[en] As part of the site investigations for the disposal of spent nuclear fuel, the construction of underground rock characterisation facility ONKALO began in July 2004. The HZ20 system is considered the most important hydrogeological feature to be intersected by ONKALO. Excavation through the HZ20 system took place in 2008-2009 and took a total of about six months. A hydrogeological analysis was performed to study the hydraulic effects of the temporary groundwater inflow through the drilled holes and the more continuous leakage through the fractures into the tunnel. A hydrogeological analysis of the head observations and interpreted pressure responses provides information on the extent of the HZ20 zones in the Well Characterised Area (WCA) and the hydraulic connections to the other hydrogeological zones, and indicative information on the continuity of the system outside the WCA and on the internal hydraulic properties of the HZ20 system. The results of the analysed observations support the present understanding of the HZ20 structure: a restricted extent and a non-existing or insignificant hydraulic connection to the hydrogeological HZ19 system above. In addition, a change in hydraulic properties from the central HZ20 area to the northwest HZ20E area is supported by the transient Theis analysis. Steady-state Thiem solution was applied to head data during the temporary stationary conditions after excavation through zone HZ20A and after zone HZ20B when no inflow to drillholes occurred. The results supported a restricted extent outside the WCA. Packed-off monitoring sections were classified into three types of hydraulic connections. Direct connections describe the modelled HZ20 zones applied in site scale numerical groundwater flow modelling. Minor changes are suggested for modelling of the HZ20A and HZ20B zones. Zone HZ20A could be extended to drillholes OL-KR16 and OL-KR27 and the intersection depth of zone HZ20A could be changed in drillhole OL-KR29 and that of zone HZ20B in drillhole OL-KR25. Hydraulic connections related to monitoring sections with lower fracture transmissivities were assessed to be connected to the HZ20 system along local hydrogeological zones. They are located close to the interpreted boundaries of the HZ20 system. Due to the long-term leakage of the HZ20 system into the tunnel, several packed-off sections with a minor hydraulic connection were observed. These kinds of monitoring sections were interpreted to be connected to the HZ20 system along fractures with low transmissivity. A rough estimation of the transmissivity was carried out using Theis solution. As a result, a mean transmissivity of 1.5x10^-5 m²/s with a standard deviation of 3. 10^-6 m²/s was obtained for the central area of the HZ20 system. (orig.)