MICROMORPHOLOGY OF PEDOGENICALLY DERIVED FRACTURE FILLS IN BANDELIER-TUFF, NEW-MEXICO

Fractures in the Bandelier Tuff are potential paths for water movement and transport of contaminants from waste disposal sites and other con taminated areas at Los Alamos National Laboratory, Los Alamos, NM. Con taminants transported in this way could ultimately be found in Los Ala mos drinking water or in the Rio Grande, which hows through heavily po pulated areas in both the USA and Mexico. We conducted this study to d etermine (i) the morphology and origin of soil-like material in the fr actures, and (ii) the likelihood of significant water movement through the fractures. We examined thin sections of fracture fills, soils, an d tuffs to obtain fabric and mineralogical data, and collected field d ata on soil horizons, color, texture, structure, clay films, and root abundance. Fracture fills consist of clay, CaCO3, or combinations of t he 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 mas sive microcrystalline calcite, which completely fills some fractures, and laminae or infillings between clay laminae or boob in clay-dominat ed fractures. The carbonate was precipitated after clay deposition, su ggesting a change to a more arid climate. Weaker development of argill ans and carbonate features in the soils suggests that the fractures ma y be derived from older soils that have been stripped by erosion. The presence of live roots throughout the fracture fills indicates the pre sence of water, but the smectitic clay and massive carbonate make it u nlikely that significant water movement is now taking place through th e fractures. The potential creation of new macropores by a variety of processes, however, including seismic activity and biologic disturbanc e, could allow rapid water movement and contaminant transport.