is the dominating rock type. The annual precipitation and runoff are 560 and 150 mm, respectively.
Groundwater levels in QD are very shallow; on average less than 0.7 m below ground during 50% of the time. Shallow groundwater levels imply a strong interaction between evapotranspiration, soil moisture and groundwater. Diurnal fluctuations of the groundwater levels, driven by evapotranspiration cycles, are evident in many groundwater wells. Furthermore, groundwater level measurements in the vicinity of the lakes show that the lakes may act as recharge sources to till aquifers in the riparian zone during summer.
There is a close correlation between the topography and the groundwater levels in the QD. For groundwater levels in the upper bedrock there is no such strong coupling to the topography. This is most evident in the central part of the study area, where the groundwater level are low and the gradients very small in the bedrock, indicating a high transmissivity. Here, the groundwater levels in the till in general are considerably higher than in the bedrock. The result is that local, small-scale recharge and discharge areas, involving groundwater flow systems restricted to QD, will overlay the more large-scale flow systems associated with groundwater flow in the bedrock. Also in the middle of Lake Bolundsfjärden, located in the central part of the study area, the lake level and the groundwater level in till are considerably higher than the levels in the bedrock down to 200 m depth, indicating a downward flow gradient from the lake and QD to the bedrock.
The flow systems around and below the lakes seem quite complex. The lake water/groundwater level relationship, under natural as well as disturbed conditions, indicates that the lake sediments and the underlying till have low vertical hydraulic conductivities. The groundwaters below the lakes have relict marine chemical signatures, whereas groundwaters in the riparian zones are fresh.