New Mexico Geological Society Annual Spring Meeting — Abstracts

A seismic hazards investigation and the need to understand hydrologic transport through fractured bedrock motivated fracture characterization in the Tshirege Member of the Bandelier Tuff (welded and non-welded units 3, 3T, and 4) at material disposal area P (MDA-P) at Los Alamos National Laboratory. We measured 454 fractures along eight traverses to characterize fracture distribution, orientation, and aperture in three dimensions. Removal of over 50,000 cu yd of waste from MDA-P exposed 87% of the bedrock along traverses. Fractures show a preferred orientation of N15W with some scatter due to cooling joints. Background fracture density in the non-welded tuff is 20 fractures/ 100 ft, representing a tectonic signature. Apertures in all units are bimodal (0-4 mm and 10-14 mm); 79% of large apertures (20-110 mm) are in welded units 3 and 3T. Higher fracture densities (to 40 fractures/ 100 ft) and larger apertures in the welded tuffs demonstrate the influence of cooling joints. Only a few fractures are open; 89% are filled with clay and roots. The fractures dissecting MDA-P have a mean trace length of only 9.5 ft, and therefore are not large structural features. Geologic mapping adjacent to the study area indicates that fracturing may be related to monoclinal folding associated with deformation in the hanging wall of the Pajarito fault. Characterizing fractures at MDA-P also establishes a foundation for modeling flow and transport in fractured Bandelier Tuff.