Geology in the Vicinity of the TYBO and BENHAM Underground Nuclear Tests, Pahute Mesa, Nevada Test Site

Abstract

Recent radiochemical evidence from groundwater characterization and monitoring wells in the vicinity of the TYBO and BENHAM underground nuclear tests in Area 20 of the Nevada Test Site, suggests that migration of radionuclides within groundwater beneath this portion of Area 20 may be more rapid than previously thought. In order to gain a better understanding of the hydrogeologic conditions in the TYBO-BENHAM area for more accurate flow and transport modeling, a reevaluation of the subsurface geologic environment in the vicinity of the two underground tests was conducted. Eight existing drill holes provided subsurface control for the area. These holes included groundwater characterization and monitoring wells, exploratory holes, and large-diameter emplacement holes used for underground nuclear weapons tests. Detailed and consistent geologic descriptions of these holes were produced by updating existing geologic descriptions with data from petrographic, chemical, and mineralogic analyses, and current stratigraphic concepts of the region. The updated descriptions, along with surface geologic data, were used to develop a detailed geologic model of the TYBO-BENHAM area. This model is represented by diagrams that correlate stratigraphic, lithologic, and alteration intervals between holes, and by isopach and structure maps and geologic cross sections. Regional data outside the TYBO-BENHAM area were included in the isopach and structure maps to better evaluate the geology of the TYBO-BENHAM area in a regional context. The geologic model was then evaluated with regard to groundwater flow and radionuclide migration to assess the model's implications for flow and transport modeling. Implications include: 1) confirmation of the general hydrogeology of the area described in previous studies; 2) the presence of two previously unrecognized buried faults that could act as zones of enhanced permeability within aquifers; and 3) secondary alteration within tuff confining units that is much more complex than previously described.iv This page intentionally left blank.

Development of Phenomenological Models of Underground Nuclear Tests on Pahute Mesa, Nevada Test Site - BENHAM and TYBO

“…After drilling and geophysical logging, the working point was further refined to place it below a sharp break in velocity and density data at 1180 m so that the experimenters would have the 122 m of homogeneous media required for their tests. Prothro and Warren (1999) recently updated stratigraphic and lithologic logs in support of UGTA modeling near BENHAM and TYBO. Contacts and descriptions show only minor differences in interpretations from work at the time of the BENHAM test.…”

Completion Report for Well ER-EC-14, Corrective Action Units 101 and 102: Central and Western Pahute Mesa

“…Additional information about Well ER-EC-14 is provided in Table 1-1. The underground nuclear tests closest to and generally upgradient from Well ER-EC-14 are TYBO (U-20y) and BELMONT (U-20as) (Figure 1-2). The TYBO test, located 9.5 km (5.9 mi) to the north-northeast, was conducted below the regional water table (Prothro and Warren, 2001 …”

Well Completion Report for Well ER-20-11, Corrective Action Units 101 and 102: Central and Western Pahute Mesa

“…Lava of the rhyolite of Benham occurs throughout the area. It is exposed at the surface along the up-thrown side of the Boxcar fault northeast of the well site (map unit Tpu on Figure 5-1, and Trpq in Byers and Cummings, 1967) and is present in all wells drilled in the area west of the Boxcar fault (Prothro and Warren, 2001;DOE/NV, 2000a;2000c;NNSA/NSO, 2010a;2010b).…”