Results of monitoring at Olkiluoto in 2008. Hydrology

The impact of the ONKALO construction is monitored by measuring and observing numerous different parameters of 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, 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 Environment Report. Mainly the monitoring has been carried out according to plan. The previous Monitoring Report contained the results until the end of 2007, and this Report presents the results for the year 2008. A significant change in the Monitoring Programme was performed while most of the open drillholes were packed-off before excavation of the ONKALO access tunnel through the hydrogeological HZ20 zones began in Jun 2008. Prior to packing-off, open drillholes connected the main hydrogeological features, HZ19 and HZ20 systems, to each other. Due to packing-off open drillholes, number of flow logging and hydraulic testing monitoring measurements has considerably decreased. According to the observations carried out in shallow observation tubes in the overburden and in shallow drillholes in the bedrock, the construction of ONKALO has not caused any certain changes in groundwater level. However, weak indications of decrease in groundwater level have been observed. The effects on head deeper in the bedrock have been both short-term and long-term and in 2008 these were mostly connected to excavation of the tunnel trough the HZ20 zones. In most cases, short-term changes have been caused by temporary leakages due to the probe holes and grouting holes drilled from ONKALO. In other drillholes except packed-off sections connected to the HZ20 system, long-term changes i.e. decrease in pressure heads near ONKALO have remained on the same order of magnitude, c 1 m, as a year ago. Head decrease in the HZ20 system varies up to 9 m. The amount of flow-based monitoring by PFL-tool was reduced in 2008 and only two drillholes were measured in 2008. Some changes in flow and calculated head values are most likely caused by direct connections along HZ19 zones and other local connections to ONKALO. Leakages into ONKALO also likely increased salinity in OL-KR27 at depths where HZ19 zones intersect the hole. Changes in hydraulic conductivities were measured by HTU-tool only in OL-KR31, where changes were minor. Measurements in OL-KR32 began in 2008. (orig.)