Results of monitoring at Olkiluoto in 2010. Hydrology

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 boreholes, cross drillhole 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. Monitoring has primarily been carried out according to plan. This Report presents the results for the year 2010. A significant change took place in the Monitoring Programme when most of the open drillholes were packed-off before the excavation of the ONKALO access tunnel through the hydrogeological HZ20 zones began in June 2008. Prior to packing-off, open drillholes connected the main hydrogeological features, the HZ19 and HZ20 systems, to each other. Due to the packing-off of open drillholes, the number of flow logging and hydraulic testing (HTU) measurements has decreased considerably. The mapping of water leakages and moisture conditions in tunnel walls and roof has been continued. Some changes have been observed in the pattern of moisture content. The changes have probably been caused by shotcreting, postgrouting and possibly also by seasonal effects. The changes have so far not been analysed. 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 decrease in groundwater level have been observed. Effects on the head deeper in the bedrock have been both short-term and long-term. In 2010, 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 between 10 and 12 m within the HZ20 system and between 1 and 2 m within the HZ19 system. Due to the intersection of the new HZ056 zone, a long-term drawdown of over 20 m occurred and the groundwater level has decreased below measurable drillhole depth in three monitoring section. Flow-based monitoring using the PFL-tool was carried out in four drillholes in 2010. The several locally strong changes in flow and calculated head are most likely caused by direct connections along the HZ19 and HZ20 zones as well as along zone HZ056. In addition, several other local connections to ONKALO were revealed by the PFLtool. Some minor changes in in situ EC values were also observed. Changes in hydraulic conductivities were measured with the HTU-tool in two drillholes and with the Slug-test in 29 shallow holes. (orig.)