Uncertainty and Sensitivity Analyses of Groundwater Travel Time in a Two-Dimensional Variably-Saturated Fractured Geolog
- PDF / 509,909 Bytes
- 8 Pages / 414.72 x 648 pts Page_size
- 63 Downloads / 174 Views
UNCERTAINTY AND SENSITIVITY ANALYSES OF GROUNDWATER TRAVEL TIME IN A TWO-DIMENSIONAL VARIABLY-SATURATED FRACTURED GEOLOGIC MEDIUM
A. B. GUREGHIAN AND B. SAGAR Center for Nuclear Waste Regulatory Analyses 6220 Culebra Road San Antonio, Texas 78228-0510 ABSTRACT
This paper presents a method for sensitivity and uncertainty analyses of a hypothetical nuclear waste repository located in a layered and fractured unconfined aquifer. Groundwater travel time (GWTlT) has been selected as the performance measure. The repository is located in the unsaturated zone, and the source of aquifer recharge is due solely to steady infiltration impinging uniformly over the surface area that is to be modeled. The equivalent porous media concept is adopted to model the fractured zone in the flow field. The evaluation of pathlines and travel time of water particles in the flow domain is performed based on a Lagrangian concept. The Bubnov-Galerkin finite-element method is employed to solve the primary flow problem (nonlinear), the equation of motion, and the adjoint sensitivity equations. The matrix equations are solved with a Gaussian elimination technique using sparse matrix solvers. The sensitivity measure corresponds to the first derivative of the performance measure (GWTT) with respect to the parameters of the system. The uncertainty in the computed GWTT is quantified by using the firstorder second-moment (FOSM) approach, a probabilistic method that relies on the mean and variance of the system parameters and the sensitivity of the performance measure with respect to these parameters. A test case corresponding to a layered and fractured, unconfined aquifer is then presented to illustrate the various features of the method. INTRODUCTION
Groundwater travel time (GWTT) is a subsystem performance measure established by the U.S. Nuclear Regulatory Commission for evaluating a potential high-level nuclear waste (HLW) repository in a geologic medium (1OCFR60.113). The GWT'I is defined to provide an estimate of water travel time associated with the fastest likely path of radionuclides that might be released from the repository horizon and transported to the accessible environment. This paper presents a method for estimating the sensitivity and uncertainty of the GWTT, based on a deterministic method of analysis. The approach to our calculation proceeds in four steps, as follows: (i) The hydrologic analysis consists of the steady-state calculation of the water pressure head and velocity distribution in the variably-saturated, fractured geologic medium, including an exact determination of the water-table height at the seepage face of the flow domain. Of the existing approaches in modeling groundwater flow through fractured medial the equivalent porous medium approach is the one adopted in this work. (ii) The pore velocity distribution then is used to obtain the preferential flow paths and GWTTs for a set of particles released from the same repository location. While particle pathlines and travel times are Lagrangian quantities and velocities ar
Data Loading...
