The influence of topography, stratigraphy, and barometric venting on the hydrology of unsaturated Bandelier Tuff

Abstract

This paper presents a preliminary analysis of observations of vadose zone hydrology below two mesas of the Pajarito Plateau near Los Alamos, New Mexico. Abnormal quantities of moisture extend to depths as large as 90ft beneath asphalt-covered areas on the mesas. Comparison with moisture profiles beneath unpaved ground with and without vegetation indicates that atmospheric venting removes moisture from the deep subsurface. At several locations, the profiles of hydraulic head as a function of depth also suggest that the atmosphere may be removing moisture. The profiles reveal intervals where the gradient is such that moisture would flow upward (reversed gradient). A reversed gradient could be generated if moisture were removed from a particular horizon faster than it arrived by conduction and/or vapor diffusion. The vertical intervals with reversed gradient present barriers to downward aqueous transport of contaminants. At some locations, the barriers may be associated with atmospheric venting through nearby excavated disposal pits or open disposal shafts. However, the barriers also occur in stratigraphic horizons that are exposed to the atmosphere at the canyon walls, suggesting that venting may occur at the sides of the mesas. Although continuing investigations of subsurface air flow and rock properties have not yet revealed a model for natural venting that could remove the moisture far from ground surface or a canyon wall, a barrier might be maintained by barometric pumping via fractures or high-permeability layers.