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| New Mexico Water Science Center |
By Edward L. Nickerson
U.S. Geological Survey Scientific Investigations Report 2005-5248
This report is available as a pdf.
In 2003, El Paso Water Utilities installed six deep piezometers at three sites in the lower Mesilla Valley. This report, which was written in cooperation with El Paso Water Utilities, presents the results of an investigation to describe piezometers and ground-water-quality characteristics at these new sites. The report presents piezometer-location and completion information, selected borehole-geophysical logs, water-level data, water-quality data, and water-quality characteristics, including the interpretation of isotope data to estimate the source and apparent age of ground water. Methods of investigation included collection of drilling and piezometer-construction information and analysis of ground-water-quality data collected by the El Paso Water Utilities and the U.S. Geological Survey.
The LMV-1 site is located approximately 1 mile eastsoutheast of Vinton, Texas, along the eastern margin of the lower Mesilla Valley. Piezometer LMV-1A (JL-49-04-498) is completed in the middle Santa Fe Group hydrostratigraphic unit; the screened interval is from 280 to 290 feet below land surface. Piezometer LMV-1B (JL-49-04-499) is completed in the lower Santa Fe Group hydrostratigraphic unit; the screened interval is from 640 to 650 feet below land surface. The freshwater zone, with dissolved-solids concentrations of 1,000 milligrams per liter or less, is estimated to extend from about 430 to 660 feet below land surface.
The LMV-2 site is located in the lower Mesilla Valley approximately 3 miles northwest of Anthony, New Mexico. Piezometer LMV-2A (26S.03E.20.423A) is completed in the middle Santa Fe Group hydrostratigraphic unit; the screened interval is from 680 to 690 feet below land surface. Piezometer LMV-2B (26S.03E.20.423B) is completed in the lower Santa Fe Group hydrostratigraphic unit; the screened interval is from 1,860 to 1,870 feet below land surface. The freshwater zone is estimated to extend from the water table to about 1,900 feet below land surface. Dissolved arsenic concentrations in LMV- 2A and LMV-2B exceeded the proposed U.S. Environmental Protection Agency primary drinking-water standard.
The LMV-3 site is located on the west side of the lower Mesilla Valley approximately 0.8 mile southeast of La Union, New Mexico. Piezometer LMV-3B (27S.03E.20.432D) is completed in the lower Santa Fe Group hydrostratigraphic unit; the screened interval is from 1,745 to 1,755 feet below land surface. The freshwater zone is estimated to extend from less than 190 feet to about 1,320 feet below land surface. The concentration of dissolved arsenic in LMV-3B also exceeded the proposed U.S. Environmental Protection Agency primary drinking-water standard.
Water-quality samples collected by the U.S. Geological Survey from the LMV piezometers were analyzed for the stable isotope ratios δ2H, δ18O, and δ13C. Sample LMV-1A plots slightly above the Global Meteoric Water Line, and its isotopic composition may represent source water from precipitation as mountain-front recharge along the Franklin Mountains. The δ13C compositions in samples from LMV-1A, LMV-2A, LMV- 2B, and LMV-3B ranged from –13.71 to –4.99 per mil. The wide range in values indicates that different processes have affected the carbon compositions and may represent differences in recharge origin and chemical reactions along the groundwater flow path.
Analytical results for the radioactive isotopes tritium (3H) and carbon-14 (14C) were used to estimate the age of ground water in the LMV piezometers. The small tritium values (less than 0.5 picocurie per liter) are indicative of ground water that was isolated from the atmosphere prior to 1954 atmospheric nuclear weapons testing and that has received no post-1954 contribution from recharge. The 14C values ranged from 2.61 to 28.42 percent modern carbon and were used to calculate the apparent age of ground water at the LMV sites. Uncorrected apparent ages ranged from 10,400 to 30,100 years before present and represent maximum calculated ages. Corrected apparent ages ranged from 4,670 to 24,400 years before present and represent minimum calculated ages assuming maximum carbonate dissolution.
Abstract
Introduction
Purpose and scope
Hydrogeologic setting
Methods of investigation
Drilling and piezometer construction
Ground-water sample collection and analysis
Acknowledgments
Description of piezometers and ground-water-quality characteristics
LMV-1 site
LMV-2 site
LMV-3 site
Stable isotopes and source of ground water
Radioactive isotopes and age of ground water
Summary
References cited
Appendix
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For information about water-resource studies in the State of New Mexico, please visit our Web site at http://nm.water.usgs.gov/.
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