Modeling Non-Isothermal Behavior of Alternative Conceptual Models of Flow at Yucca Mountain, Nevada - State of Nevada Funded Research
Abstract
Ground-water temperature and piezometric head data from a number of wells at the Yucca Mountain Site are correlated. This correlation appears to be controlled by hydrogeologic structures and discontinuities in the saturated zone at this site. In the assessments of ground-water pathways in complex systems such as the Yucca Mountain saturated zone, heat may be used as a flow path tracer. Thermal, hydraulic head, and chemical data serve to constrain the results of analyses that may have non-unique solutions. Heat and chemical tracers can give a much better indication of the ground-water flow directions than hydraulic head alone. A conceptual model of flow was developed in which geologic structure plays a key role in the hydraulic head and temperature distributions. The conceptual hydrogeologic model proposed for Yucca Mountain consists of multiple hydrostratigraphic layers on which a complex flow system is superimposed. For our modeling purpose, the flow field consist of two distinct hydrologic systems the deeper, partially confined carbonate system and the overlying upper unconfined system. In the northern part of the Yucca Mountain Site, the piezometric head in the unconfined system is much higher than that to the south. Beneath Yucca Mountain and to the east beneath Fortymile Wash, piezometric head distribution is predominantly flat, which is believed to be the result of the highly fractured volcanic tuff aquifer system. There are very few direct measurements of the piezometric head in the carbonate aquifer in the vicinity of Yucca Mountain Site. There appears to be a generally upward hydraulic gradient in this aquifer. To the south, flow occurs primarily upward due to breaches in the confining unit. Flow in these areas is consequently warmer. Both hydrologic systems are transient in nature; however, the thermal regime by comparison may be assumed to be in a steady state. Therefore, highly conductive discontinuities such as fault zones can be examined to identify thermal disturbance caused by convective transport of heat. These analyses help better define the hydrologic system. A highly simplified numerical model was constructed to evaluate flow paths to the accessible environment based on this conceptualization of the coupled heat and ground-water flow at the Yucca Mountain Site. The numerical model is fully three-dimensional, evaluates thermal transport, and explicitly considers geologic structures as controls on the flow field. The latest model results indicate differences exist in flow path direction and velocity when compared to the latest DOE results. We also believe that further differences in interpretation exist with respect to the distribution of infiltration through the unsaturated zone. This paper will detail the conceptual and numerical model set up and latest results. The model calibration results are also presented.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....8575L