Dw. Davenport et al., MICROMORPHOLOGY OF PEDOGENICALLY DERIVED FRACTURE FILLS IN BANDELIER-TUFF, NEW-MEXICO, Soil Science Society of America journal, 59(6), 1995, pp. 1672-1683
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.