Characterization of Bedrock Hydrogeology at the Olkiluoto Site Using Surface Based and Underground Data

Posiva Oy is responsible for implementing the program for geological disposal of spent nuclear fuel in Finland. Olkiluoto in Eurajoki has been selected as the primary site for the repository, subject to further detailed characterization which is currently focused on the construction of an underground rock characterization and research facility (the ONKALO). An essential part of the site investigation program is analysis of groundwater flow and solute transport to depth by means of numerical flow modeling as part of the 2011 site descriptive model. Groundwater flow in the crystalline rocks at Olkiluoto takes place predominantly in the void space of the interconnected fractures in the bedrock. Many of the hydraulic characteristics of flow through such a system can be represented by hydrogeological Discrete Fracture Network (hydro-DFN) models, since they capture some of the details of fracture geometry, size, connectivity and openings. The paper describes how field data is used to derive such models, ultimately for use in safety assessment. Surface based site investigations at Olkiluoto include 53 vertical or inclined deep core drilled boreholes and 27 shallower sub-vertical boreholes. The characterization of fractures is a key objective. Fracture geometrical data (fracture positions and orientations) are determined from drill core mapping and/or borehole TV images, while hydraulic data are determined with the Posiva Flow Log (PFL). The PFL method is a geophysical logging device developed to detect continuously flowing fractures in sparsely fractured crystalline bedrock by means of difference flow logging, providing the effective transmissivity of individual fracture intercepts. A methodology developed originally in the Swedish program for constraining hydrogeological DFN models based on the PFL high resolution hydraulic test data has been extended to incorporate data from underground. The approach integrates deterministic models of the main hydro-structures with a stochastic model of the bedrock between. An exploration of conceptual uncertainties in the representation of the very scarce distribution of flow at repository depth, c. 400m, has been performed. This has been greatly informed by the analysis of 15 additional sub-horizontal boreholes drilled ahead of the ONKALO tunnel excavations. Conditions for PFL hydraulic tests performed in these pilot holes provide a lower detection limit and reveal the very low magnitude of groundwater flow that may be expected around the repository depositional volumes.