Geology topics

Joseph A. Hevesi

Publications and source records attributed to Joseph A. Hevesi.

22 records · Page 2Linked to original sources

USGS1993

Influence of long term climate change on net infiltration at Yucca Mountain, Nevada

Net infiltration and recharge at Yucca Mountain, Nevada, a potential site for a high level nuclear waste repository, are determined both by the rock properties and past and future changes in climate. A 1-dimensional model was constructed to represent a borehole being drilled through the unsaturated zone. The rock properties were matched to the lithologies expected to be encountered in the borehole. As current paleoclimate theory assumes that 18O increases with wetter and cooler global climates, a past climate scenario, built on depletion of 18O from ocean sediments was used as a basis for climate change over the past 700,000 years. The climate change was simulated by assigning net infiltration values as a linear function of 8O. Assuming the rock properties, lithologies and climate scenarios are correct, simulations indicated that Yucca Mountain is not in steady state equilibrium at the surface (<75 meters) when compared to measured data, but that the system could be at steady state conditions at depths of >250 meters. Based on the cyclic climate inputs, the near surface is currently in a long term drying trend (for the last 3,000 years) yet recharge into the water table is continuing to occur at an average rate equivalent to the average input rate of the climate model, indicating that conditions at depth are damped out over very long time periods. The Paintbrush Tuff nonwelded units, positioned between the Tiva Canyon and Topopah Spring welded Tuff Members, do not appear to act as capillary barrier and therefore would not perch water. The low porosity vitric caprock and basal vitrophyre of the Topopah Spring Member, however, act as restrictive layers. The higher porosity rock directly above the caprock reduces the potential for the caprock to perch water leaving the basal vitrophyre as the most likely location for perched water to develop.

Conference Paper↗

USGS1993

Influence of seasonal climatic variability on shallow infiltration at Yucca Mountain

To analyze infiltration and the redistribution of moisture in alluvial deposits at Yucca Mountain, water content profiles at a 13.5 m deep borehole were measured at monthly intervals using a neutron moisture probe. Increases in water content to a maximum depth of 1.8 m in response to winter season precipitation were noted. Below a depth of 1.8 m, a gradual drying trend was indicated. A simulation study showed that, although small amounts of water may be percolating through the deep nonwetted ones of the profile, the influence of climatic variability on infiltration through thick alluvial deposits at Yucca Mountain is greatly mitigated by evapotranspiration.

Conference Paper↗

USGS1992

Precipitation estimation in mountainous terrain using multivariate geostatistics. Part I: structural analysis

Values of average annual precipitation (AAP) are desired for hydrologic studies within a watershed containing Yucca Mountain, Nevada, a potential site for a high-level nuclear-waste repository. Reliable values of AAP are not yet available for most areas within this watershed because of a sparsity of precipitation measurements and the need to obtain measurements over a sufficient length of time. To estimate AAP over the entire watershed, historical precipitation data and station elevations were obtained from a network of 62 stations in southern Nevada and southeastern California. Multivariate geostatistics (cokriging) was selected as an estimation method because of a significant (p = 0.05) correlation of r = .75 between the natural log of AAP and station elevation. A sample direct variogram for the transformed variable, TAAP = ln [(AAP) 1000], was fitted with an isotropic, spherical model defined by a small nugget value of 5000, a range of 190 000 ft, and a sill value equal to the sample variance of 163 151. Elevations for 1531 additional locations were obtained from topographic maps to improve the accuracy of cokriged estimates. A sample direct variogram for elevation was fitted with an isotropic model consisting of a nugget value of 5500 and three nested transition structures: a Gaussian structure with a range of 61 000 ft, a spherical structure with a range of 70 000 ft, and a quasi-stationary, linear structure. The use of an isotropic, stationary model for elevation was considered valid within a sliding-neighborhood radius of 120 000 ft. The problem of fitting a positive-definite, nonlinear model of coregionalization to an inconsistent sample cross variogram for TAAP and elevation was solved by a modified use of the Cauchy-Schwarz inequality. A selected cross-variogram model consisted of two nested structures: a Gaussian structure with a range of 61 000 ft and a spherical structure with a range of 190 000 ft. Cross validation was used for model selection and for comparing the geostatistical model with six alternate estimation methods. Multivariate geostatistics provided the best cross-validation results.

Nevada↗

USGS1992

Precipitation estimation in mountainous terrain using multivariate geostatistics. Part II: isohyetal maps

Values of average annual precipitation (AAP) may be important for hydrologic characterization of a potential high-level nuclear-waste repository site at Yucca Mountain, Nevada. Reliable measurements of AAP are sparse in the vicinity of Yucca Mountain, and estimates of AAP were needed for an isohyetal mapping over a 2600-square-mile watershed containing Yucca Mountain. Estimates were obtained with a multivariate geostatistical model developed using AAP and elevation data from a network of 42 precipitation stations in southern Nevada and southeastern California. An additional 1531 elevations were obtained to improve estimation accuracy. Isohyets representing estimates obtained using univariate geostatistics (kriging) defined a smooth and continuous surface. Isohyets representing estimates obtained using multivariate geostatistics (cokriging) defined an irregular surface that more accurately represented expected local orographic influences on AAP. Cokriging results included a maximum estimate within the study area of 335 mm at an elevation of 7400 ft, an average estimate of 157 mm for the study area, and an average estimate of 172 mm at eight locations in the vicinity of the potential repository site. Kriging estimates tended to be lower in comparison because the increased AAP expected for remote mountainous topography was not adequately represented by the available sample. Regression results between cokriging estimates and elevation were similar to regression results between measured AAP and elevation. The position of the cokriging 250-mm isohyet relative to the boundaries of pinyon pine and juniper woodlands provided indirect evidence of improved estimation accuracy because the cokriging result agreed well with investigations by others concerning the relationship between elevation, vegetation, and climate in the Great Basin. Calculated estimation variances were also mapped and compared to evaluate improvements in estimation accuracy. Cokriging estimation variances were reduced by an average of 54% relative to kriging variances within the study area. Cokriging reduced estimation variances at the potential repository site by 55% relative to kriging. The usefulness of an existing network of stations for measuring AAP within the study area was evaluated using cokriging variances, and twenty additional stations were located for the purpose of improving the accuracy of future isohyetal mappings. Using the expanded network of stations, the maximum cokriging estimation variance within the study area was reduced by 78% relative to the existing network, and the average estimation variance was reduced by 52%.

Nevada↗