Characterisation of discharge areas of radionuclides originating from nuclear waste repositories Lars Marklund and Anders Wörman Department of Land and Water Resource Engineering, The Royal Institute of Technology (KTH), Teknikringen 76, SE - 100 44 Stockholm, Stockholm, Sweden INTRODUCTION If leakages in nuclear waste repositories located at great depth in crystalline bedrock arise, radionuclides will reach the biosphere and cause a risk of radiological impact (SKB, 2006). The extent of the radiological impact depends on in which landscape elements the radionuclides emerge. Both biological and hydrological pathways near ground surface differ widely depending on the landscape element. This affects both accumulation and bioavailability of the radionuclides. The quaternary deposits are the geological strata where radionuclides first can contaminate the biosphere, upon migrating from a leaking waste repository. Hence, a good understanding of the accumulation and transport in the overburden is critical to the potential radiological consequences and to the safety assessment of nuclear waste repositories. In this study, we investigate if there are certain landscape elements that will generally act as discharge areas for radionuclides leaking from a subsurface deposit of nuclear waste. We also characterize the typical properties that distinguish these areas from others. Understanding the processes controlling the clustering of discharge to certain areas is an additional topic of study. Landscape topography is the most important driving force for groundwater flow (Marklund et al., 2008). Because groundwater is the main transporting agent for migrating radionuclides, the topography will determine the flowpaths of leaking radionuclides. How topography and heterogeneities in the subsurface affect the discharge distribution of the radionuclides is the main scope of this study. METHODS To address these issues, we developed both a numerical simulation model and an analytical model based on an exact solution for 3-d groundwater flow. The analyses we present here are based on a site-specific data from two different areas in Sweden, Forsmark/Östhammar, Uppland, and Oskarshamn/Simpevarp, Småland. The Swedish Nuclear Waste Management Company (SKB) has selected these two areas as candidate areas for deep repository for nuclear waste and the areas are currently subject to site investigations. Numerical model The three-dimensional numerical simulations were performed using a computational package in a Matlab® environment, developed by Wörman et al. (2005). The problem is formulated based on a (Darcy) continuum formulation for the groundwater flow and is solved using central finite differences. In Forsmark the anisotropy and the heterogeneity of the bedrock have been modelled. This was done by combining deterministically defined fracture zones (on the 1-20 km scale) with stochastically defined fracture populations (on the 1-1000 m scale). Data for fracture zones and