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

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1994

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Localized flow in fissured sediments in arid settings has important implications for waste disposal in these regions. Fissures are surface features or gullies that are underlain by partially open or sediment-filled fractures. The objectives of this study were to compare unsaturated flow beneath different fissures, investigate the vertical and lateral extent of increased flow associated with fissured sediments, and examine different techniques for evaluating flow in fissured zones. Boreholes were drilled directly beneath four fissures and at distances of 10 and 50 m from the fissures. Sediment samples were analyzed for hydraulic parameters such as water content and water potential and environmental tracers such as Cl, 36Cl, 3H, 2H, and 18O. A trench was dug beneath one fissure for detailed sampling. Electromagnetic induction was used to measure apparent electrical conductivity in transects perpendicular to the fissures. Unsaturated flow is relatively high beneath fissures, as evidenced by higher water potentials and lower chloride concentrations there than in surrounding sediments. The lateral extent of high water flux was restricted to the zone directly beneath one fissure but extended to profiles 10 m from two other fissures. The profiles 50 m distant from all fissures had low water fluxes, as indicated by low water potentials and high maximum chloride concentrations. The vertical extent of high water fluxes was restricted to the upper 10 to 20 m, as shown by water potential and chloride fronts within the upper 10 m zone beneath one fissure and by chloride fronts in the upper 20 m zone beneath and 10 m distant from another fissure. Additional evidence for localized water flux was provided by high tritium levels, less-enriched 2H and 18O, and higher plant water potentials in fissured sediments than in nonfissured sediments. Apparent electrical conductivity was higher in two of the four fissures. Multiple independent lines of evidence indicate that subsurface water fluxes are higher at shallow depths beneath fissures; however, the various techniques differ in their effectiveness in delineating higher water fluxes beneath fissures. Multiple profiles drilled in one fissure indicate that there is large variability in flow along this fissure that is attributed to topographic variations and degree of ponding.

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