Migrace uranu a dalsich stopovych prvku v bedrichovskem vodarenskem tunelu v Jizerskych horach

The Bedřichov water-supply tunnel in the Krkonoše-Jizera Composite Massif is important due to its unusual length (6 km) and the technical attributes of the method of construction (a combination of the tunnel boring method - TBM - and drill and blast technology). The tunnel walls are particularly appropriate for the long-term monitoring of the rock environment (Fig. 1). Trace element parameters in groundwater allow modelling of the evolution of both far-field and excavation-damaged zones (EDZ) for follow-up studies of underground radioactive waste repositories. The migration process of uranium, one of the major radionuclide carriers, was tested by analysing groundwater trace elements and measuring the flow rates of groundwater springs in the Bedřichov water-supply tunnel (section A). Surface water percolating through Jizera Granite in the Krkonoše-Jizera Composite Massif, along fractures up to a depth of 150 m in the tunnel, becomes enriched with a number of mobile chemical elements and compounds generated by granite oxidation. Ca-HCO₃SO₄type groundwater in the Bedřichov tunnel is characterized by average mineralization of 117.51 mg/l with pH about 7.5 and temperature between 5 to 7 ^°C (Němeček 1982, Klomínský et al. 2008).
The samples were taken from six different groundwater spring sites (Fig. 2) - three dripping ones (V1, V2-1, and V3), three with continual fluxes (V4-1, V5, V6) and from one central drain collecting all outflows from the tunnel in section A (V8). Inflow to the groundwater sample sites was measured in ml/s. A sample from the Josefův Důl water reservoir was used for estimation of the uranium and associated trace element content in the 22 600 000 m³of the dam water. All samples were analysed for 72 chemical elements (Hokr et al. 2010). Results over detection limits are shown in Table 1.
Higher uranium concentrations have been registered in water springs from the central part of the tunnel (V2 and V3), where the overlying granite thickness is the highest (Fig. 3). The longer retention period results in a higher mineral saturation. Balvín et al. (2012) estimated the transit time of the groundwater inflow in the Bedřichov water-supply tunnel, according to the ²H and ¹⁸Oisotopes method, at 24.9 to 30.9 months.
Occurrences of uranyl-sulphate-carbonate (schröckingerite) on the Bedřichov tunnel walls correspond with places with intense evaporation, e.g. in places with abundant microfractures in the EDZ (Excavation Damage Zone) tunnel section A constructed by classic mining drill and blast technology (Fig. 4). The total amount of uranium released from the Jizera Granite during 30 years of operation of the Bedřichov tunnel (section A) is estimated to be about 40 kilograms, including 2.5 kg of uranium bonded in schröckingerite on the tunnel walls (Table 2).
The migration of uranium from the Jizera Granite in and out of the Bedřichov water-supply tunnel (section A) represents a continuous groundwater leaching process over the life of the tunnel. The existence of the similar environment over whole area of the Krkonoše-Jizera Composite Massif is indicated by the trace element content in water from the Josefův Důl water reservoir (Table 1).