The presence and approximate rates of deep percolation beneath areas of native vegetation, irrigated fields, and the Amargosa-River channel in the Amargosa Desert of southern Nevada were evaluated using the chloride mass-balance method and inferred downward velocities of chloride and nitrate peaks. Estimates of deep-percolation rates in the Amargosa Desert are needed for the analysis of regional ground-water flow and transport. An understanding of regional flow patterns is important because ground water originating on the Nevada Test Site may pass through the area before discharging from springs at lower elevations in the Amargosa Desert and in Death Valley. Nine boreholes 10 to 16 meters deep were cored nearly continuously using a hollow-stem auger designed for gravelly sediments. Two boreholes were drilled in each of three irrigated fields in the Amargosa-Farms area, two in the Amargosa-River channel, and one in an undisturbed area of native vegetation. Data from previously cored boreholes beneath undisturbed, native vegetation were compared with the new data to further assess deep percolation under current climatic conditions and provide information on spatial variability.
The profiles beneath native vegetation were characterized by large amounts of accumulated chloride just below the root zone with almost no further accumulation at greater depths. This pattern is typical of profiles beneath interfluvial areas in arid alluvial basins of the southwestern United States, where salts have been accumulating since the end of the Pleistocene. The profiles beneath irrigated fields and the Amargosa-River channel contained more than twice the volume of water compared to profiles beneath native vegetation, consistent with active deep percolation beneath these sites. Chloride profiles beneath two older fields (cultivated since the 1960’s) as well as the upstream Amargosa-River site were indicative of long-term, quasi-steady deep percolation. Chloride profiles beneath the newest field (cultivated since 1993), the downstream Amargosa-River site, and the edge of an older field were indicative of recently active deep percolation moving previously accumulated salts from the upper profile to greater depths.
Results clearly indicate that deep percolation and ground-water recharge occur
not only beneath areas of irrigation but also beneath ephemeral stream channels,
despite the arid climate and infrequency of runoff. Rates of deep percolation
beneath irrigated fields ranged from 0.1 to 0.5 m/yr. Estimated rates of deep
percolation beneath the Amargosa-River channel ranged from 0.02 to 0.15 m/yr.
Only a few decades are needed for excess irrigation water to move through the
unsaturated zone and recharge ground water. Assuming vertical, one-dimensional
flow, the estimated time for irrigation-return flow to reach the water table
beneath the irrigated fields ranged from about 10 to 70 years. In contrast,
infiltration from present-day runoff takes centuries to move through the unsaturated
zone and reach the water table. The estimated time for water to reach the water
table beneath the channel ranged from 140 to 1000 years. These values represent
minimum times, as they do not take lateral flow into account. The estimated
fraction of irrigation water becoming deep percolation averaged 8 to 16 percent.
Similar fractions of infiltration from ephemeral flow events were estimated
to become deep percolation beneath the normally dry Amargosa-River channel.
In areas where flood-induced channel migration occurs at sub-centennial frequencies,
residence times in the unsaturated zone beneath the Amargosa channel could be
longer. Estimates of deep percolation presented herein provide a basis for evaluating
the importance of recharge from irrigation and channel infiltration in models
of ground-water flow from the Nevada Test Site.
Abstract
Introduction
Purpose and Scope
Acknowledgments
Methods of Estimating Deep Percolation
Theory and Assumptions
Applicability of Assumptions
Collection and Analysis of Sediments
Analysis of Depositional Sources and Uncertainty
Atmospheric Deposition
Irrigation Water
Amargosa-River Water
Uncertainty in Deep-Percolation Estimates
Uncertainty in Atmospheric Deposition
Uncertainty in Agricultural Deposition
Uncertainty in Channel Deposition
Analysis of Unsaturated-Zone Profiles
Vertical Profiles of Sediments, Water Status, and Selected Anions
Sediments
Water Potential and Water Content
Chloride, Nitrate, and Sulfate
Cumulative Water and Cumulative Chloride
Cumulative Water
Cumulative Chloride
Cumulative Chloride versus Cumulative Water
Estimated Rates of Deep Percolation
Beneath Native Vegetation
Beneath Fields
Beneath the Amargosa-River Channel
As a Percentage of Irrigation
As a Percentage of Channel Infiltration
Summary and Conclusions
References Cited
Appendix A—Description of sediments
Appendix B—Water content and water potential of sediments, and selected chemical properties of water extracts
Appendix C—Chloride, sulfate, and nitrate concentrations in pore water
For additional information contact:
District Chief
U.S. Geological Survey
333 W. Nye Lane, Suite 203
Carson City, NV 89706-3810
Email: GS-W-NVpublic-info@usgs.gov
Copies of this report can be purchased from:
U.S. Geological Survey,
Information Services
Box 25286
Federal Center
Denver, CO 80225-0286
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