Continuation of Geologic and Hydrologic Studies of Fort Hancock ,Texas

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Scanlon, Bridget R.

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Three chemical tracer studies have been conducted to evaluate the flux of water in the unsaturated zone beneath the Chihuahuan Desert in West Texas and to compare the results with the flux predicted from physical attributes of the system. First, the spatial variability in soil moisture flux was evaluated from 10 chloride profiles located in ephemeral stream and interstream settings. Second, thermonuclear tritium (3H) and chlorine-36 (36Cl) were measured in an ephemeral stream setting to quantify flux in the shallow unsaturated zone during the past 35 years. Third, a bromide pulse was applied over a 100-square meter area, leached by natural precipitation for 1 year, then recovered and measured in soil cores.

Specific moisture fluxes based on chloride mass balance ranged from 0.01 to 1 millimeter per year (4 x 10^4 to 0.04 inches per year). Specific moisture flux based on the 0.5-meter depth of the 36Cl peak was 1.4 millimeters per year (0.06 inches per year), or 0.5% of the mean annual precipitation rate. The 36Cl peak was intensified by repeated alternation of the flux vector from downward (infiltration) to upward (evapotranspiration); therefore, the distribution of 36Cl cannot be simulated with unidirectional water movement. The 3H tracer penetrated much deeper than did the 36Cl tracer, yielding a specific moisture flux of 7 millimeters per year (0.3 inches per year). The deeper penetration of 3H relative to 36Cl is attributed to downward vapor transport controlled by summer temperature gradients.

In contrast to the chemical tracer data that record cumulative downward-directed flux, water potential gradients indicate predominantly upward water movement under isothermal conditions, except in the shallow (1 meter [3 feet]) subsurface immediately after precipitation events. In the shallow unsaturated zone where downward- and upward-directed fluxes of liquid and vapor phases occur, interpretation of chemical tracer data is critical for delineating the net movement of water during a long time period.


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