Results of monitoring at Olkiluoto in 2012 - hydrology and hydrogeology

The impact of the construction of ONKALO is monitored by measuring and observing numerous different parameters related to hydrology, geochemistry, environment, rock mechanics and foreign materials. The Hydrological Monitoring Programme consists of the following parameters: groundwater level, hydraulic head, flow conditions in open drillholes, transverse flow, hydraulic conductivity, groundwater salinity (in situ EC), precipitation (including snow), sea-water level, surface flow (runoff), infiltration, ground frost, leakages in tunnels, and water balance in the tunnel system and in Korvensuo Reservoir. This Report focuses on hydrogeological parameters. Other parameters, like precipitation, ground frost etc. will be reported in the Monitoring Report of Environment. Updated monitoring program was introduced in the beginning of 2012. The updated program will be used for the period before repository operation. Only minor changes were implemented. Monitoring has been carried out according to plan. This Report presents the results for the year 2012. The access tunnel was excavated from chainage 4913 m to chainage 4987 m in 2012. In addition, demonstration tunnel 2 from chainage 65 m to 101 m and some technical facilities were excavated. Total inflow into ONKALO down to chainage 4580 m including shaft ONK-KU2 down to level -427m was 36 l/min at the end of 2012. The mapping of water leakages and moisture conditions on the tunnel walls and the ceiling has been continued. The general pattern of leakages has remained similar during the construction of ONKALO. Most significant differences are caused by seasonal effects like condensation of warm ventilation air on tunnel walls and ceiling. The changes observed in the groundwater level in shallow observation tubes in the overburden and in shallow drillholes in the bedrock are not necessarily caused by the construction of ONKALO. However, weak indications of a local decrease in groundwater level have been observed. Effects on the head deeper in the bedrock have been both short-term and long-term. In 2012, short-term effects were mostly connected to the drilling of pre-grouting holes in the shafts running through the HZ20 zones. Long-term changes, i.e. a decrease in pressure heads near ONKALO, vary averagely between 11 and 13 m within the HZ20 system and between 1 and 2 m within the HZ19 system. Due to the intersection of the HZ056 zone and a few local features, long-term drawdowns of over 20 m have occurred and the groundwater level has decreased below the measurable drillhole depth in ten monitoring sections. Flow-based monitoring using the PFL-tools (DIFF-tool and TRANS-tool) was carried out in seven drillholes in 2012. The changes detected in flow and calculated head are most likely caused by direct connections to ONKALO along the HZ19 and HZ20 zones or along their possible extensions. Also, weak indications of other connections to ONKALO were revealed by the PFL-tool. Both increasing and decreasing changes in in situ EC values were observed. Hydraulic conductivity was measured with the HTUtool in two drillholes and changes over earlier results were minor. Slug-tests were performed in 19 shallow holes and changes over earlier results were minimal. (orig.)