Environmental and Applied Tracers as Indicators of ... Transport in the Chihuahuan Desert, Texas. Digital Download

RI0207D. Environmental and Applied Tracers as Indicators of Liquid and Vapor Transport in the Chihuahuan Desert, Texas, by B. R. Scanlon. 51 p., 27 figs., 5 tables, 1992. doi.org/10.23867/RI0207D. Downloadable PDF.


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ABSTRACT
Chemical and physical approaches are used to study unsaturated flow; however, few studies include an in-depth analysis of data from both approaches. Detailed chemical tracer studies were conducted at a site in the Chihuahuan Desert of Texas, and the results were compared with hydraulic attributes of the system. Estimated soil-moisture fluxes from both chemical and physical approaches were compared to better understand the hydrologic processes in the unsaturated zone.

Chemical tracer studies included mass balance of the chloride ion. Chloride profiles were measured in ephemeral stream and interstream settings. Moisture fluxes were calculated from measured chloride concentrations on the basis of a steady-state flow model. Chlorine 36 and tritium had been enriched in the atmosphere by weapons tests conducted in the 1950's and 1960's, and the distribution of these radionuclides in the shallow (<2 m) unsaturated zone was used to quantify flux in an ephemeral stream during the past 35 yr. In addition to environmental tracers, a bromide pulse was applied over a 100-m² area, leached by natural precipitation for 1 yr, then recovered and measured in soil cores.

The chloride profiles are characterized by low chloride concentrations near the surface, which increase to a maximum of 1,900 to 9,300 g m^-3 at depths of 1.3 to 4.7 m and gradually decrease with depth below the peak. Calculated moisture fluxes are inversely proportional to chloride concentrations in the soil water because a constant chloride accession rate was assumed throughout the study area, Reductions in chloride concentrations below the peak are attributed to differences in moisture flux as a result of paleoclimatic variations. Moisture fluxes based on the ³⁶Cl and ³H peak depths were 1.4 and 7 mm yr^-l, respectively. The artificially applied bromide pulse penetrated to a depth of 0.3 m during the 1-yr period; however, much of this water movement may have occurred during the initial application of the tracer with irrigation water.

The chemical tracer data record cumulative downward-directed fluxes that contrast with the generally upward driving forces for liquid water movement indicated by the hydraulic data. If water movement were upward, the highest tracer concentrations should occur at land surface; however, maximum ³⁶Cl/Cl ratio and ³H and chloride concentrations occur at depths of 0.5 m, 1.4 m, and 1.3 to 4.7 m, respectively. The discrepancy between the chemical and hydraulic data could be resolved if the tracers moved down by concentration-driven diffusion. Analysis of the ³⁶Cl profile suggests that diffusion alone cannot account for the distribution of 36GI and advection is also involved. Downward percolation is episodic and is better estimated by long-term average fluxes from the chemical tracer data rather than from the limited time period represented by the soil physics monitoring.

The higher moisture flux indicated by the ³H profile relative to that indicated by the ³⁶Cl profile is attributed to enhanced downward movement of ³H in the vapor phase and suggests a vapor flux of approximately 6 mm yr^-I. The vapor flux hypothesis was tested using nonisothermal liquid and vapor flow simulations with the computer code SPLaSHWaTr2. Simulations of 5-day periods in the winter and summer were conducted to represent extremes in the temperature gradients. The calculated vapor flux was 2 to 8 orders of magnitude greater than the liquid flux for the periods simulated. Predicted vapor fluxes were upward in the top 0.04 m of the unsaturated none in the summer and winter periods in response to steep water potential gradients induced by surface evaporation. Below the evaporation front, from depths of 0.15 to 1 m, downward vapor fluxes in the summer were much greater than generally upward vapor fluxes in the winter. These results suggest a net downward vapor flux on an annual basis that is consistent with the chemical tracer results.


Keywords: advection, bomb ³⁶Cl, bomb ³H, bromide, chloride mass balance, diffusion, numerical modeling, vapor transport, Chihuahuan Desert, Texas


CONTENTS

Abstract

Introduction

Background

Previous Work

Purpose of Study

Site Description

Chloride Mass Balance

Anthropogenic Radioisotopes

Applied Tracers

Physical Hydrology

Methods

Field Methods
Laboratory Methods

Numerical Modeling of Non isothermal Water Transport

Governing Equations

Model Input

Results and Discussion

Soil Texture

Chloride Mass Balance, 36CI, 3H, and Bromide Relative Importance of Advection and Diffusion

Validity of Assumptions of Chloride Mass Balance Approach

Solute Transport Parameters

Comparison of Hydraulic and Chemical Approaches

Numerical Simulations of Liquid and Vapor Flux

Comparison of Hueco Bolson Chemical Tracer Data with Data from Other Regions
Chloride Profiles

Distribution of 36CI and 3H

Implications for Site Assessment for Radioactive Waste Disposal

Conclusions

Acknowledgments

References

Appendix: Nomenclature

Figures

1. Location of the study area

2. Temporal variations in precipitation recorded in El Paso and Fort Hancock

3. Location of sampled boreholes and Ca Br 2 irrigation plot

4. Pre-bomb 36Cl/Cl fallout ratios (atoms 36Cl 10-12 atoms Cl) calculated with an atmospheric box model

5. Temporal variations in predicted bomb 36Cl fallout and in bomb 3H fallout
6. Variations in observed soil and air temperatures measured with thermistors and psychrometersfor June 17 and January 17, 1990

7. Variations in observed solar radiation, calculated longwave radiation, and observed air temperature, wind speed, and absolute humidity for June 17 and January 17, 1990

8. Moisture-retention curves for different soil textures

9. Liquid hydraulic conductivity and isothermal and thermal vapor diffusivity as a function of water potential and temperature for different soil textures

10. Soil texture, moisture content, chloride concentration, fitted cubic spline, and calculated moisture flux from six boreholes

11. Vertical profile of 36Cl/CI ratios and 3H concentrations

12. Comparison of the predicted depth of the 36Cl/CI peak based on chloride mass balance data from boreholes 50 and 51 with the observed peal< depth in borehole 51
13. Comparison of the predicted depth of the 3H peak based on chloride mass balance data from boreholes 50 and 51 with the observed peak depth in borehole 52

14. Profiles of bromide and moisture content sampled within 1 OO-m2 test plot 1 yr after artificial application of bromide with an irrigation system
15. Comparison of the calculated 36Cl profile based on diffusion from the upper 0.2 m with the observed 36CI profile

16. Calculated chloride mass balance age

17. Approximation of the 36CI fallout with a step function that was used in the analysis of advective-dispersive transport of 36CI
18. Comparison of the calculated 36CI profile based on an analytical solution of the advective-dispersive equation with the observed 36CI profile
19. Diurnal variations in predicted soil temperatures for June 17and January 17, 1990

20. Comparison of observed and predicted soil temperatures for 1200 hr and 2400 hr on June 17, 1990

21. Comparison of observed and predicted soil temperatures for 1400 hr and 2400 hr on January 17, 1990
22. Diurnal variations in predicted water potentials for June 17 and January 17, 1990

23. Diurnal variations in predicted isothermal vapor flux for June 17 and January 17, 1990

24. Diurnal variations in predicted thermal vapor flux for June 17 and January 17, 1990

25. Diurnal variations in predicted total vapor flux for June 17 and January 17, 1990

26. Net vapor and liquid flux calculated for 5-day simulation period for June 17 and January 17, 1990, for a layered soil system and a uniform sandy loam soil
27. Comparison of 36Cl/CI profiles sampled in Texas, New Mexico, and Nevada

Tables

1. Grain size, soil texture, and volumetric moisture content of soil samples

2. Volumetric moisture content, chloride concentration, moisture flux, moisture velocity, age, cumulative chloride, and cumulative moisture content of soil samples

3. Soil texture, volumetric moisture content, chloride concentration, 36Cl/CI ratios, and 36CI concentrations in samples from borehole 51, and 3H concentrations in samples from borehole

4. Comparison of physical and chloride data from Texas, New Mexico, and Australia
5. Comparison of physical and chemical data from Texas, New Mexico, Nevada, South Australia, and Cyprus



Citation
Scanlon, B. R., 1992, Environmental and Applied Tracers as Indicators of Liquid and Vapor Transport in the Chihuahuan Desert, Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 207, 51 p.