Chlorine-36 data at Yucca Mountain: statistical tests of conceptual models for unsaturated-zone flow.
An extensive set of chlorine-36 (36Cl) data has been collected in the Exploratory Studies Facility (ESF), an 8-km-long tunnel at Yucca Mountain, Nevada, for the purpose of developing and testing conceptual models of flow and transport in the unsaturated zone (UZ) at this site. At several locations, the measured values of 36Cl/Cl ratios for salts leached from rock samples are high enough to provide strong evidence that at least a small component of bomb-pulse 36Cl, fallout from atmospheric testing of nuclear devices in the 1950s and 1960s, was measured, implying that some fraction of the water traveled from the ground surface through 200-300 m of unsaturated rock to the level of the ESF during the last 50 years. These data are analyzed here using a formal statistical approach based on log-linear models to evaluate alternative conceptual models for the distribution of such fast flow paths. The most significant determinant of the presence of bomb-pulse 36Cl in a sample from the welded Topopah Spring unit (TSw) is the structural setting from which the sample was collected. Our analysis generally supports the conceptual model that a fault that cuts through the nonwelded Paintbrush tuff unit (PTn) that overlies the TSw is required in order for bomb-pulse 36Cl to be transmitted to the sample depth in less than 50 years. Away from PTn-cutting faults, the ages of water samples at the ESF appear to be a strong function of the thickness of the nonwelded tuff between the ground surface and the ESF, due to slow matrix flow in that unit.
Estimating recharge at Yucca Mountain, Nevada, USA: comparison of methods
Abstract. Obtaining values of net infiltration, groundwater travel time, and recharge is necessary at the Yucca Mountain site, Nevada, USA, in order to evaluate the expected performance of a potential repository as a containment system for high-level radioactive waste. However, the geologic complexities of this site, its low precipitation and net infiltration, with numerous mechanisms operating simultaneously to move water through the system, provide many challenges for the estimation of the spatial distribution of recharge. A variety of methods appropriate for arid environments has been applied, including water-balance techniques, calculations using Darcy's law in the unsaturated zone, a soil-physics method applied to neutron-hole water-content data, inverse modeling of thermal profiles in boreholes extending through the thick unsaturated zone, chloride mass balance, atmospheric radionuclides, and empirical approaches. These methods indicate that near-surface infiltration rates at Yucca Mountain are highly variable in time and space, with local (point) values ranging from zero to several hundred millimeters per year. Spatially distributed net-infiltration values average 5 mm/year, with the highest values approaching 20 mm/year near Yucca Crest. Site-scale recharge estimates range from less than 1 to about 12 mm/year. These results have been incorporated into a site-scale model that has been calibrated using these data sets that reflect infiltration processes acting on highly variable temporal and spatial scales. The modeling study predicts highly non-uniform recharge at the water table, distributed significantly differently from the non-uniform infiltration pattern at the surface.Résumé. Les valeurs d'infiltration nette, de temps de parcours de l'eau souterraine et de la recharge sont nécessaires sur le site de Yucca Mountain, Nevada, USA, pour évaluer les capacités d'un site de stockage potentiel pour le confinement de déchets hautement radioactifs. Cependant, la complexité géologique du site, les faibles valeurs de précipitation et d'infiltration nette, associées à de nombreux mécanismes agissant simultanément pour faire se déplacer l'eau dans le système, posent de nombreux problèmes pour estimer la distribution spatiale de la recharge. Un ensemble de méthodes adaptées aux environnements arides a été mis en œuvre, comprenant les techniques de bilan hydrologique, des calculs s'appuyant sur la loi de Darcy en milieu non saturé, une méthode de physique des sols appliquée aux données de teneur en eau par mesure neutronique, une modélisation inverse des profils thermiques en forage portant sur l'épaisse zone non saturée, le bilan de masse des chlorures, les radionucléides atmosphériques et des approches empiriques. Ces méthodes indiquent que les taux d'infiltration au voisinage de la surface à Yucca Mountain sont très variables dans le temps et dans l'espace, avec des valeurs locales, ponctuelles, comprises entre 0 et plusieurs centaines de mm/an. Les valeurs d'infiltration nette distribuées dans l'espace sont en moyenne de 5 mm/an, les plus fortes approchant 20 mm/an. Ces résultats ont été introduits dans un modèle à l'échelle du site qui a été calibré au moyen de ces jeux de données, reflétant les processus d'infiltration agissant à des échelles de temps et d'espace à forte variabilité. L'étude par modélisation prédit une recharge fortement non uniforme de la nappe, dont la distribution est significativement différente de l'organisation de l'infiltration non uniforme en surface.Resumen. Se necesitan conocer los valores de infiltración neta, el tiempo de tránsito de las aguas subterráneas y la recarga en el emplazamiento de Yucca Mountain, Nevada, USA, con el fin de evaluar el comportamiento esperado de un repositorio potencial como sistema de contención de residuos de alta radioactividad. Sin embargo, la complejidad geológica del lugar y los pequeños valores de precipitación e infiltración neta, junto con los numerosos mecanismos que operan simultáneamente en relación con el movimiento del agua en el sistema, representan un enorme reto para la estimación de la distribución espacial de la recarga. Se ha aplicado diversos métodos considerados apropiados para medios áridos: técnicas de balance de agua, cálculos basados en la ley de Darcy en la zona no saturada, un método de mecánica de suelos para conocer el contenido de agua con sondas de neutrones en sondeos, modelación inversa de los perfiles de sondeos a lo largo de una zona no saturada gruesa, balance de masas de cloruros, radionucleidos atmosféricos y técnicas ambientales. Estos métodos indican que, cerca de la superficie, las tasas de infiltracón en Yucca Mountain son muy variables en el tiempo y el espacio, presentando valores locales (puntuales) comprendidos entre cero y varios cientos de mm/a. Los valores espaciales de la infiltración neta tienen un promedio de 5 mm/a, con máximos cercanos a 20 mm/a. Estos resultados han sido incorporados a un modelo que se ha calibrado con los datos existentes, que reflejan la elevada variabilidad temporal y espacial de los procesos de infiltración. El modelo predice una recarga muy poco uniforme, distribuida de forma significativamente diferente al esquema no uniforme de infiltración en superficie.
The Inca Occupation of the South Coast of Peru
like the others, however; it forks into several nearly parallel ravines of which the largest are Huayuri, Rio Grande-Palpa, Ingenio (called Collao in ancient times), Nasca proper (the ancient Caxamarca), Taruga, and Poroma. Due to peculiarities of the local topography, the irrigated areas in Ica and the ravines of Nasca are located inland, at the foot of the mountains, rather than at the mouths of the rivers as is more usual in Peruvian coastal valleys. No sharp natural or cultural boundary separates the valley of Chincha from the valleys of Cafiete (ancient Huarco and Runahuana), Asia, and Mala, the next valleys going up the coast to the northwest. At the other end of the area we are considering, however, there is a sharp break; beyond Yauca the Andean foothills reach out to the sea, natural conditions are quite different, and there are important differences in the archaeological picture as well. The historical sources which mention the south coast are not particularly helpful, except in so far as they provide us with a few dates. To begin with, no chronicler gives an explicit account of the political organization of the south coast at the time it came into contact with the Incas. It can, perhaps, be inferred from Cabello Balboa's statements about common policy toward the Inca invaders that there was some sort of political connection between the valleys of Mala, Cafiete, and Chincha, but the connection need have been no more than a temporary military alliance. Cabello makes no mention at all of Pisco. The valleys of Ica, Nasca, and 125