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.
Calibrating hydrogeologic parameters for the 3-D site-scale unsaturated zone model of Yucca Mountain, Nevada
An important issue in the evaluation of the unsaturated zone (UZ) at Yucca Mountain, Nevada is the calibration of the parameters used in the 3-D site-scale numerical flow model. The hydrogeologic parameters are calibrated using an inversion code (ITOUGH2) to fit measured core sample and in situ data according to statistical criteria. The available data include saturations, water potentials, pneumatic pressure records, and pore water age estimates based on geochemical analyses. Five 1-D columns are extracted from the 3-D site-scale model and used in an inversion that matches data from the five boreholes simultaneously. This results in layer-averaged fracture and matrix permeabilities and van Genuchten fitting parameters. The columns are then refined and individual inversions are performed with each 1-D model to develop a spatially varying parameter set. A number of sensitivity studies are performed to evaluate the effects of modeling assumptions on the calibrated parameters. Several fracture-matrix interaction formulations are also investigated during the study. The results show that the calibrated parameter set concluded to be most consistent with the conceptual model provides a good fit to the ambient data. The study also shows that the choice of the fracture-matrix interaction formulation has a significant effect on the calibrated parameter set.
Time and Heritage
As a historian studying what can be considered a form of intellectual history, I have slowly come to adopt Michel de Certeau’s observation made at the end of the 1980s that ‘objectifying the past, for the last three centuries, has undoubtedly left unconsidered time within a discipline that has continued to use it as a taxonomic instrument’. To a certain degree, time has become commonplace for the historian who has preserved or instrumentalized it. It is not considered because it is inconceivable, but because we do not think of it or, more simply, we do not think about it. As a historian attempting to pay attention to the time I’m living in, I have thus, like many others, noticed the swift development of the category of the present until it has become obvious that the present is omnipresent. This is what I refer to here as ‘presentism’.