Regional groundwater flow pattern in fractured bedrock with a high-level radioactive waste repository: numerical simulation of the Xinchang site, northwestern China

The Xinchang site in northwest China has been selected as a potential deep geological repository for spent high-level radioactive waste (HLW) due to its low-permeability granite bedrock. Predicting the characteristics of long-term flow systems is of priority for improving the suitability assessment for a HLW disposal repository. In this study, a three-dimensional saturated-unsaturated groundwater flow numerical model, using the software TOUGH3/EOS9 with an improved evaporation module, was developed. It was subsequently used to reveal the regional groundwater flow patterns in the bedrock fracture network and to provide scientific support for the safety assessment of the potential HLW disposal repository at the Xinchang site. The regional groundwater flow system associated with the Xinchang site can be characterized by three hydrogeological units. The groundwater at Xinchang site cannot flow into the Hexi Corridor due to the high surface topography and low-permeability F11 fault, but it flows southward to Yuanyang Ditch and then discharges from the east and south boundaries. Changes in precipitation, fault permeability and fault connectivity only cause fluctuation of the water table in local areas, and do not impact the regional groundwater flow patterns. Topography is the main factor determining regional groundwater flow. The proposed model is expected to serve as a powerful tool for studying groundwater flow patterns in fractured bedrock regions and provides the foundation for deeper analysis of nuclides migration in terms of distance and time in the Xinchang area.