EVALUATION OF A PALEO-HYDROGEOLOGICAL MODEL OF THE YUCCA MOUNTAIN NUCLEAR WASTE DISPOSAL SITE BY MEANS OF NUMERIC THERMAL MODELING

The unsaturated zone (UZ) of Yucca Mountain, Nevada, has persisted during the last 11.6 myr. It is known, from fluid inclusion studies and U-Pb dating of secondary minerals, that waters with T up to 70-90°C circulated through the UZ for 4 to 6 myr, starting at 10 Ma. The model adopted by U.S. DOE explains elevated temperatures by conductive heating of the UZ by shallow crustal silicic magma chamber(s) associated with the Timber Mountain caldera system, located 7-9 km to the N.

The “conductive heating” model was evaluated by means of numeric simulations (codes HEAT 3D and HYDROTHERM). Conductive and advective (by moving water) heat transfer modes were modeled. The generalized UZ cooling trajectory derived from paleo-temperature data and radiometric ages was used for calibration. Uncertainties related to poorly constrained parameters (e.g., dimensions and depth of emplacement of magma, residence time(s), properties of deep-seated rocks, etc.) were handled by using non-conservative (“favorable” for the tested model) values. Temperature was monitored at a reference point (RP) located at a depth of 0.25 km in 7 km from a disc-shaped model pluton (r=15 km, h=7 km, T = 900°C).

We were unable to find any combination of parameters that would return the target T-time trajectory. In conduction-only simulations, even in the case of shallow pluton emplacement (2.5 km) preceded by 4 Ma-preheating of the crust, the thermal perturbations at RP did not exceed 4°C. Higher temperatures were reached by "emplacing" the pluton under or adjacent to RP. Yet, the heat loss occurred too fast to be consistent with the mineralogical data. Simulations involving advection showed coupling of the topography- and buoyancy-driven flow patterns. At certain parameters, T up to 54°C was obtained at RP. The peak T occurred earlier than in conduction-only simulations (e.g., at 10 Ka rather than at 200 Ka). We conclude that neither timing nor the temperatures of thermal waters that circulated through and deposited secondary minerals in the Yucca Mountain UZ can be explained by the conductive heating of the rocks by a Miocene magma body.

The work was supported by grant RFBR 04-05-64347.