Evaluation of Subsurface Flow in Fissured Sediments in the Chihuahuan Desert, Texas

Fissures are surface features, or gullies, some of which are underlain by sediment-filled fractures. A previous study of subsurface flow beneath a fissure showed higher water fluxes beneath the fissure, which was attributed to infiltration of ponded water in the fissure. This study was conducted to investigate the vertical and lateral extent of increased flow associated with fissured sediments, to compare subsurface flow beneath fissures of different maturity, and to examine different techniques for evaluating flow in fissured zones. Boreholes were drilled directly beneath four fissures and at distances of 10 m and 50 m from the fissures, and soil samples were analyzed for various soil physics parameters and environmental tracer distribution. Electromagnetic induction was used to map apparent conductivity in transects perpendicular to the fissures. Fissures had higher water potentials and lower chloride concentration than surrounding sediments. Zones of high flux were restricted to the area directly beneath some fissures, whereas others also had high fluxes in the profiles 10 m distant from the fissure. Water potential and chloride fronts were found beneath two of the fissures in the upper 20-m zone, which indicates that most of the flow occurred in this zone. Water flux estimates, based on the position of the chloride front and an assumed age of the fissures of 50 yr, ranged from 28 to 48 mm yr^-1. High tritium levels were found throughout the fissured profiles (to maximum depth of 26.4 m) and in some cases in the profiles 10 m distant from the fissure, indicating post-1952 water. The occurrence of high tritium levels beneath the chloride front in one fissure indicates that some of the water is flowing preferentially. Minimum estimates of water flux based on the tritium data ranged from 28 to 120 mm yr^-1. Stable isotopes of oxygen and hydrogen were less enriched beneath the fissure, which is consistent with higher fluxes beneath the fissure. Plant water potentials were of limited use in delineating fissure flow. Apparent conductivities were higher across two fissures, whereas the other two fissures did not show any variation in apparent conductivity. The higher conductivity in some fissures is attributed to higher water content. Multiple independent lines of evidence indicate that subsurface water fluxes are higher beneath fissures.