Micromorphology of Pedogenically Derived Fracture Fills in Bandelier Tuff, New Mexico

Abstract

AbstractFractures in the Bandelier Tuff are potential paths for water movement and transport of contaminants from waste disposal sites and other contaminated areas at Los Alamos National Laboratory, Los Alamos, NM. Contaminants transported in this way could ultimately be found in Los Alamos drinking water or in the Rio Grande, which flows through heavily populated areas in both the USA and Mexico. We conducted this study to determine (i) the morphology and origin of soil‐like material in the fractures, and (ii) the likelihood of significant water movement through the fractures. We examined thin sections of fracture fills, soils, and tuffs to obtain fabric and mineralogical data, and collected field data on soil horizons, color, texture, structure, clay films, and root abundance. Fracture fills consist of clay, CaCO3, or combinations of the two with minor inclusions of tuff and sand grains. Clay consists of thick, highly oriented argillans aligned parallel to fracture walls, and of discrete books in fracture interiors. Carbonate consists of massive microcrystalline calcite, which completely fills some fractures, and laminae or infillings between clay laminae or books in clay‐dominated fractures. The carbonate was precipitated after clay deposition, suggesting a change to a more arid climate. Weaker development of argillans and carbonate features in the soils suggests that the fractures may be derived from older soils that have been stripped by erosion. The presence of live roots throughout the fracture fills indicates the presence of water, but the smectitic clay and massive carbonate make it unlikely that significant water movement is now taking place through the fractures. The potential creation of new macropores by a variety of processes, however, including seismic activity and biologic disturbance, could allow rapid water movement and contaminant transport.