ANALYSIS OF FOCUSED UNSATURATED FLOW BENEATH FISSURES IN THE CHIHUAHUAN DESERT, TEXAS, USA

Localized flow beneath fissures in arid settings has important implica tions for waste disposal in these regions. Fissures are surface featur es or gullies that are underlain by sediment filled fractures. The obj ectives of this study were to compare unsaturated flow beneath differe nt fissures, investigate the vertical and lateral extent of increased flow associated with fissures, and examine different techniques for ev aluating flow in zones containing fissures. Boreholes were drilled dir ectly beneath four Fissures and at horizontal distances of 10 and 50 m from the fissures. Physical parameters such as water content and wate r potential were analyzed in sediment samples and water potential was analyzed in plant samples. Environmental tracers such as Cl, Cl-36/Cl, H-3, D, and O-18 were analyzed in sediment samples. A trench was dug beneath one fissure for detailed sampling. Electromagnetic induction w as used to measure apparent electrical conductivity in transects perpe ndicular to the fissures. Unsaturated flow is relatively higher beneat h fissures, as evidenced by higher water potentials and lower chloride concentrations there than in surrounding sediments. The lateral exten t of high water flux was restricted to the zone directly beneath one f issure but extended to profiles 10 m from two other fissures. The prof iles 50 m from all fissures had low water fluxes, as indicated by low water potentials and high maximum chloride concentrations, The vertica l extent of high water fluxes was restricted to the upper 10 to 20 m z one, as shown by water potential and chloride fronts within the upper IO m zone beneath one fissure and by chloride fronts in the upper 20 m zone beneath and 10 m from another fissure. Additional evidence for l ocalized water Bur was provided by less enriched D and O-18, and highe r plant water potentials in sediments beneath fissures relative to sed iments adjacent to fissures. High tritium levels were found in all sam pled profiles and cannot readily he explained. Apparent electrical con ductivity 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 diff er in their effectiveness in delineating higher water fluxes beneath f issures. 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 pending. (C) 1997 Elsevier Scienc e B.V.