Numerical Modeling of Deep Borehole Disposal Performance: Influence of Regional Hydrology

Abstract:

Long-term waste isolation at a deep borehole disposal facility is most favorable at a site where the crystalline basement is hydraulically isolated and groundwater flow is negligible. Site suitability guidelines include evidence of lack of fluid flow in basement, for example lack of significant topographic relief, or evidence of ancient and/or saline groundwater at depth. However, lack of local topographic relief does not preclude regional hydraulic gradients created by recharge and discharge at distant outcrops; and precisely because of hydraulic isolation, the crystalline basement has the potential to be over- or under-pressured relative to overlying units. In the absence of previous boreholes in the area of a potential site, hydraulic gradients at depth are difficult to predict, and the possibility remains that a deep borehole drilled for the disposal of waste will encounter vertical or lateral driving forces for fluid flow. This study asks the question: How large a driving force can be tolerated while still maintaining repository performance? We use PFLOTRAN (an open source, massively parallel subsurface flow and reactive transport code) and a 3-D model domain (representing a disposal borehole in crystalline basement overlain by sedimentary strata) to examine the influence of horizontal and vertical hydraulic gradients on the long-term performance of a deep borehole radioactive waste repository. Simulations include steady-state lateral hydraulic gradients and transient vertical hydraulic gradients, and predict radionuclide concentrations in an overlying aquifer to quantify the potential influence of regional hydraulic gradients on repository performance.