Contributions of Thermodynamic and Mass Transport Modeling to Evaluation of Groundwater Flow and Groundwater Travel Time
- PDF / 619,531 Bytes
- 8 Pages / 414.72 x 648 pts Page_size
- 23 Downloads / 206 Views
CONTRIBUTIONS OF THERMODYNAMIC AND MASS TRANSPORT MODELING TO EVALUATION OF GROUNDWATER FLOW AND GROUNDWATER TRAVEL TIME AT YUCCA MOUNTAIN, NEVADA WILLIAM M. MURPHY Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute, 6220 Culebra Rd., San Antonio, TX 78238 USA ABSTRACT Constraints on groundwater flow and groundwater travel time (GWTT) at the proposed HLW repository site at Yucca Mountain, Nevada, U.S.A., can be provided through thermodynamic modeling of relations between gas, liquid, and solid phases, and mass 14 transport modeling. In the unsaturated zone at Yucca Mountain the observed distribution of C can be reasonably represented by a mass transport and mass transfer model representing diffusion of 14 CO2 in the gas phase from the ground surface and equilibrium between aqueous and gas phases, in the absence of net advective flow. This mechanism could also account for aqueous phase 14C activities in the unsaturated zone. Thermodynamic interpretations indicate that groundwaters extracted from boreholes in the water saturated tuffaceous aquifer at Yucca Mountain are undersaturated with respect to calcite, providing evidence that groundwater flow bypasses zones of rock containing calcite and therefore must be channelized. This conclusion is supported by published isotopic data for calcites and groundwaters. INTRODUCTION United States Nuclear Regulatory Commission regulations for licensing a geologic repository for high-level radioactive waste (HLW) require evaluations of radionuclide transport, which would occur by groundwater flow, and of groundwater travel time (GWTT) as a site characteristic and as measure of site performance [1]. Groundwater flow and GWTT at the proposed HLW repository site at Yucca Mountain, Nevada, U.S.A., have been addressed through hydrologic modeling and statistical analyses of hydraulic parameters (e.g., [2]), and through analyses of soluble radioactive tracers including 3H, 14C, and 36 C1 (e.g., [3,4,5]). Thermodynamic modeling of relations between gas, liquid, and solid phases, and mass transport modeling can also contribute to evaluations of groundwater flow and GWTT. The objective of research reported here is to apply theoretical geochemical analyses to interpret specific data from the Yucca Mountain area relating to groundwater flow. Examples are drawn from the unsaturated zone, which is the proposed repository location, and from the underlying saturated zone. Radioisotopic analyses of groundwater samples provide direct geochemical evidence for groundwater travel times. 3H, 14C, and 36 C1 have cosmogenic sources and are also products of testing of explosive nuclear devices. These radioisotopes have diverse half lives of 12.4, 5.73x10 3, and 3.01x10 5 years, respectively. They are highly soluble in groundwater characteristic of the Yucca Mountain site, and isotopes of carbon and hydrogen are also prevalent in the gas phase of the hydrologically unsaturated zone at Yucca Mountain. 3 H generated by atmospheric testing of nuclear devices increased in precipitation
Data Loading...
