Updated strategy and test of new concepts for groundwater flow modelling in Forsmark in preparation of site descriptive modelling stage 2.2

As part of the preliminary Site Descriptive Modelling (SDM version 1.2) for the Initial Site Investigation (ISI) stage at Forsmark, Simpevarp and Laxemar, a methodology was developed for constructing hydrogeological models of the crystalline bedrock. The methodology achieved reasonable success given the restricted amounts and types of data available at the time. Notwithstanding, several issues of concern have surfaced following the reviews of the preliminary site descriptions of the three sites. Possible solutions to parts of the problems have been discussed internally for a longer time and an integrated view and strategy forward has been formulated. The 'new strategy' is not a complete shift in methodology, however, but a refocusing on and clarification of the key aspects that the hydrogeological SDM needs to accomplish. In broad terms the basic principle of the 'new strategy' suggested is to develop an overall conceptual model that first establishes the major flowing deformation zones, and then gradually approaches determination of the hydraulic properties of the bedrock outside these zones in the potential repository volume. On each scale, the focus of the description should be on features/structures of significance on that scale. Clearly, a detailed (although statistical) description of the repository and canister deposition hole scale is the end goal, but this approach (which also is more the traditional approach in hydrogeology) is judged to provide a much better motivated overall geometrical description. Furthermore, the 'new strategy' puts more emphasis on field testing (e.g. interference tests) and data analyses and less on numerical simulation and calibration. That is, before extensive (and costly) simulations and model calibrations are made it needs to be clearly understood what could be the potential gains of carrying them out. This report presents the conceptual model development for Forsmark in preparation of the site descriptive modelling in stage 2.2. Four 'pre-modelling' exercises have been performed to develop and test the methodology issues. These exercises are not directly aimed at a model update, but more intended to provide some insight into new aspects of the modelling methodology and the use of field data, and therefore provide background support also for the upcoming final SDM. Some of the issues treated for the SDM of the Forsmark site are also of interest for the SDM of the Laxemar site