Stress control of hydraulic conductivity in fracture-saturated Swedish bedrock Christopher J. Talbot * , Manhal Sirat 1 Hans Ramberg Tectonic Laboratory, Institute of Earth Science, Uppsala University, Villavagen 16, SE-752 36 Uppsala, Sweden Abstract The granitoid bedrock exposed in the A È spo È Hard Rock Laboratory is the best known rock mass in Sweden but is probably generally similar to most of the country in many characteristics relevant to the long term isolation of spent nuclear fuel. Like most Precambrian rocks in the Fennoscandian shield, the rocks of A È spo È are saturated by fractures with a large range of orientations after long and complicated deformation histories. The ca. 11,000 fractures documented throughout the A È spo È HRL belong to six sets with distinctive orientations. Only about 8% of the total fracture population were wet when they were excavated. Although the wet fractures belong to the same six sets as the total population, their numbers in each set change abruptly at a particular gently dipping fracture zone 240 m deep in the laboratory. We correlate the numbers of wet fractures in particular sets to different regimes in the current stress ®eld. Most wet fractures are subhorizontal in a stress regime prone to thrusting above a depth of 240 m and subvertical with NW trends in an underlying stress regime prone to wrench faulting. Faults favourably orientated for slip or dilation in the ambient stress ®eld have recently been shown to be the most active groundwater ¯ow pathways near a plate boundary. Here we show a similar relationship for old fractures in a Precambrian shield embedded deep in the Eurasian plate. However, the stress regimes, and thus the anisotropy of transmissivity, are complicated by post-glacial uplift in the Fennoscandian shield and can change 908 within a few metres at depths and locations on the scale of tens and hundreds of metres. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Fractures; Stress regimes; Hydraulic conductivity; A È spo È HRL; Sweden 1. Introduction Groundwater ¯ow must be understood at the site mined in crystalline bedrock soon to be chosen for the isolation of Sweden's spent nuclear fuels. When the spent fuel eventually releases radioactive phases through the three engineered barriers copper canister, bentonite and other back®lls) their transport and dilution through, or precipitation in, the geological barrier will be in groundwater ¯owing along complex fracture patterns in rocks with long and complicated histories. Most of Sweden is part of a Precambrian shield that has not been near a plate boundary for about 60 million years. When it was ®rst proposed to store Sweden's spent nuclear fuel deep in Swedish bedrock, those bedrocks were considered to represent the essence of long term geological stability and therefore likely to have large volumes with low hydraulic Engineering Geology 61 2001) 145±153 0013-7952/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S0013-795201)00047-3 www.elsevier.com/locate/enggeo * Corresponding author. Tel.: 146-18-471-25-70; fax: 146-18- 471-25-91. E-mail address: christopher.talbot@geo.uu.se C.J. Talbot). 1 Present address: Department of Civil Engineering, University of Hawaii at Manoa, 2540 Dole Street, 383 Holmes Hall, Honolulu, Hawaii 96822.