Groundwater circulation patterns in bedrock aquifers from a pre-selected area of high-level radioactive waste repository based on two-dimensional numerical simulation

Abstract

Deep geological disposal is internationally accepted as a feasible and safe way to dispose the high-level radioactive waste (HLW) in low permeability fractured bedrocks. It is essential to understand the regimes of groundwater flow, circulation, and predict the characteristics of long-term flow systems for site selection and safety assessment of deep geological HLW disposal by using numerical simulation technology. In this study, TOUGH3/EOS9 software with an improved evaporation module is used to develop a two-dimensional variably saturated numerical model of a typical profile at Xinchang site, which is considered as a potential deep geological repository for spent HLW in China. Results reveal that almost 99% of the precipitation is discharged through evaporation, and less than 1% infiltrates into shallow groundwater. The groundwater flow pattern is closely related to topography and lithological distribution, and the groundwater flow in Xinchang rock mass cannot discharge to the south Hexi Corridor. Groundwater in the typical profile is divided into three types of groundwater flow system, namely local, intermediate and regional flow system, and each has its own characteristics based on aspects of flow path, cycle amount, cycle path, residence time and discharge position. The circulation flux of these three systems accounts for 79.25%, 13.24% and 7.51% of the total groundwater quantity of Xinchang site, respectively. Meanwhile, different-depth models were constructed to validate the influence of bottom boundary depths on groundwater characteristics. Increasing the bottom boundary depth has limited influence on the maximum circulation depth of regional flow system, but negligible impact on circulation flux. This study can enhance the understanding of groundwater circulation and evolution in fractured bedrock aquifers with low permeability, and provide the feasibility of geological repository for spent HLW for the view point of groundwater flow.