Calibration of regional palaeohydrogeology and sensitivity analysis using hydrochemistry data in site investigations

A transient coupled regional model of groundwater flow and solute transport has been developed, which allows the use of hydrochemical data to calibrate the model input parameters. The methodology has been illustrated using examples from the Simpevarp area in south-eastern Sweden which is being considered for geological disposal of spent nuclear fuel. The 3-dimensional model includes descriptions of spatial heterogeneity, density driven flow, rock matrix diffusion and transport and mixing of different water types, and has been simulated between 8000 BC and 2000 AD. Present-day analyses of major elemental ions and stable isotopes have been used to calibrate the model, which has then been cross checked against measured hydraulic conductivities, and against the hydrochemical interpretation of reference water mixing fractions. The key hydrogeological model sensitivities have been identified using the calibrated model and are found to include high sensitivity to the top surface flow boundary condition, the influence of variations in fracture transmissivity in different orientations (anisotropy), spatial heterogeneity in the deterministic regional deformation zones and the spacing between water bearing fractures (in terms of its effect on matrix diffusion).