Radioprotection, vol. 46,n ◦ 6 (2011) S661–S667 C  EDP Sciences, 2011 DOI: 10.1051 / radiopro / 20116729s Groundwater diversion from a deep-rock repository for spent nuclear fuel: Ecohydrological assessment of environmental impacts K. Werner 1 ,P. Collinder 2 , S. Berglund 3 and E. Bosson 3 1 EmpTec, Larmvägen 8, SE-187 75 Täby, Sweden 2 Ekologigruppen AB, Åsögatan 121, SE-116 24 Stockholm, Sweden 3 Swedish Nuclear Fuel and Waste Management Co, Box 250, SE-101 24 Stockholm, Sweden Abstract. Forsmark in Mideastern Sweden is chosen as site for the planned Swedish deep-rock repository for spent nuclear fuel. The construction, operation and decommissioning of the repository require assessments of various types of risks for health and environment on a wide range of temporal scales. This study is focused on the assessment of environmental impacts during the relatively short construction and operation phases, during which the repository will be kept drained by means of groundwater diversion. This diversion may influence hydrogeological and hydrological conditions and thereby cause ecological and other types of consequences in the surroundings of the repository. We here describe methodology and some main findings from the ecohydrological assessment of consequences for aquatic and terrestrial ecosystems in Forsmark. The assessment combines the results of a number of steps, including ecological field inventories and predictive water-flow modelling. The results of the study show that subsurface disposal of radioactive waste is an important application of ecohydrological concepts in assessments of sites with nature values. 1. INTRODUCTION The Swedish Nuclear Fuel and Waste Management Co (SKB) is responsible for the management and disposal of radioactive waste from Swedish nuclear power plants. According to the so-called KBS-3 method, copper canisters with a cast-iron insert containing spent nuclear fuel are to be enclosed by bentonite clay and deposited at a depth of approximately 500 metres in granitic bedrock [1]. Some 12,000 tonnes of spent nuclear fuel are forecasted to arise from the Swedish nuclear power programme, corresponding to roughly 6,000 canisters. The construction, operation and decommissioning of a deep-rock repository require assessments of various types of risks for health and environment. These assessments concern risks on a wide range of temporal scales, such as long-term risks associated with potential post-decommissioning radionuclide releases and subsequent transport towards and within the biosphere [2]. This study is focused on the assessment of environmental impacts, other than radionuclide releases and transport, during the relatively short (less than 100 years) construction and operation phases of the repository. In particular, during these phases the repository will be kept drained by means of groundwater diversion. This diversion may influence the hydrogeological and hydrological system at the site, which in turn could cause ecological and other types of consequences in the surroundings of the repository. In the following, we describe the methodology used in the Swedish repository programme to predict environmental impacts, in the form of ecological consequences that may be caused by groundwater diversion during construction and operation of a deep-rock repository. The methodology is illustrated using data and information from Forsmark in Mideastern Sweden. Forsmark, which is chosen by SKB as site for the planned repository, is very valuable from a nature conservation point of view. The assessment methodology requires data and information on site-specific ecological, hydrogeological