Investigators:
Barry Hibbs
Chris Eastoe
Fred Phillips
James Hogan


Overview

NSF, through a "Glue Grant", has recently funded a joint research program between SAHRA and CEA-CREST. CEA-CREST (The Center for Environmental Analysis at Cal State-LA) promotes the development and testing of theories predicting natural and anthropogenic changes in ecosystems, with a particular emphasis on Southern California and the southwestern U.S. (Additional information on the CEA-CREST Glue Grant is in Appendix F). One effort of the Glue Grant is focused on understanding the causes of groundwater and surface water salinization in the region around City of El Paso/Ciudad Juarez international metroplex.

The Rio Grande and the trans-boundary aquifers of the Las Cruces NM/ El Paso TX/ Ciudad Juarez ,MX region are the sole water resources for the 2 million people and 178,000 acres of irrigated agriculture of the region. This region, like many in the southwest, is experiencing dramatic population growth and is projected to reach 3.2 million by 2010 (Lowery, 1995). Issues of water quantity and water quality are extremely important for this region due to degradation of surface waters in the Rio Grande (Figure 1), overpumping of the local aquifers, and salinization of groundwater resources (Figure 2).

Water supply and water quality problems facing the City of El Paso and Ciudad Juárez are complex and interrelated. Over-pumping of shared water resources from the Hueco Bolson has resulted in excessive drawdown of the water table, encroachment of brackish groundwater, and the early retirement of wells because chloride now exceeds the maximum recommended limit of 250 mg/L (see below). Chloride data in time series tend to correlate to drawdown in wells. Water quality decline threatens existing freshwater resources in these aquifers, which are being extracted at a rate 15 to 20 times the rate of natural recharge.

(Time series diagrams comparing drawdown and salinity increases in the Hueco Bolson aquifer, El Paso and Juarez)

Most of the previous studies of salinization were based on anecdotal analysis. To determine the causes of salinization more accurately, the CEA-CREST - SAHRA research team is age-dating groundwaters with radioisotopes (carbon-14 and tritium), tracking stream/aquifer interactions with stable isotopes (oxygen, hydrogen, carbon, and sulfur), and assessing mixing of saline and fresh waters with halides (chloride, iodide, and bromide). Other institutions collaborating in this study include New Mexico State University, Universidad Autónoma de Ciudad Juárez, and United Nations University. Governmental agencies are also participating in the project by providing existing data, access to water wells, and other support services. Participating agencies include El Paso Water Utilities, US Environmental Protection Agency, US Army Fort Bliss, International Boundary and Water Commission, Comisión Internacional de Limites y Aguas, Comisión Nacional del Agua, and Junta Municipal de Agua y Saneamiento de Ciudad Juárez.

Scope of Work

Our research is focused around the following four questions:

1. What are the flowpaths and residence times of water within aquifers of the El Paso region (e.g. Hueco-Tularosa aquifer, Rio Grande - Rio Bravo aquifer, Mesilla Bolson aquifer)?

The spatial and temporal dynamics of aquifer systems are difficult to address directly because they occurs entirely below the land surface. Over the next two years, we propose to collect and analyze a suite of isotopic tracers from the aquifers of the El Paso region. These tracers will enable us to understand the spatial dynamics of the system by tracing water from areas of recharge to regions of discharge, as well as the temporal dynamics by understanding the residence time of groundwaters within the aquifer system.

Because of the large population and irrigated agriculture in this region, there are an abundance of wells providing good spatial coverage of the aquifer system. We plan to measure O and H isotopes in order to trace water sources within the aquifer system, ³H to identify regions of recent recharge, and ¹⁴C to constrain the residence time of groundwater. Additionally, we may employ a suite of solute isotopes (e.g. B, Sr, S) if distinct regions of the aquifer have specific solute sources and isotopic compositions. Our plan is to start with the US portion of these aquifers, but we are optimistic that we can extend our work into Mexico to provide complete coverage of these trans-boundary aquifers.

2. How do aquifer flowpaths control the introduction of solutes and ultimately the salinization of groundwaters?

As groundwater moves along the basin axis from recharge areas toward discharge areas in the Rio Grande Valley, the groundwater evolves hydrochemically. Groundwater in areas of active recharge is characterized by fairly low concentrations of dissolved ions, dominantly calcium, magnesium, and bicarbonate. As groundwater moves laterally along the basin axis there is an increase in total dissolved solids. In general, cation exchange processes results in an increase in sodium ions, whereas the relative concentrations of calcium and magnesium ions decrease. Sulfate and chloride ions also increase due to dissolution of evaporite minerals, such as gypsum.

Salinization of potable groundwater in El Paso and Juarez also coincides with drawdown in the aquifer (Figure 2), with some wells now exceeding the USEPA maximum recommended limit for chloride (250 mg/L Cl). Most of the information on salinization of water wells in the Hueco Bolson is anecdotal (Hibbs, 1999). Pumping may induce inferior quality water to move to wells where drawdown cones have reversed the natural hydraulic gradient (Figure 3). Several possible sources of saline waters have been suggested, these include: upconing of saline groundwater; leakage of saline groundwater from mud interbeds; downward movement of saline groundwater from the brackish zone near the Rio Grande; and lateral migration from the saline groundwaters along the axis of the basin (Figure 4).

Our studies will use a variety of geochemical and isotopic tracers to answer questions about increasing salinity in the developed parts of this threatened aquifer. Boron isotopes, for example, may be used to help discriminate between mixing end-member waters such as background groundwater, water from anthropogenic sources such as treated municipal wastewater, and irrigation-affected water (Bassett et al., 1995). Oxygen and hydrogen isotopes will be used to help distinguish between other saline end-members, including evaporated water in the Rio Grande alluvium and deeper basinal groundwaters. Comparison of ionic ratios of Cl, Br, and I may also be very helpful in providing insights on mixing mechanisms and sources of salinity.

A better understanding of the processes responsible for salinization of water wells will allow local water resources managers to make informed decisions about use of these wells, which is particularly useful to a broad range of SAHRA researchers. For example, where salinization of wells is caused by a brackish water zone against the top portion of a multi-level screen (Figure 4e and 4f), it will be possible to isolate this part of the formation by casing-off the upper well screen. Likewise, identification of strata that are less likely to be affected by salinity will be of great utility in identifying proper injection zones for artificial recharge wells.

3. What is the interaction between the surface water of the Rio Grande and the associated groundwaters of the Rio Grande aquifer? What role does saline groundwater play in the salinity increases in the Rio Grande?

As mentioned previously, the Rio Grande exhibits large, and localized, salinity increases below El Paso. With an increased understanding of the flowpaths of the aquifer systems we can begin to address stream-aquifer interactions between the groundwater system and Rio Grande. Specifically, we should be able to identify regions where groundwater discharges to the river system, and vice-versa. By combining our understanding of isotopic and geochemical changes in the river system (work currently being done within SAHRA-TA2), with the information about the groundwater systems (this proposed collaboration), we can calculate fluxes of water and solutes from the groundwater system to the river system. Ultimately, such geochemical and isotopic information will constrain physical models of groundwater-surface H²O interactions.

4. How has climate change affected groundwater recharge in this region? How much of the groundwater in storage today is Pleistocene water?

Geologic evidence of climate change in the El Paso region (paleontological and geomorphologic data) indicates that the region was significantly cooler and moister during the pluvial periods of the late Pleistocene Epoch. The late Wisconsinan glacial period from 25,000 to 14,000 years b.p. was a time of cooler and moist climate throughout the region (Wells et al., 1982; Hall, 1985). Evidence from other regional aquifers indicates that recharge rates were generally much higher during the late Pleistocene (Phillips et al., 1986; Swift, 1993). Evidence from radioisotopes and stable isotopes of oxygen and hydrogen appear to corroborate this, indicating that some of the groundwater in adjacent basins (Eagle Flat and Red Light Draw) was recharged more than 14,000 years b.p. (Darling et al., 1998). Recharge today may be a fraction of the recharge during the late Pleistocene, resulting in a lowering of regional water table elevations as the climate changed.

Accordingly, we are using ¹⁴C to constrain the age of groundwaters and to quantify the amount of groundwater in the Hueco Bolson recharged during the late Pleistocene. We also plan to measure O and H isotopes (*¹⁸O and *D) because these have proven useful for identifying recharge of precipitation that fell during the cooler climate of the Pleistocene (Darling et al., 1998). Such work will help us understand groundwater availability issues in the context of groundwater extraction that is occurring today. This work will also have important ramifications for other hydrogeologic issues in this region, such as waste disposal. There are presently several research projects within SAHRA's TA2 that are examining groundwater recharge, both the present day recharge mechanisms and how recharge rates have changed since the late Pleistocene. As such, questions of groundwater recharge may provide an area for future collaborations.

Activities and Results

Formal agreements for collecting groundwater samples on the U.S. side of the international border were established with El Paso Water Utilities and US Army Fort Bliss. Both entities operate a large number of water wells. Formal agreements were also established with Universidad Autónoma de Ciudad Juárez (UACJ) for collaborating on the study. UACJ, in turn, was given permission to sample water wells operated by Mexican governmental entities. Water supply companies for Horizon City, Fabens, and Tornillo (Lower Valley cities) were contacted about sampling their deeper water supply wells. Discussions have been positive, and Memoranda of Understanding (MOUs) are being prepared to sample these wells.

To date, forty-three groundwater samples have been collected from the Hueco Bolson aquifer and flanking highlands. Twenty of these wells are located in the Mexican portion of the aquifer. Three deeper wells were sampled in the Lower Valley, four wells were sampled in the Franklin and Hueco Mountains, and two wells were sampled in the New Mexico part of the Hueco Bolson. Stable isotope, radioisotope, and general minerals analysis have been performed on these samples. The data show promising variations in all parameters. Additional sampling is currently underway.

Plans

This project began in Fall 2001. We have subsequently recruited the graduate students for this project, established many agreements for collecting groundwater samples on both sides of the border, and begun initial sampling and analysis. We will continue collection and analysis throughout 2003. We plan to perform an initial analysis of the isotopic results in order to refine our sampling strategy. We hope to evaluate the role of saline groundwater in the solute balance of the Rio Grande in this region.

Shared Resources

SAW Group Laboratories and Field Equipment, Cal State-LA
Dr. Hibbs' wet chemistry laboratory in the Department of Geological Sciences is equipped to measure the full suite of standard inorganic constituents and many trace elements. Analytical equipment includes a new Dionex DX600 Ion Chromatograph with UV detector, a Perkin-Elmer 5000 Atomic Absorption Spectrophotometer, and a Hach DR/4000 UV-VIS spectrophotometer. Hibbs' wet chemistry lab is one of four water quality labs operated by CEA's SAW Group. Additional equipment includes a ThermoFinnigan LC/MS/MS system, a Perkin-Elmer Series II CHN Analyzer, a Micrometrics Surface Area Analyzer, and a Dionex Solvent Extraction System, all recently funded by Cal-State LA to support SAW Group activities. Other equipment includes an ICP-MS, HPLC, additional AA and IC units, and a gas chromatograph.

In addition to standard hydrogeological field equipment (ph/Eh meters, conductivity meters, E-lines, sampling apparatus), Dr. Hibbs has a number of field instruments that may be used in this study, including a new Sting Earth Resistivity Meter, a Bison Geopro Reflection/Refraction Seismograph, a new Turner Designs Fluorometer, and two new Marsh-McBirney Electromagnetic Flowmeters.

Geographical Information Systems, Cal State-LA
The Center for Spatial Analysis and Remote Sensing (CSARS) offers considerable computational power in service to CEA research and figures prominently in CEA plans for student recruitment and training. CSARS has a total of twenty?five Pentium PC's that run Windows NT versions of GIS software with all key extensions, and various image processing software. The workstations have been heavily used for classroom GIS teaching, outreach related workshops, and research activities. CSARS also has UltraSparc Sun workstations. Installed software packages include ArcView, Arc/Info, ENVI/IDL, Fortran, C, and JAVA compilers. These software packages can be accessed from CSARS PC's through X?windows. A Sun Workstation is dedicated to license manager software for GIS and image processing. The Virtual Center for Spatial Analysis and Remote Sensing (VCASRS) is a second distributed computing facility linked with CSARS. It includes numerous workstations for spatial analysis, and instrumentation devoted to remote and close-up sensing of landscapes. Two Sun workstations are heavily used by the image processing and GIS tasks related to CEA Components, a NASA-BOREAS project and an NSF-CRUI project.

Laboratory of Isotope Geochemistry, University of Arizona
The Laboratory of Isotope Geochemistry conducts research which focuses on low-temperature isotopic and hydrogeochemical studies. Experimental instrumentation includes six low-level beta counters for radiocarbon and tritium measurements, three isotope ratio mass spectrometers for measurements of the stable isotopes of carbon, hydrogen, oxygen, sulfur and chlorine, and an automated device for detecting O and H isotopes in water.

The lab is currently involved in several SAHRA related projects including: identification of the sources of active ground-water recharge in the Tucson basin by identifying the bomb-pulse tritium in ground water and by radiocarbon measurements; study of the origin of sulfate in ground water; using O and H isotopes in water to trace water sources to, and quantify evaporative losses from, the Rio Grande; and stable chlorine isotope analysis to understand diffusion processes in desert vadose zones.

The lab, led by Dr. Austin Long, has five permanent personnel and receives funding through sample analysis in conjunction with numerous projects. Dr. Chris Eastoe, a permanent research staff member, will serve as the main collaborator and contact for this proposed project.


References

Bassett, R.L., Buszka, P.M., Davidson, G.R., and Chong-Diaz, D., 1995, Identification of groundwater solute sources using boron isotopic composition: Environmental Science & Technology, 2:.2915-22.

Darling, B.K., Hibbs, B.J., and Sharp, J.M., Jr., 1998, Environmental isotopes as indicators of the residence time of ground waters in the Eagle Flat and Red Light Draw Basins of Trans-Pecos, Texas: in The Search Continues into the 21st Century, ed. by W.D. DeMis and M.K. Nelis, West Texas Geological Society Publ. #98-105, p.259-270.

Hall, S.A., 1985, Quaternary pollen analysis and vegetational history of the southwest, in Pollen Records of Late-Quaternary North America, ed. by V.M Bryant., Jr., and R.G. Holloway, Dallas, Texas, American Association of Stratigraphic Palynologists, p.95-123.

Hibbs, B.J., 2000, Numerical simulation of ground-water flow and aquifer flow capacity in a Chihuahuan Desert aquifer, Hydrological Science and Technology, 16(4):.200-212.

Hibbs, B.J., 1999, Water quality and hydrogeologic issues along the City of El Paso/Ciudad Juarez corridor - international case study, Environmental & Engineering Geosciences (5)1:.27-39.

Hibbs, B.J., and Boghici, R., 1999, On the Rio Grande aquifer: flow relationships, salinization, and environmental problems from El Paso to Fort Quitman, Texas, Environmental & Engineering Geosciences, 5(1): 51-59.

Lowery, N.A., 1995, Binational water management, a case study of the Binational Water Program/Program Binacional de Agua for the El Paso/Ciudad Juarez Region: in Jensen, R., ed., Proceedings of the 24th Water for Texas Conference, Research Leads the Way, p.625-633.

Phillips, F.M.; L.A. Peeters; M.K. Tansey; and S.N. Davis, 1986, Paleoclimatic inferences from an isotopic investigation of ground water in the central San Juan Basin, New Mexico: Quat. Res., 26: 179-193.

Swift, P.N., 1993, Long-term climate variability at the Waste Isolation Pilot Plant, southeastern New Mexico, U.S.A, Environment Management, 17: 83-97.

Wells, S.G., Bullard, T.F., and Smith, L.N., 1982, Origin and evolution of deserts in the Basin and Range and Colorado Plateau Provinces of western North America, in The geological story of the world's desert: Stria, v.17, p.101-111.