Five-Year Interim Report of the United States – Mexico 

Transboundary Aquifer Assessment Program: 2007 – 2012

By William M. Alley, editor

Open-File Report 2013–1059 

U.S. Department of the Interior

U.S. Geological Survey
ii

U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2013 

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Suggested citation:

Alley, W.M., ed., 2013, Five-year interim report of the United States – Mexico Transboundary Aquifer Assessment 

Program: 2007 – 2012: U.S. Geological Survey Open-File Report 2013–1059, 31 p. 

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endorsement by the U.S. Government. 

Although this report is in the public domain, permission must be secured from the individual 

copyright owners to reproduce any copyrighted material contained within this report.
iii

Contents

Executive Summary....................................................................................................................................................... 1

Introduction.................................................................................................................................................................... 3

Arizona/Sonora Section of the Transboundary Aquifer Assessment Program............................................................... 5 

 by James Callegary, Sharon B. Megdal, Christopher A. Scott, and Prescott L. Vandervoet

Program Overview ..................................................................................................................................................... 5

Partners ..................................................................................................................................................................... 7

Binational Activities .................................................................................................................................................... 9

Accomplishments..................................................................................................................................................... 11

Future Plans (subject to funding level)..................................................................................................................... 13

References............................................................................................................................................................... 13

Project Publications.............................................................................................................................................. 15

(A) Papers and Reports .................................................................................................................................... 15

(B) Presentations and Proceedings .................................................................................................................. 16

Mesilla Basin/Conejos-Médanos Section of the Transboundary Aquifer Assessment Program .................................. 19

 by Zhuping Sheng, Michael Darr, J. Phillip King, John Bumgarner, and Ari Michelsen

Program Overview ................................................................................................................................................... 19

Partners ................................................................................................................................................................... 22

Binational Activities .................................................................................................................................................. 22

Accomplishments..................................................................................................................................................... 23

Future Plans (subject to funding level)..................................................................................................................... 27

References............................................................................................................................................................... 29

Project Publications.............................................................................................................................................. 30

Figures

1. Map showing the locations of Upper Santa Cruz and Upper San Pedro River Basins............................................... 6 

2. Map showing the locations of the Mesilla Basin/Conejos-Médanos and Hueco Bolson aquifer systems. ................ 20

3. Photograph showing project team members at the binational meeting at IBWC, August 20, 2009.......................... 23

Tables

1. Key partners in the Arizona/Sonora Transboundary Aquifer Assessment Program (TAAP). ..................................... 8 

2. Key partners in the Mesilla Basin/Conejos-Médanos Transboundary Aquifer Assessment Program (TAAP).......... 22
Five-Year Interim Report of the United States – Mexico 

Transboundary Aquifer Assessment Program: 2007 – 2012

By William M. Alley, editor

Executive Summary 

Transboundary aquifers are an essential, and in many cases, singular source of water for United 

States – Mexico border communities, particularly in arid regions. Declining water levels, deteriorating 

water quality, and increasing use of groundwater resources by municipal, industrial, and agricultural

water users on both sides of the international border have raised concerns about the long-term 

availability of this supply. Water quantity and quality are determining and limiting factors that 

ultimately control agriculture, future economic development, population growth, human health, and 

ecological conditions along the border. Knowledge about the extent, depletion rates, and quality of 

transboundary aquifers, however, is limited and, in some areas, completely absent. 

The U.S. – Mexico Transboundary Aquifer Assessment Act (Public Law 109-448), referred to in 

this report as “the Act,” was signed into law by the President of the United States on December 22, 

2006, to conduct binational scientific research to systematically assess priority transboundary aquifers 

and to address water information needs of border communities. The Act authorizes the Secretary of the 

Interior, through the U.S. Geological Survey (USGS), to collaborate with the States of Arizona, New 

Mexico, and Texas through their Water Resources Research Institutes (WRRIs) and with the 

International Boundary and Water Commission (IBWC), stakeholders, and Mexican counterparts to 

provide new information and a scientific foundation for State and local officials to address pressing 

water-resource challenges along the U.S. – Mexico border. Objectives of the U.S. – Mexico 

Transboundary Aquifer Assessment Program are to

 

• Develop binational information and shared databases on groundwater quantity and quality;

• Assess the extent, availability, and movement of water in transboundary aquifers and the 

interaction with surface water;

 

• Develop and improve groundwater-flow information for binational aquifers to facilitate waterresource assessment and planning; 

• Analyze trends in groundwater quality, including salinity, nutrients, toxins, and pathogens; 
2

 

• Assess temporal and spatial variability of subsidence related to aquifer depletion; 

 

• Apply new data, models, and information to evaluate strategies to protect water quality and 

enhance supplies; and

• Provide useful information to decision makers, including assessments of groundwater 

management institutions and policies.

The Act states that not later than 5 years after the date of its enactment, and on completion of the 

program in fiscal year (FY) 2016, the Secretary of the Department of the Interior “shall submit to the 

appropriate water resource agency in the Participating States, an interim and final report.” This interim 

report describes the U.S. – Mexico Transboundary Aquifer Assessment Program (TAAP) activities and 

accomplishments to date. 

Activities in support of the Act are carried out through the TAAP, a unique Federal agency – 

university – binational partnership. The program is authorized for a total of $50 million for FY 2007 

through 2016. Appropriations were $500,000 for FY 2008 and 2009, and $1 million for FY 2010. No 

funding was provided in FY 2011 and 2012. The USGS received 50 percent of all funds, and the three 

State WWRIs (Arizona, New Mexico, and Texas) shared the other 50 percent equally. Part of the WRRI 

funding was matched with contributions from Mexico to support the binational research. 

The Act specified the Hueco Bolson and Mesilla Basin as priority aquifers for New Mexico and 

Texas, and allowed for additional aquifers to be identified for these two States. Based on the increasing 

use of the aquifer, lack of information, and stakeholder input, activities have focused on the Mesilla 

Basin in the United States and its counterpart in Mexico—the Conejos-Médanos aquifer system. For 

Arizona, the Act limits study to the Santa Cruz and San Pedro aquifers in south-central Arizona and 

northern Sonora. Accomplishments of the program to date include 

• Binational research plans were developed in collaboration with State and local stakeholders to ensure the 

program meets user needs. 

 

• A U.S. – Mexico Joint Cooperative Agreement was developed to allow the sharing of transboundary 

aquifer data, models, and information and to coordinate and collaborate on binational scientific 

investigations. The agreement was signed by the U.S. and Mexican Sections of the IBWC. Prior to this 

agreement, there had been limited and unofficial sharing of information or coordination of 

investigations. 

• More than 24 binational meetings were held to develop the agreement, exchange information, and jointly 

plan and coordinate aquifer investigations. 

 

• Existing U.S. hydrologic and geologic data, well records, associated databases, and geographic 

information systems were assembled, compiled, and analyzed to help determine information needs. 

 

• Activities for the Santa Cruz and San Pedro aquifers focused on improved hydrogeologic 

characterization linked to institutional and sociodemographic assessments. Building on existing studies 

of both aquifers, the Arizona /Sonora team compiled and integrated water-level and geologic log data 

and undertook geophysical surveys to improve the definition of the hydrostratigraphic units, determine 
3

hydraulic parameters, analyze groundwater levels, and provide an initial characterization of aquifer 

hydrogeochemistry. 

• The Mexican National Water Commission (CONAGUA), the Mexican Geological Service, and the 

University of Sonora, with funding support from the Mexican Federal Government, conducted parallel 

collaborative research for the aquifers in Arizona/Sonora through the TAAP to develop and provide 

compatible aquifer information. 

• Activities for the Mesilla Basin focused on (1) updating the geohydrologic framework for the U.S. and 

Mexican parts of the basin; (2) advances in the development of an integrated hydrologic model to 

account for agricultural water-budget components, the largest water-use category in the basin; 

(3) evaluation of surface-water and groundwater interaction and interbasin exchange with neighboring 

aquifers; and (4) water-quality sampling for isotopic and environmental trace constituents in selected

wells to assist with a water-quality and age-dating analysis. 

• A regional hydrogeologic reconnaissance study of the Conejos-Médanos aquifer system, with new 

aquifer and geohydrologic information, was completed by the Mexican Geological Service. 

Introduction

The U.S. – Mexico Transboundary Aquifer Assessment Act (Public Law 109-448), referred to in 

this report as “the Act,” was signed into law by the President of the United States on December 22, 

2006. The Act authorizes the Secretary of the Interior, through the U.S. Geological Survey (USGS), to 

collaborate with the States of Arizona, New Mexico, and Texas through their Water Resources Research 

Institutes (WRRIs) to conduct hydrogeologic characterization, mapping, and modeling of priority 

transboundary aquifers. The Act specified the Hueco Bolson and Mesilla Basin as priority aquifers for 

New Mexico and Texas and allowed for additional aquifers to be identified for these two States.

Activities to date have focused on the Mesilla Basin in the United States and its equivalent in Mexico—

the Conejos-Médanos aquifer system. For Arizona, the Act limits the selection of priority aquifers to the 

Santa Cruz and San Pedro aquifers. 

The Act states that not later than 5 years after the date of its enactment, and on completion of the 

program in fiscal year 2016, the Secretary of the Interior “shall submit to the appropriate water resource 

agency in the Participating States, an interim and final report, respectively, that describes (1) any 

activities carried out under the program; (2) any conclusions of the Secretary relating to the status of 

priority transboundary aquifers; and (3) the level of participation in the program of entities in Mexico.” 

This interim report addresses items (1) and (3). The Transboundary Aquifer Assessment Program 

(TAAP) is well positioned to describe the status of priority transboundary aquifers as future funding 

enables the program to carry out planned activities. 

Funding for the TAAP was split equally among the States of Arizona, New Mexico, and Texas. 

Furthermore, each State’s funds are split between the USGS and the State’s WRRI, located at one of the 

State’s universities as stipulated by the Water Resources Research Act. The WRRIs have active, 

ongoing partnerships with the USGS, State agencies, and Mexico. This mechanism for TAAP 

implementation is a unique model of Federal agency – university – binational partnership for program 

implementation.

Six binational meetings were held with the Commissioners and Principal Engineers of the U.S. 

and Mexican Sections of the International Boundary and Water Commission (IBWC), Mexican National 
4

Water Commission, USGS, and the three WRRIs to negotiate and finalize an agreement for scientific 

exchange, coordination, and collaboration between U.S. and Mexican agencies, organizations, and 

scientists regarding the TAAP. An agreement was signed by IBWC and CILA (Note: U.S. and Mexican 

Sections of IBWC are identified as IBWC and CILA (Comisión Internacional de Limites y Aquas), 

respectively) on August 19, 2009, in a ceremony on the International Bridge between El Paso and 

Juarez. This agreement allows USGS and WRRIs to coordinate the activities of the binational technical 

group participating in the project through the IBWC/CILA. 

Activities of the Arizona/Sonora and Mesilla Basin/Conejos-Médanos sections of the TAAP are 

described in this interim report. The work is the joint effort of many individuals. The authors appreciate 

the efforts of three key individuals who unfortunately passed away during the course of the program: 

Carlos Marin, Commissioner, International Boundary and Water Commission; Arturo Herrera, 

Commissioner, Comisión Internacional de Limites y Aguas; and Bobby Creel, New Mexico Water 

Resources Research Institute. 
5

Arizona/Sonora Section of the Transboundary Aquifer Assessment Program

By James Callegary,

1 Sharon B. Megdal,2 Christopher A. Scott,3,4 and Prescott L. Vandervoet4

Program Overview

The Upper Santa Cruz and the Upper San Pedro Rivers are binational watercourses that cross the 

international border between the State of Arizona and Sonora, Mexico. The basins that support these 

rivers are adjacent, with the San Pedro lying to the east of the Santa Cruz. Both the Santa Cruz and San 

Pedro Rivers flow north to join with the Gila River, which in turn forms part of the lower Colorado 

River Basin (Figure 1). Because of the semiarid climate with low rainfall and high evaporation, both the 

Santa Cruz and San Pedro Rivers are ephemeral in certain reaches. Human populations within each 

basin have depended on these rivers for thousands of years (Logan, 2002; Ferguson and ColwellChanthaphonh, 2006). 

The San Pedro River Basin is one of the most important flyways for migrating birds in North 

America with more than 400 species identified. In 2004, Congress recognized the importance of the 

river at the continental scale in passing Section 321 of Public Law 108-136, which requires the 

restoration and maintenance of sustainable yield in the regional aquifer with the goal of maintaining 

flow in the San Pedro River (Upper San Pedro Partnership, 2004). 

 

1U.S. Geological Survey.

2Water Resources Research Center, University of Arizona.

3School of Geography and Development, University of Arizona.

4Udall Center for Studies in Public Policy, University of Arizona.
6

Figure 1. Locations of Upper Santa Cruz and Upper San Pedro River Basins 

(adapted from Megdal and Scott, 2011).
7

Groundwater pumping, primarily for agricultural and urban development, increased through the 

20th century in both basins. From 1990 to 2004, groundwater levels declined more than 30 feet in some 

areas (Arizona Department of Water Resources, 2011a and b). Mining activities, both historic and 

current, are substantial sources of water use and contamination in both basins. The Cananea mine, 

located in the Sonoran part of the San Pedro Basin, extracts ore from one of the largest copper deposits 

in the world. Similarly, a proposed copper mine called El Pilar is located in the Sonoran part of the 

Santa Cruz Basin, within a mile of the river (M3 Engineering & Technology Corporation, 2009). 

The largest urban centers in the two basins are Sierra Vista and Nogales in Arizona and Cananea 

and Nogales in Sonora. The twin border cities of Nogales, Arizona, and Nogales, Sonora, have a 

combined population of more than 270,000; the population of Sierra Vista is about 60,000 and Cananea 

is about 33,000 (Arizona Department of Commerce, 2010; and Instituto Nacional de Estadística y 

Geografía, 2010). Currently (2012) the city of Cananea discharges raw sewage into the Upper San Pedro 

River. Plans are in place to build a treatment plant, but delays have plagued the project. Most sewage for 

the twin Nogales cities is treated in the United States. Flows in the downstream part of the Santa Cruz 

River are maintained by the effluent discharged from the plant. River seepage of wastewater effluent is 

an important source of recharge to the aquifer and helps to maintain groundwater levels (Nelson, 2007). 

Mexico is in the process of building a treatment plant that will treat and discharge about 50 percent of 

the wastewater into a river basin south of Nogales, Sonora. This treatment plant will almost certainly 

have substantial effects on flow in the river, the riparian ecosystem, and groundwater levels in the Santa 

Cruz aquifer.

Both the Mexican section of the San Pedro River Basin and the Sierra Vista subwatershed (the 

uppermost section of the river basin in the United States) are considered to be in groundwater deficit, 

because more water is removed by pumping from the regional aquifers than is recharged annually

(Upper San Pedro Partnership, 2004; Comisión Nacional del Agua, 2009). Parts of the Upper Santa 

Cruz Basin have been designated as an Active Management Area (AMA) by the Arizona Department of 

Water Resources (ADWR). This designation represents the strictest level of water management possible 

under Arizona State law. According to ADWR, “the management goal of the Santa Cruz AMA is to 

maintain a safe-yield condition...and to prevent local water tables from experiencing long term 

declines” (Arizona Department of Water Resources, 2012). “Safe yield” is achieved when no more 

water is withdrawn annually from an aquifer than is replenished annually. In the Mexican section of the 

basin, less groundwater is withdrawn for human use than is naturally recharged (Comisión Nacional del 

Agua, 2009). The USGS Arizona Water Science Center and the University of Arizona Water Resources 

Research Center, both located in Tucson, were the designated entities to assume responsibility for 

carrying out the assessment of the Upper Santa Cruz and Upper San Pedro River Basins.

Partners

The Arizona/Sonora team of the TAAP has worked continually to engage pertinent stakeholders 

as partners for designing and carrying out assessment activities in the shared aquifer regions. The goal 

of the Arizona/Sonora team has been to engage all stakeholders within the region under investigation, 

regardless of national setting/origin. Partners are listed in Table 1.

Initial meetings among the U.S. collaborators began in early September 2007, with Mexican 

counterparts joining shortly thereafter, to address concerns related to groundwater assessment. This 

effort has been ongoing and primarily focused on engagement of stakeholders within each basin. 
8

Table 1. Key partners in the Arizona/Sonora Transboundary Aquifer Assessment Program (TAAP). 

[CILA, Comisión Internacional de Limites y Aquas; UNESCO, United Nations Educational, Scientific and Cultural 

Organization]

Binational Federal Government State government Local/municipal 

government

International Boundary

and Water Commission 

(IBWC and CILA) 

Water Committee of the 

Arizona-Sonora 

Commission

Mexican National 

Water Commission 

(CONAGUA)

U.S. Bureau of 

Reclamation

Arizona Department of 

Water Resources 

(ADWR)

Arizona Department of 

Environmental Quality 

(ADEQ)

Sonoran State

Water Commission 

(CEA)

City of Nogales, Sonora

City of Nogales, 

Arizona

City of Sierra Vista, 

Arizona

Nongovernmental International Academic

Friends of the Santa 

Cruz River (FOSCR)

Upper San Pedro 

Partnership (USPP)

Internationally Shared 

Aquifer Resources 

Management (ISARM) 

Program, UNESCO

University of Sonora 

(UNISON)

College of Sonora 

(COLSON)

Sonoran Technical 

Institute (ITSON)

On November 3–4, 2009, a workshop was held in Tucson, Arizona, with the goal of defining 

and reviewing work plans and programmatic priorities. A diverse set of stakeholders representing local, 

national, and international levels of management and assessment agencies and organizations attended, 

including a representative from the United Nations Educational, Scientific and Cultural Organization 

(UNESCO) Internationally Shared Aquifer Resources Management (ISARM) Program. The first day of 

the event was dedicated to presentations related to the Santa Cruz and San Pedro shared aquifers, 

Federal-level groundwater management and assessment issues from the United States and Mexico, as 

well as developments in shared aquifer resource management as defined by UNESCO. The second day 

of the workshop focused on breakout group discussions with the goal of acquiring workshop participant 

feedback on the Arizona/Sonora work plan. 

Arizona/Sonora team members have maintained close communication with counterparts in New 

Mexico and Texas regarding TAAP activities carried out in those States. Concerted efforts have been 

made to engage Mexican, and particularly Sonoran, counterparts through participation at a variety of 

professional-level conferences. In addition to attendance at the bi-State Arizona-Mexico Commission 

meetings, held semiannually in either Arizona or Sonora, Arizona/Sonora team members participated in 

the 2009 Mexican Geohydrological Congress, hosted by the Sonoran section of the Mexican 

Geohydrological Association (AGM) in San Carlos, Sonora. Similarly, the Arizona/Sonora team 

organized and moderated two sessions in the 2010 Arizona Hydrological Society annual meeting related 

to transboundary groundwater with presentations by Arizonan and Sonoran team members. 
9

Binational Activities

The role of the IBWC (both U.S. and Mexican Sections) was solidified by the Joint Report of the 

Principal Engineers Regarding the Joint Cooperative Process United States – Mexico for the 

Transboundary Aquifer Assessment Program, signed on August 19, 2009. 

Following the signing of the Joint Report, a series of meetings between U.S. and Mexican 

stakeholders was held at which the goals and initial extent of hydrological studies on a regional basis 

were outlined. Texas and New Mexico were to collaborate with Chihuahua to assess the Mesilla 

Basin/Conejos-Médanos aquifer system, and Arizona was to engage Sonora in assessing the Upper 

Santa Cruz and San Pedro River Basins. An agreement on first stage funding for assessment of the 

basins in Sonora, Mexico, was reached during a meeting in Rio Rico, Arizona, on May 18, 2010. The 

Mexican National Water Commission (CONAGUA) then set forth to engage researchers and faculty at 

the University of Sonora (UNISON), Campus Hermosillo, and to develop work plans for the Santa Cruz 

and San Pedro aquifers in Sonora. Hydrological studies to be carried out in the Santa Cruz and San 

Pedro River Basin aquifers in Sonora, Mexico, are outlined in the work plans submitted by the UNISON 

and International Boundary and Water Commission – Mexican Section (CILA) for each location. 

Arizona/Sonora team members at the USGS and the University of Arizona (UA) Water Resources 

Research Center reviewed the work plan for each aquifer; each work plan is included as a technical 

annex within the Memorandum of Understanding (MOU) between the UA and International Boundary 

and Water Commission – U.S. Section (IBWC). Work plans for the Santa Cruz and San Pedro aquifers 

in Sonora were reviewed during late summer 2010. Upon finalization, IBWC and the Office of Research 

and Contract Analysis at the UA began drafting an MOU to outline funding transfer and project 

oversight protocol, which ties the transfer of funds to UA approval of UNISON’s progress on work plan

goals. The MOU between the UA and IBWC was signed by IBWC contracting personnel on January 6, 

2011, and by UA contracting representatives on January 10, 2011. 

Counterparts at UNISON began preliminary background data gathering and analysis prior to 

final signing by the UA and IBWC. As required in the legislation authorizing TAAP, funding to Mexico 

was to be matched dollar for dollar by the Mexican Government through CONAGUA. Funding for 

hydrological studies in the binational basins for TAAP-related work was to begin at $80,000 per basin 

($160,000 total, given that two Arizona/Sonora basins were being evaluated). Since the UA-IBWC 

agreement was signed, however, the Mexican Government has assumed financial responsibility for the 

entire $160,000 (equivalent to approximately 2 million pesos). The U.S. contribution of $80,000 was 

reprogrammed to develop reports on both aquifers to be written by U.S. and Mexican teams, as detailed 

in binational planning meetings held in June, August, and December 2011. 

As defined by the MOUs between IBWC-UA and CILA-UNISON, the IBWC will serve as the 

official binational repository for all data collected within the Mexican sections of the Santa Cruz and 

San Pedro Basins. Following a review period, the data will be made available to the public. 

The work plan definition for assessment activities in Mexico identified the following items as 

priority for initial studies:

a. Compilation, integration, summary, and critical analysis of prior studies of 

the aquifer.

a. Description of the physical geography and geological, geomorphological, 

climatological, and hydrological analyses.

c. Compilation and integration of information related to wells, such as depth, water levels, and 

geologic logs. 
10

d. Definition of the hydrostratigraphic units, determination of the aquifer properties, and 

analysis of groundwater-level behavior. 

e. Geophysical surveys.

f. Initial characterization of the hydrogeochemistry of the aquifer.

g. Development of a conceptual hydrogeologic model. 

Initial technical meetings between researchers from both Arizona and Sonora have begun to 

align research priorities and data collection to facilitate cross-border comparisons. Continued data 

collection for hydrogeologic modeling is a priority, especially in the Upper Santa Cruz River Basin near 

the border cities of Nogales, Arizona, and Nogales, Sonora. Aquifer properties of the Nogales 

Formation are being reevaluated for groundwater movement and storage on the basis of preliminary 

geologic mapping results. 

Work plans for assessment activities in Arizona originating from the USGS (Part 1) and the 

Water Resources Research Center at the UA (Part 2) are summarized below:

Part 1. Hydrological Modeling Framework (USGS to take the lead on all activities except for 

item f.): Design the protocol for data collection and synthesis for the creation of a binational 

coupled surface-water and groundwater-flow model. 

a. Geophysical studies requiring a combination of airborne and surface- and ground-based 

surveys to identify the three-dimensional distribution of hydrogeologically relevant structure

and properties in each basin. 

b. Surface- and groundwater-flow measurements and analyses: It is imperative to maintain 

current USGS streamflow gaging stations along the rivers and the ADWR groundwater-level 

monitoring network. The re-introduction of new or historical streamflow gages also would 

be beneficial. Both CILA and USGS have identified such locations along the Nogales Wash

and Santa Cruz River in Sonora. 

c. Coupled groundwater and surface-water flow model for each binational basin: Currently 

only separate groundwater and surface-water models exist and most stop at the international 

border. The model resolution typically is different on each side of the border, partly because 

of asymmetric data collection, which can lead to conflicting or misleading results.

d. Assessment of the effect of recently installed flood-detention basins on surface-water quality 

and recharge in the Nogales Wash subbasin. 

e. Increased precipitation monitoring networks in Ambos Nogales watershed: These networks 

serve the dual purpose of being used as required input for groundwater modeling and helping 

with flood warning. 

f. Binational coordination for modeling: Engage stakeholders in Arizona and Sonora, as well as 

Federal agencies responsible for each region, in the design of and data collection for

binational hydrological models. 

Part 2. Vulnerability Assessment: With a focus on urban areas, evolving vulnerability in 

Cananea, Sierra Vista, and Ambos Nogales is an important issue for transboundary groundwater 

due to dependence on the resource. 
11

a. Data collection of land use, zoning, economic and population growth, infrastructure, etc.: 

Develop profiles and identify corresponding pressures on groundwater resources originating 

from urban and rural areas using geographic information systems (GIS) and remote sensing 

tools, census data, and economic indicators. 

b. Urban growth characterization and effect on watershed land use and hydrology: The effect of 

land-use change, primarily due to urban growth, affects basin hydrology. Increased runoff 

and sediment in surface flow and decreased infiltration are primary results of urban growth. 

c. Binational water balances and supply/demand analysis: Urban and rural development rates 

and climate change have important effects on groundwater usage. Evolving usage for

agriculture, industrial, and residential purposes has a direct impact on groundwater pumping 

within each basin. 

d. Groundwater vulnerability to contamination: Integrate land use, climate, and hydrogeologic 

(soil type, depth to groundwater, and so forth) data to evaluate the potential for groundwater 

contamination in the Santa Cruz and San Pedro aquifer systems.

e. Water-quality assessments: These include industrial and other contaminants originating in 

urban areas and contaminants associated with mining operations from Cananea and 

wastewater treatment from Ambos Nogales. 

f. Assessment of institutional asymmetries and binational cooperation frameworks: Because of

differing groundwater management strategies between the United States and Mexico, it is 

imperative to understand how cross-border cooperation can best function. 

g. Improved links with international best practices (via ISARM): Continue engagement with 

global and regional (Americas) ISARM initiatives. Participate as a case study in order to 

provide other ISARM participants with information on the TAAP activities as well as learn 

from other shared resource scenarios.

Accomplishments

• Organization and management of workshop, “Developing a Workplan for the Santa Cruz and San 

Pedro Aquifers” November 3–4, 2009, Tucson, Arizona. 

• Development and signing of an MOU with IBWC for transfer of funding and data related to 

hydrological assessment of the Santa Cruz and San Pedro aquifers in Sonora, Mexico. 

• Presentations and materials for publication (see Project Publications list below).

• Initial work on hydrogeologic framework model for Upper Santa Cruz Basin: 

◦ Development of binational wells database with geologic log information. 

◦ Initial modeling of geophysical data for determination of structure and distribution of 

hydrogeologic units. 

◦ Consultation with USGS geologic mapping project to reevaluate and remap geologic 

units (Nogales Formation). Initial results indicate the need to reevaluate estimated 

hydraulic properties of the deepest sedimentary layer in the aquifer. 
12

• Collaboration with the USGS – Mexico Border Environmental Health Initiative 

(http://borderhealth.cr.usgs.gov/projectindex.html) to support studies of groundwater contamination, 

including the following: 

◦ Groundwater vulnerability to contamination: USGS and University of Arizona scientists and 

students are using the U.S. Environmental Protection Agency DRASTIC model, which takes 

into consideration a range of variables related to infiltration and recharge potential. 

Contaminants originating from households, mining, agriculture, and industry, including the 

Nogales International Wastewater Treatment Plant, may be released into the Santa Cruz and 

San Pedro Basins where migration into groundwater used for potable supply may occur. 

◦ Development of a binational geochemical database with information on the location and 

concentration of contaminants and other chemical constituents in groundwater, surface water, 

soils, and bed sediments. 

◦ Groundwater contaminant transport modeling (using USGS MODFLOW/MT3D) of the 

portion of the aquifer downstream from the Nogales International Wastewater Treatment 

Plant. The model is being used to study the movement of contaminants, such as nitrate, 

arsenic, and cadmium, from surface-water sources to wells.

• With combined funding from the U.S. Environmental Protection Agency Border 2012 Program, the 

Arizona Department of Water Resources, and TAAP, a new stream gage was installed and operated 

in Nogales Wash the main tributary to the Santa Cruz River flowing out of the twin cities of Nogales. 

Surface drainage from this urban area may be contaminated, and this streamflow is a source of 

recharge to the aquifer system. 

• Runoff-recharge measurement and modeling in the Sierra Vista subwatershed of the Upper San Pedro 

Basin, being carried out by USGS scientists, UA faculty, and a Ph.D. student: 

◦ Maintenance of a network of 11 gages in ephemeral stream channels in Sierra Vista and on 

the Fort Huachuca Military Installation. Data are to be used to update estimates of aquifer 

recharge.

◦ Support of the development of a surface-water model using KINEROS (Woolhiser and 

others, 1990) that incorporates measured rainfall and streamflow information to estimate 

infiltration and recharge via soil and stream channels.

• Analysis of population growth and related groundwater demand in the Ambos Nogales region. 

• Development and distribution of fact sheets in English and Spanish (see project Web page link 

below). 

• Development of project Web page (with English and Spanish versions) hosted by The University of 

Arizona Water Resources Research Center (WRRC; http://wrrc.arizona.edu/taap/). 

• Support for six graduate research assistants. 

• Funding support for laboratory analysis of surface-water sampling by the local nongovernmental 

organization, Friends of the Santa Cruz River, maintaining a multiyear sampling record.

 

• Creation of two databases, in English (and partially in Spanish), compiling metadata related to 

previous studies and publications related to the binational Upper Santa Cruz and San Pedro River 

aquifers (available at http://wrrc.arizona.edu/taap/).
13

• Geophysical surveys: USGS scientists working with the UA Geophysics Field Methods course 

brought to light new information on the distribution of water-bearing and impermeable units within 

the section of the aquifer from which Nogales, Arizona, extracts about 50 percent of its water.

Future Plans (subject to funding level)

• Continue analysis of population growth and related groundwater demand in the Ambos Nogales 

region. 

• Compilation and analysis of data created by UNISON researchers.

• Hydrogeologic framework model – Upper Santa Cruz Basin. 

◦ Finish ground-based geophysical surveys. 

◦ Create three-dimensional model of subsurface. 

• Binational monitoring. 

◦ Water quality: Santa Cruz River water is primarily treated effluent. Evaluate whether residual 

chemicals are contaminating wells. After assessing results of previous work, sample wells in 

Nogales, Rio Rico, and Tubac areas in which Arizona Department of Environmental Quality 

(ADEQ) found trichloroethylene and emerging contaminants. 

◦ Expand and maintain groundwater-level network in Santa Cruz and San Pedro Basins.

◦ Maintain and add streamflow gages in both basins on both sides of the border. 

◦ Conduct annual seepage runs in San Pedro and Santa Cruz Basins.

◦ Expand and maintain precipitation monitoring network: Install rain and snow gages in the 

Huachuca and Santa Rita Mountains. 

• Data analysis. 

◦ Process, analyze, and publish water-quality data, including trends and exceedances of 

standards.

◦ Process, analyze, and publish geophysical data. 

◦ Process ephemeral channel streamflow data from the Sierra Vista and Fort Huachuca gages.

◦ Analyze groundwater-level and streamflow data. 

• Modeling. 

◦ Create and calibrate binational groundwater/surface-water flow model for both basins in

Mexico and the United States.

◦ Perform contaminant transport modeling with new binational groundwater/surface-water flow 

models.

References

Arizona Department of Commerce, 2010, 2006–2055 Commerce population projections, accessed 

May 2011, at http://www.workforce.az.gov/population-projections.aspx. 

Arizona Department of Water Resources, 2011a, Arizona water atlas, volume 3, Southeastern Arizona 

Planning Area, 598 p., accessed March 2013, at 

http://www.azwater.gov/azdwr/StatewidePlanning/WaterAtlas/SEArizona/default.htm. 
14

Arizona Department of Water Resources, 2011b, Arizona water atlas, volume 8, Active Management 

Areas, 467 p., accessed March 2013, at 

http://www.azwater.gov/azdwr/StatewidePlanning/WaterAtlas/ActiveManagementAreas/default.htm. 

Arizona Department of Water Resources, 2012, Active Management Areas (AMAs) & Irrigation Nonexpansion Areas (INAs), accessed May 2012, at 

http://www.azwater.gov/AzDWR/WaterManagement/AMAs/. 

Comisión Nacional del Agua, 2009, Actualización de la disponibilidad media anual de agua subterránea 

Acuífero (2616) Río San Pedro Estado de Sonora, accessed March 2013 at 

http://www.conagua.gob.mx/Conagua07/Aguasubterranea/pdf/DR_2616.pdf. 

Ferguson, T.J., and Colwell-Chanthaphonh, Chip, 2006, History is in the land: Multivocal tribal 

traditions in Arizona’s San Pedro Valley: Tucson, University of Arizona Press. 

 

Instituto Nacional de Estadística y Geografía, 2010, Cuéntame: Información por entidad – Sonora – 

División Municipal, accessed July 2010, at 

http://cuentame.inegi.org.mx/monografias/informacion/son/territorio/div_municipal.aspx?tema=me&

e=26. 

Logan, M.F., 2002, The lessening stream: The environmental history of the Santa Cruz River: Tucson, 

University of Arizona Press. 

M3 Engineering & Technology Corporation, 2009, Feasibility study: El Pilar project, Sonora, Mexico, 

REVISION 0. 43-101F1 Technical Study Report, accessed March 2012, at: 

http://www.mercatorminerals.com/i/pdf/techreport/Rapport_technique_El_Pilar.pdf. 

Megdal, S.B., and Scott, C.A., 2011, The importance of institutional asymmetries to the development of 

binational aquifer assessment programs: The Arizona-Sonora experience: Water, v. 3, p. 949–963, 

doi:10.3390/w3030949. 

Nelson, K.M., 2007, Groundwater flow model of the Santa Cruz Active Management Area along the 

effluent-dominated Santa Cruz River, Santa Cruz and Pima Counties, Arizona: Hydrology Division, 

Arizona Department of Water Resources, Model Report 14. 

Upper San Pedro Partnership, 2004, Water management of the regional aquifer in the Sierra Vista 

subwatershed, Arizona, 2007 Report to Congress, accessed October 2010, at 

http://www.usppartnership.com/lib_study_321.htm. 

Woolhiser, D.A., Smith, R.E., and Goodrich, D.C., 1990, KINEROS, A kinematic runoff and erosion 

model: Documentation and user manual: U.S. Department of Agriculture, Agricultural Research 

Service, ARS-77, 130 p. 
15

Project Publications

(A) Papers and Reports

Browning-Aiken, Anne, Scott, C.A., Varady, R.G., and Megdal, S.B., 2010, Spanning transboundary 

waters in North America (Atravesando las aguas transfronterizas en América del Norte, Traverser les 

eaux transfrontières en Amérique du Nord): Newsletter of International Network of Basin 

Organizations. 

Culbertson, Chris, Lytle, W.E., McMillan, M.M., Sternberg, B.K., and Withers, K.B., 2010, Transient 

electromagnetic (TEM) investigation of microbasin morphology along the Santa Cruz River, Nogales, 

Arizona: University of Arizona, Geophysical Field Methods Spring, 2010 Course Report, accessed 

June 19, 2012, at http://www.lasi.arizona.edu/GEN%20416-516%202010%20Final%20Report.pdf. 

Lincicome, A.D., 2011, DRASTIC groundwater vulnerability assessment of the Santa Cruz and San 

Pedro aquifers: The University of Arizona, Soil, Water, and Environmental Sciences, Master’s Thesis. 

McAndrew, R.M., 2011, Subsurface contaminant transport modeling in the upper Santa Cruz Basin: 

The University of Arizona, Hydrology and Water Resources, Master’s Thesis.

Megdal, Sharon, Varady, Robert, Vandervoet, Prescott, and Scott, Chris, 2012, Water allocation under 

uncertainty: Groundwater and effluent in the United States – Mexico transboundary Santa Cruz River 

Basin: The Third United Nations World Water Development Report (WWDR4) Case Studies 

Volume, UNESCO.

Milman, A., and Scott, C.A., 2010, Beneath the surface: Intranational institutions and management of 

the United States – Mexico transboundary Santa Cruz aquifer: Environment and Planning C: 

Government and Policy, v. 28, p. 528–551. 

Scott, C.A., 2011, The water-energy-climate nexus: Resources and policy outlook for aquifers in 

Mexico: Water Resources Research, v. 47, doi:10.1029/2011WR010805. 

Scott, C.A., Megdal, Sharon, Oroz, L.A., Callegary, James, and Vandervoet, Prescott, 2012, Effects of 

climate change and population growth on the transboundary Santa Cruz aquifer: Climate Research, 

v. 51, p. 159–170, doi: 10.3354/cr01061. 

Scott, C.A., and Pasqualetti, M.J., 2010, Energy and water resources scarcity: Critical infrastructure for 

growth and economic development in Arizona and Sonora: Natural Resources Journal, v. 50, no. 3, 

p. 645–682. 

Scott, C.A., and Pineda-Pablos, N., 2011, Innovating resource regimes: Water, wastewater, and the 

institutional dynamics of urban hydraulic reach in northwest Mexico: Geoforum, v. 42, no. 4, 

p. 439–450, doi:10.1016/j.geoforum.2011.02.003. 

 
16

Stewart, A.M., Callegary, J.B., Smith, C.F., Gupta, H.V., Leenhouts, J.M., and Fritzinger, R.A., Use of 

the continuous slope-area method to estimate runoff in a network of ephemeral channels, southeast 

Arizona, U.S.A.: Journal of Hydrology, v. 472–473, p. 148–158. 

Wilder, M., Scott, C.A., Pineda-Pablos, N., Varady, R.G., Garfin, G.M., and McEvoy, J., 2010, 

Adapting across boundaries: Climate change, social learning, and resilience in the U.S. – Mexico 

border region: Annals of the Association of American Geographers, v. 100, no. 4, p. 917–928. 

(B) Presentations and Proceedings 

Callegary, J.B., Megdal, S.B., Scott, C.A., and Vandervoet, Prescott, 2009, Transboundary Aquifer 

Assessment Program: National Ground Water Association’s 2009 Ground Water Summit, Tucson, 

Arizona, April 20–23, 2009. 

Callegary, J.B., Paretti, Nick, Gray, Floyd, Norman, Laura, Beisner, Kim, Eddelman, Katie, Matti, 

Logan, Papoulias, Diana, van Riper, Charles, Bell, J.R., McAndrew, Rose, Lincicome, Alexis, 

Brusseau, Mark, Alvarez, Dave, Scott, Chris, Vandervoet, Prescott, and Megdal, Sharon, 2011, 

Linking hydrology, geology, chemistry, and biology in the Upper Santa Cruz River Basin: Santa Cruz 

River Researchers’ Day, Tucson, Arizona, March 29, 2011. 

Callegary, J.B., Withers, K., Gray, F., Page, W., Vandervoet, Prescott, McAndrew, R., Caldeira, F., 

Scott, Chris, and Megdal, Sharon, 2009, Desarrollo inicial del marco hidrogeológico para el lado 

estadounidense de la Cuenca Alta del Río Santa Cruz: VII National Groundwater Congress of the 

Mexican Geohydrologic Association, San Carlos, Sonora, Mexico, November 11–13, 2009. 

Eastoe, C., and Callegary, J.B., 2010, Sources of shallow riparian groundwater in the San Pedro and 

Santa Cruz Basins, southern Arizona: A review of stable isotope evidence, Arizona Hydrological 

Society, Tucson, Arizona, September 1–4, 2010. 

McAndrew, R.M., Callegary, J.B., and Brusseau, M.L., 2011, Groundwater contaminant transport 

modeling in the Upper Santa Cruz Basin: Santa Cruz River Researchers’ Day, Tucson, Arizona, 

March 29, 2011. 

McAndrew, R.M., Callegary, J.B., Brusseau, M.L., and Gray, Floyd, 2009, Contaminant transport and 

predictive modeling in the Upper Santa Cruz Basin, 2009 Geological Society of America Annual 

Meeting, Portland, Oregon, October 18–21, 2009. 

Megdal, S., Sención, R., Scott, C.A., Díaz, F., Oroz, L., Callegary, J.B., Varady, R.G., 2010a, 

Evaluación institucional de los acuíferos transfronterizos Santa Cruz y San Pedro en la frontera 

México – Estados Unidos: UNESCO-IAH-UNEP Conference, Paris, France, December 6–8, 2010. 

Megdal, S., Sención, R., Scott, C.A., Díaz, F., Oroz, L.A., Callegary, James, and Varady, R.G., 2010b, 

Institutional assessment of the transboundary Santa Cruz and San Pedro Aquifers on the United States 
17

– Mexico border: Pre-Proceedings for the UNESCO-IAH-UNEP Conference on Transboundary 

Aquifers: Challenges and New Directions (ISARM 2010), Paris, France, December 6–8, 2010. 

Megdal, S.B., 2008, Front-row view of federal water lawmaking shows process works – U.S. Mexico 

Transboundary Aquifer Assessment Act pondered, passed and signed, in Norman, L.M., Hirsch, D. 

D., and Ward, A.W., eds., Proceedings of a USGS Workshop on facing tomorrow's challenges along 

the U.S. – Mexico border: Monitoring, modeling, and forecasting change within the Arizona-Sonora 

transboundary watersheds: U.S Geological Survey Circular 1322. 

Megdal, S.B., 2009a, Developing a framework for closer binational cooperation: 2009: Box within map 

‘Transboundary Aquifers of the World – Update 2009 – Special edition for the 5th World Water 

Forum, Istanbul, Turkey, IGRAC, UNESCO, accessed March 2013 at 

http://www.isarm.net/publications/319#. 

Megdal, S.B., 2009b, The U.S. – Mexico Transboundary Aquifer Assessment Program: The ArizonaSonora portion as a case study: World Water Week, Stockholm, Sweden, August 20, 2009. 

Megdal, S.B., 2010, Institutional mechanisms for the assessment and management of transboundary 

aquifers: The importance of partnerships: 19th session of the International Hydrological Programme 

Intergovernmental Council, Paris, France, July 7, 2010. 

Prichard, A., 2011, Interbasin water transfers at the U.S. – Mexico border city of Nogales, Sonora: 

Implications for aquifers and water security: Environmental Research Grad Blitz, University of 

Arizona Institute for the Environment, Tucson, Arizona, February 1, 2011. 

Scott, C.A., 2009, Evaluación de acuíferos transfronterizos en la frontera Estados Unidos – México en el 

contexto de cambio climático y crecimiento de la demanda de agua en áreas urbanas: VII Congreso 

Nacional de Aguas Subterráneas, Asociación Geohidrológica Mexicana (VII National Congress on 

Groundwater, Mexican Geohydrological Association), San Carlos, Sonora, November 10–13, 2009. 

Scott, C.A., 2010, Arizona – Sonora transboundary aquifer assessment , University of Arizona, 

Maricopa County Cooperative Extension, Phoenix, Arizona, May 27, 2010. 

Scott, C.A., Megdal, S., Oroz, L.A., Mexía, M., and Ramos, H., 2010, Building shared vision: 

Assessment of transboundary aquifers along the United States – Mexico border, in Proceedings of 

International Conference on Water Scarcity, Global Changes, and Groundwater Management 

Responses, University of California – Irvine, UNESCO, USGS, Irvine, Calif., December 1–5, 2008. 

Scott, C.A., Megdal, S.B., Callegary, J., and Vandervoet, P., 2008, The United States – Mexico 

Transboundary Aquifer Assessment Program: Water Resources Research Center Seminar Series, 

University of Arizona, Tucson, Arizona, December 8, 2008. 

Scott, C.A., and Vandervoet, P.L., 2010, Transboundary aquifer assessment, management and policy: 

Session organized and moderated at the annual symposium of the Arizona Hydrological Society,

Tucson, Arizona, September 3–4, 2010. 
18

Stewart, A.M., Callegary, J.B., Smith, C.F., Gupta, H.V., and Leenhouts, J.M., 2010, Use of the 

continuous slope-area method to quantify runoff and transmission losses in a network of ephemeral 

stream channels in Southeast Arizona : New Mexico Water Resources Research Institute Water 

Research Symposium, Socorro, New Mexico, August 2, 2010. 

Stewart, A.M., Gupta, H.V., Goodrich, D.C., Callegary, J.B., and Montenegro, E., 2010, Optimized 

numerical modeling to estimate runoff and infiltration in ephemeral stream channels, Southeast 

Arizona, Abstract H33G-1246 poster presented at 2010 Fall Meeting, AGU, San Francisco, Calif., 

December 13–17, 2010. 

Vandervoet, P., Megdal, S.B., and Scott, C.A., The state of the aquifers: The transboundary Santa Cruz 

and San Pedro River Basins: Dryland Hydrology: Global Challenges, Local Solutions, Arizona 

Hydrological Society Annual Symposium, Westin La Paloma Resort, Tucson, Ariz., September 2, 

2010. 

Vandervoet, Prescott, 2008, El Programa de Evaluación de Acuíferos Transfronterizos, Arizona – 

Mexico Commission Plenary Session, Water Committee, Hermosillo, Sonora, December 5, 2008. 

Vandervoet, Prescott, 2009a, TAAPing the sources: Santa Cruz database creation for the Transboundary 

Aquifer Assessment Program: Santa Cruz River Researchers’ Day, Tucson, Ariz., March 27, 2009. 

Vandervoet, Prescott, 2009b, The Transboundary Aquifer Assessment Program, Arizona: A case study 

in binational cooperation, Association for Borderlands Studies Annual Meeting, Albuquerque, N.M., 

April 18, 2009. 

Vandervoet, Prescott, 2010a, Assessment of the binational Upper Santa Cruz River aquifer: Designing a 

workplan, Second Annual Santa Cruz River Researchers’ Day, Tucson, Ariz., March 19, 2010. 

Vandervoet, Prescott, 2010b, El Programa de Evaluación de Acuíferos Transfronterizos: creación y 

divulgación de recursos académicos: Coloquio Binacional sobre Desarrollo Humano transfronterizo 

en la región Sonora – Arizona, Universidad de Sonora: Unidad Nogales, Nogales, Sonora, May 26, 

2010. 

Vandervoet, Prescott, Callegary, J.B., Megdal, S.B., and Scott, C.A., 2009, Stakeholder involvement 

and priority-setting for transboundary aquifer assessment: The case of the Santa Cruz and San Pedro 

River aquifers: Water Resources Research Center 2009 Annual Conference: Best Practices for 

Stakeholder Engagement and Water Resources Planning. University of Arizona, Tucson, Ariz., March 

17, 2009. 

Varady, R.G., Scott, C.A., and Megdal, S.B., Transboundary aquifer institutions, policies, and 

governance: A preliminary inquiry: Pre-Proceedings for the UNESCO-IAH-UNEP Conference on 

Transboundary Aquifers: Challenges and New Directions (ISARM 2010), Paris, France, December 6–

8, 2010. 
19

Mesilla Basin/Conejos-Médanos Section of the Transboundary Aquifer 

Assessment Program

By Zhuping Sheng,5 Michael Darr,

6 J. Phillip King,7 John Bumgarner,8 and Ari Michelsen5

Program Overview

The Mesilla Basin/Conejos-Médanos aquifer system constitutes one of the largest transboundary 

aquifer systems in the Rio Grande/Rio Bravo Basin region of the United States and Mexico (Figure 2). 

The part of the Mesilla Basin occupied by the entrenched valley of the Rio Grande, the Mesilla Valley, 

contributes to the largest rechargeable groundwater reservoir in the region and includes the western part 

of the El Paso (Texas) – Ciudad Juárez (Chihuahua) – Las Cruces (New Mexico) metropolitan area with 

a population of about 2.5 million (Creel and others, 2006). Because of stakeholders’ needs for a better 

understanding of groundwater availability, use, and quality, as well as a long history of prior 

multiagency water-resource investigations, the binational Mesilla Basin/Conejos-Médanos aquifer 

system was selected for primary initial focus of the New Mexico – Texas assessment program. 

Scientists from the New Mexico State University Water Resources Research Institute 

(NMWRRI), the Texas Water Resources Institute (TWRI), part of Texas AgriLife Research, the Texas 

A&M University system, and the USGS Texas and New Mexico Water Science Centers conducted 

studies in collaboration with Federal and State agencies and organizations, as well as with Mexican 

partners. New and existing data were collected and evaluated in order to understand the availability and 

use of groundwater in the Mesilla Basin/Conejos-Médanos aquifer system, and evaluate strategies to 

protect water quality and enhance water supplies for sustainable economic development. 

 

5Texas AgriLife Research & Extension Center, El Paso, Texas.

6U.S. Geological Survey.

7Department of Civil Engineering, New Mexico State University.

8U.S. Geological Survey.
20

Figure 2. Locations of the Mesilla Basin/Conejos-Médanos and Hueco Bolson aquifer systems.
21

A joint binational, USGS and WRRI work plan was developed for comprehensive assessment of 

the Mesilla Basin/Conejos-Médanos aquifer system and reviewed by stakeholders. The work plan 

identified the following general goals and strategies: 

a. Develop and implement an integrated scientific approach to assess priority transboundary 

aquifers, including evaluation of available data and publications, creation or enhancement of 

a GIS, and the implementation of field studies where additional data are needed; 

b. Consider the expansion of modification of existing agreements, as appropriate, between the 

USGS, the WRRIs, the States, and authorities in Mexico to conduct joint scientific 

investigations, archive and share relevant data, and carry out other activities consistent with 

the program; and 

c. Produce scientific products that can be widely distributed and that provide the scientific 

information needed by water managers and natural resource agencies on both sides of the 

U.S. – Mexico border to effectively accomplish the missions of the managers and agencies. 

Specific tasks to accomplish the general objectives also were described in the work plan. Each objective 

is described in the “Accomplishments” section of this report, along with an update of the status. 

 Research findings have been shared with stakeholders and presented at regional, national, and 

international conferences. Though funding for this program has not been provided since fiscal year (FY) 

2010, a study plan for possible research has been developed by research team members to complete the 

assessment of the Mesilla Basin/Conejos-Médanos aquifer system and expand the research efforts into 

other aquifers, including the Hueco Bolson and Presidio Basin in west Texas and the Mimbres Basin in 

southwest New Mexico.
22

Partners

In the first year (FY 2008) of this project, the project team of the New Mexico and Texas 

WRRIs and USGS Water Science Centers focused efforts on coordination with stakeholders and invited 

them to form a Steering Committee to identify priority issues that could be addressed. The invited 

stakeholders included Federal and State agencies, city/county governments, water utilities, irrigation 

districts, regional water planning groups, and others as listed in the table below. 

Table 2. Key partners in the Mesilla Basin/Conejos-Médanos Transboundary Aquifer Assessment Program (TAAP). 

Federal agencies New Mexico State agencies

- U.S. Bureau of Reclamation - New Mexico Office of the State 

 - U.S. Bureau of Land Management Engineer/New Mexico Interstate Stream 

 - U.S. Army Corps of Engineers Commission

 - U.S. Environmental Protection Agency - Region 6 - New Mexico Environment Department

 - International Boundary & Water Commission - New Mexico State Land Office

 (U.S. Section) - New Mexico State Parks

 - Sandia National Laboratories - New Mexico Department of Agriculture

 - Los Alamos National Laboratory - New Mexico Institute of Mining and 

Texas State agencies

 Technology

- New Mexico Bureau of Geology and 

 - Texas Water Development Board Mineral Resources

 - Texas Commission on Environmental Quality - New Mexico Energy, Minerals and Natural 

 - Texas State Soil & Water Conservation Board Resources Department

 - Texas Rio Grande Compact Commissioner - Utton Transboundary Resources Center at 

 - El Paso County Water Improvement District No.1 University of New Mexico

 - El Paso Water Utilities - Border Outreach & Coordination Office at 

 - University of Texas at El Paso New Mexico State University

 - City of Anthony - City of Las Cruces

 - Keystone Heritage Park - Dona Ana County

 - The Texas State Senate - Dona Ana Mutual Domestic Water 

 - El Paso County Commissioners Consumers Association

- Town of Mesilla

 Mexican agencies

 - Mexican Geological Service (SGM)

- Village of Hatch

- Elephant Butte Irrigation District

 - International Boundary & Water Commission 

 (Mexico Section) Regional organizations

 - National Water Commission (CONAGUA) - Paso del Norte Water Task Force

 - The State of Chihuahua Central Water and Sanitation - Paso del Norte Watershed Council

 Board (JCAS) - Rio Grande Council of Governments

 - Ciudad Juárez Municipal Water and Sanitation Board - Rio Grande Compact Commission 

 (JMAS)

Binational Activities

The New Mexico and Texas Water Resources Research Institutes developed a request for 

proposals (RFP) and a proposal template, and provided these documents to Mexican researchers. 

Proposal development was initiated through the IBWC/CILA. The initial phase of proposed work was 
23

conducted in 2010 and 2011, and is summarized in the Accomplishments section (Mexican Geological 

Service, 2011a and 2011b). Additionally, eight binational meetings were held to coordinate research 

activities and provide scientific exchange and data sharing.

Figure 3. Project team members participate in binational meeting of the IBWC, August 20, 2009.

Accomplishments

The achievements made toward the objectives set in the Mesilla Joint Work Plan developed by 

the project team and reviewed by stakeholders are summarized below. 

OBJECTIVE: Organize and conduct at least one stakeholder meeting to review project progress 

and plan for future studies and technical team collaboration. 

A Steering Committee was formed to provide guidance for the project, technical assistance, and 

review of documents through the project management team. A stakeholder meeting was held on 

June 11, 2008, in El Paso, Texas. The Mesilla Basin/Conejos-Médanos aquifer system was selected as 

the primary initial focus of the New Mexico and Texas TAAP portion because of the importance and 
24

immediate need for information regarding this aquifer. The Committee also reviewed a joint work plan 

prepared by the New Mexico and Texas WRRIs and USGS. 

OBJECTIVE: Collaborate with Mexican partners to conduct research on the Mexico portion of 

the Mesilla Basin/Conejos-Médanos aquifer system. 

TAAP funding was provided through a TWRI subcontract to Mexico of mutually agreed upon 

study plans for Mexico scientists to conduct research on the Mexico portion of the Mesilla 

Basin/Conejos-Médanos aquifer system. Exchange and analysis of available United States and Mexico 

aquifer data and other information took place under this task. 

To facilitate binational cooperation, an agreement was developed through the IBWC/CILA. 

Research partners in Mexico were identified, and communications were established. Numerous 

Technical Committee meetings were held to exchange ideas, identify issues of concern by binational 

stakeholders, and develop protocol for information exchange and data sharing. The Technical 

Committees were made up of scientists from the USGS and WRRIs, as well as the Mexican Geological 

Service and the Mexican National Water Commission (CONAGUA). Technical Committee meetings 

were hosted by the IBWC/CILA. 

OBJECTIVE: Consolidate and expand the bibliography with annotations of previous and current 

research and information regarding the Mesilla Basin. 

More than 800 publications were reviewed. An annotated bibliography was developed in 

Microsoft Word and EndNote with Web links to various data sources; this document was provided to 

Mexican collaborators. Additional citations are included from the Mexican literature, which continues to 

be updated. 

OBJECTIVE: Aquire and compile GIS data.

Numerous Mesilla Basin geospatial datasets have been developed and added to the Paso del 

Norte Watershed Council coordinated database and GIS Web site-hosted data, including well

information, pumping, water-quality, and surface-water data (http://www.pdnwc.org/). 

Data are available from two ftp sites at the NMWRRI (http://river.nmsu.edu/ and 

http://wrri.nmsu.edu/wrdis/ftp.html). New GIS components related to geologic structure and stratigraphy 

have been compiled, and numerous drillers’ logs from New Mexico and Texas have been scanned and 

placed in a database. 

Preliminary elements of the Conejos-Medanos aquifer subregion have been determined as well 

as the location of the well field that will provide supplemental water supply to Ciudad Juarez (Mexican 

Geological Service, 2011a and 2011b). 

OBJECTIVE: Evaluate selected wells in the Mesilla Basin aquifer.

A comprehensive assessment of the water resources of the Rincon and Mesilla Valleys was 

performed. More than 300 wells were sampled and analyzed for major ions, and a subset of about 
25

40 wells were sampled and analyzed for trace elements and radionuclides (previously sampled in the 

1970s). Some wells sampled for major ions were sampled at earlier dates, from the 1940s to 1960s. 

Groundwater geochemical samples were collected from 44 wells in the Mesilla Basin aquifer in 

New Mexico and Texas. The groundwater samples were analyzed for major ions, trace elements, 

nutrients, organic compounds, radon, stable and radiogenic isotopes, age tracers, and bacteria. 

Geochemical sampling of wells in Mexico was accomplished by the Mexican Geological Service and 

the data were published (Mexican Geological Service, 2011a, and 2011b). 

Trace elements provide insight into the processes controlling the movement of water through the 

Mesilla Basin aquifer. Changes in water chemistry are of particular interest to New Mexico and Texas 

and local governments and entities (such as irrigation districts) that are concerned with salinity issues in 

the Lower Rio Grande Valley. Interpretation of the data could be published in subsequent years, if 

sufficient additional funding becomes available through the TAAP. 

OBJECTIVE: Expand hydrogeologic framework model. 

The New Mexico WRRI updated the existing hydrogeologic framework model on the basis of an 

evaluation of both newly acquired hydrogeologic and geophysical information, as well as a reevaluation 

of all previous work and original databases. Information on more than 100 key wells and test holes 

completed in the Mesilla Basin aquifer, commonly referred to in New Mexico as the Rio Grande floodplain alluvium/Santa Fe Group aquifer system, includes best-available interpretations on the elevation 

ranges of the four major hydrostratigraphic units (HSUs) and aquifer zones (Rio Grande alluvium and 

the Upper, Middle, and Lower Santa Fe HSUs). New structure-contour maps of hydrogeologic-model 

layer bases for the basin and Rio Grande Valley areas between Hatch and El Paso have been completed. 

These maps illustrate best-available interpretations of subsurface topography and locations of major 

structural features/bedrock boundaries for four (potential) model-layer bounding surfaces: 

• Base of the inferred “shallow-aquifer” system of the Rio Grande alluvium, 

• Upper/Middle Santa Fe (USF/MSF) HSU surface, 

• Middle/Lower Santa Fe (MSF/LSF) HSU surface, and 

• Basal contact of basin and (or) valley fill on bedrock. 

These structure maps could form a core component of future groundwater-flow models. The 

latter hydrogeologic unit includes (1) inner-river valley fill (the sole alluvial aquifer of the eastern 

Rincon Valley-Selden Canyon and El Paso Narrows areas) and (2) the upper 300 to 500 feet of 

hydrologically well-connected, Santa Fe Basin fill units in the east-central and southeastern Mesilla 

Basin area (within 3 to 5 miles of the inner Mesilla Valley). These maps extend about 9 miles south of 

the New Mexico/Chihuahua boundary into the Conejos-Médanos aquifer system area. A total of 20 

schematic cross sections have been developed or updated, with base elevation lowered to mean sea level 

where possible. 

The Mexican Geological Service (2011a and 2011b) compiled piezometric data from selected 

well sites, configured depth-to-static groundwater level, and calculated static level elevation. In general, 

the static level elevation indicates that there is equilibrium throughout most of the aquifer in Mexico that 

maintains the natural direction of the groundwater flow. The withdrawal of about 23 million gallons per 
26

day of groundwater in the well field area has led to an immediate decrease in the static levels. Well 

surveys in 2007 and 2010 show an increase in public urban use from 2 percent in 2007 to 16 percent in 

2010; the percentage of inactive wells decreased from 44 percent in 2007 to 17 percent in 2010. 

Evaluation of groundwater pumping by Mexican water users was initiated, particularly pumping 

in the Juárez well field, in May 2010. Water levels in the aquifer system, in particular near the well

field, responded quickly to the withdrawals. Within 2 months of commencing operations, water levels 

dropped as much as 16 feet in the well field. Depending on the schedule of groundwater pumping, 

water-level drawdowns may be temporary or continue to increase. The physical and chemical properties

for groundwater quality were measured in the Conejos-Médanos aquifer system, and specific-capacity 

tests were performed for this study to determine the transmissivity of the aquifer. A technical report 

summarized findings for the initial phase (Mexican Geological Service, 2011a and 2011b). Data and 

GIS coverage have been exchanged through the IBWC/CILA. 

OBJECTIVE: Develop the Farm Process for the groundwater-flow model. 

Work was conducted to further the development of an integrated hydrologic model for the 

Mesilla Basin and Lower Rio Grande Valley. The hydrologic model is based on the existing 

MODFLOW-2005 groundwater-flow model currently used by the Office of the State Engineer (S.S. 

Papadopulos & Associates, 2007), but the new model will add the Farm Process (FMP1) module 

(Schmid and Hanson, 2009), which simulates landscape and surface-water processes in addition to 

groundwater flow. 

The Farm Process model simulates farm recharge and irrigation well production and, hence, 

could supersede the former use of specified recharge and well production datasets. The streamflow 

routing package (SFR) input dataset has been revised to accommodate the surface-water deliveries 

estimated externally from the Lower Rio Grande 2007 model (S.S. Papadopulos & Associates, 2007) 

and needed internally by the Farm Process model. A preliminary version of the model was developed 

and tested, and the results are consistent with the existing Lower Rio Grande model. The Farm Process 

model could be applied in the future to more comprehensively simulate the hydrology of the Mesilla 

Basin.

OBJECTIVE: Investigate the hydrologic characterization of the aquifer at Fillmore Pass in 

collaboration with Fort Bliss. 

Interbasin exchange between adjacent aquifers, particularly at Fillmore Pass between the Hueco 

Bolson and the Mesilla Basin, are not well defined in previous groundwater-flow model studies (Frenzel 

and Kaehler, 1990; Hamilton and Maddock, 1993; Weeden and Maddock, 1999; CH2M-Hill, 2002; 

Heywood and Yager, 2003; S.S. Papadopulos & Associates, 2007). Based on a preliminary assessment 

of the current status of groundwater within the Fillmore Pass area, the project team had identified 

research needs and developed a work plan for further investigation of groundwater exchange between 

the basins. The limited borehole and well-drilling data identified unconsolidated deposits in the Upper 

and Middle Santa Fe Group with a maximum thickness of 960 feet at the Fillmore Pass gap and deep 

groundwater level at a depth of approximately 350 feet below land surface. However, the 

interconnection between the Mesilla Basin and Hueco Bolson at this location is not well defined. Longterm monitoring data made available by the USGS and other data from State agencies have been 

compiled. With assistance from Fort Bliss personnel and local well owners, additional water-level 
27

measurements have been made and water samples have been collected. Additional analysis for the 

characterization of groundwater flow is ongoing. 

OBJECTIVE: Characterize surface-water and groundwater interaction. 

Understanding the interaction of surface water and groundwater within the Mesilla Basin is 

critical to the assessment of groundwater availability and configuration of the groundwater-flow model. 

The team compiled long-term groundwater-level data and seepage investigation data both from the 

USGS. Preliminary evaluations of historic trends in groundwater-level changes, cross-formational 

exchange, and impacts of groundwater withdrawals on the interaction of groundwater and surface water 

were initiated. A detailed assessment of patterns of exchange between streams and groundwater and 

estimation of trends in gains and losses is planned. 

Future Plans (subject to funding level)

Future work is planned in the Mesilla Basin to finalize the initiatives begun under TAAP, to 

further the progress achieved to date, and publish and release all data, methodology, and results of 

analyses. Contingent upon funding, the following activities will be conducted for the Mesilla Basin: 

(1) expand groundwater-level monitoring and groundwater-quality analysis; (2) investigate streamaquifer interaction; (3) examine interbasin groundwater exchange near El Paso Gap and outflow to the 

Hueco Bolson; and (4) create a joint transboundary groundwater-flow model. These activities will 

result in an improved understanding of the water resources of the Chihuahua – Texas – New Mexico 

area and allow water-resources managers to more effectively share this critical resource.

As part of a separate program, the USGS conducts groundwater-level monitoring at more than

200 sites in the Mesilla Basin on an annual basis. Additional monitoring wells near the U.S. – Mexico 

border would help to define water-level drawdown changes recently observed in a number of key border 

area wells. A pipeline from the Conejos-Médanos well field to Ciudad Juárez, Mexico, was completed 

in 2010, and existing border wells are responding to the drawdown created by the initiation of full 

production at the well field. The Mexican Geological Service included recommendations for additional 

monitoring sites in their March 2011 report (Mexican Geological Service, 2011a and 2011b ), as well as

long-term aquifer testing at the new production wells. Finally, installation of a deep well cluster in the 

border area would allow cost-effective monitoring of the shallow, intermediate, and deep aquifer zones 

as the cones of depression from municipal pumping eventually overlap. 

Groundwater-quality samples were collected in November 2010 at 44 Mesilla Basin aquifer 

wells with supplemental funding from the USGS National Water-Quality Assessment Program. Future 

funding would be applied to the laboratory analysis of additional samples from key wells in Mexico, 

which would greatly improve the dataset and include cooperative sampling with the Mexican 

Government. Finally, the team plans to examine and interpret the results and apply these water-quality 

data to questions of groundwater-quality trends, aquifer residence times, and groundwater-flow paths. 

Work is in progress to investigate the exchanges between streamflow and the groundwater-flow 

system, using seepage investigation data, historic surface-water flows, and water-level monitoring data. 

Using statistical analysis or simple hydrologic models, patterns of stream-aquifer exchange will be 

identified and trends in river gains and losses will be determined. Movement of groundwater between 

different hydrostratigraphic units will be evaluated in relation to stream-aquifer interactions. It is 
28

anticipated that these research findings will provide guidelines for reconfiguration of streamflow or 

Farm Process modules in the groundwater-flow model. A parallel effort seeks to further characterize 

interbasin groundwater flow between the Hueco Bolson and Mesilla Basin through Fillmore Pass using 

geophysical and geochemical data. Fort Bliss and other local groundwater users are interested in these 

aspects of the study because they are important to assess local groundwater availability. Depending on 

funding levels, these important elements could continue to advance. 

Hydrologic modeling of the Mesilla Basin has long been a key aspect of TAAP Mesilla Basin 

activities. The next steps of the integrated groundwater-flow model are to refine the model to reflect 

more detailed irrigation systems (or service areas) and to allow climate, evapotranspiration, and landuse updates by using remotely sensed data. This ultimately will require enough spatial and temporal 

resolution to allow the separation of the supply and demand components of the regional hydrologic 

budget from the groundwater, surface-water, and landscape components. The refined model would 

better address water management questions about resource development and sustainability that are of 

concern to the various agencies. These improvements are already in the planning stages and some 

progress has been made to date. As the project moves forward, the integrated groundwater-flow model 

will be documented and archived periodically, in order to ensure that this important water-resources 

assessment and management tool is available for future use. 

The Conejos-Médanos well field began full operation in 2010 with initial drawdown effects

observed across the southwestern part of the basin. This new hydrologic stress is not yet accounted for 

in existing groundwater-flow models. A fully binational resource such as the Mesilla Basin/ConejosMédanos aquifer system requires a coordinated approach and a joint effort to achieve consistency and 

acceptability from all parties. Accordingly, future plans include (1) integration of the hydrogeologic 

framework across the border, (2) development of a joint conceptual water budget for the Mesilla 

Basin/Conejos-Médanos aquifer system, and (3) selection, construction, and application of a mutually 

acceptable, fully integrated hydrologic flow model that will simulate the inflows and outflows of the 

groundwater and surface water of the transboundary region. Depending on funding levels, the TAAP 

may further the development of a binational model and promote the exchange of information between 

specialists in the United States and Mexico. 

The integration of the hydrogeologic framework is a critical step in improving the understanding 

of the transboundary resource. Recently completed TAAP hydrostratigraphic studies (J.H. Hawley, New 

Mexico State University, written commun., 2011) extend across the border area and will be digitized 

and distributed. Although these studies are based on preliminary data from test holes at the ConejosMédanos well field, the recently completed TAAP study of the Conejos-Médanos aquifer system 

(Mexican Geological Service, 2011a and 2011b) provides new data and suggests a much broader aquifer 

extent toward the southeast, with important inferences on regional basin recharge and inflow of saline 

groundwater. Electromagnetic profiles in Mexico and other new data in the TAAP Conejos-Médanos 

report (Mexican Geological Service, 2011a and 2011b) will be compared with the newly published 

interpretations (J.H. Hawley, New Mexico State University, written commun., 2011) and integrated into 

a basin-wide hydrogeologic framework. Analysis of microseismic events may also allow definition of 

faults and flow boundaries, which have been found to play an important role in Mesilla Basin 

hydrogeology. The final product of joint work sessions would be an improved understanding of the 

hydrogeologic framework across the entire basin. 

Development of a joint conceptual water budget is also needed for the development of the 

groundwater-flow model. Estimates of agricultural pumpage and evapotranspiration can be extracted 

from the Farm Process, having been revised to more accurately reflect the processes of the largest water 

use in the Mesilla Basin, agriculture, and to more accurately simulate groundwater/surface-water 
29

interactions that may affect the Rio Grande. Municipal pumpage estimates can be integrated from State

and municipal agencies, including data from the Conejos-Médanos well field, which began full 

operation in mid-2010. If available, seasonal deformation using remotely sensed data could be 

incorporated to update pumping data periodically. Exchange between surface waters (the Rio Grande, 

canals, and drains) and the aquifer system, deep infiltration from agricultural applications, interbasin 

flows, cross-border groundwater flow, water-table elevations and drawdowns, mountain-front recharge, 

riparian evapotranspiration, and other discrete water budget components will be defined in joint working 

sessions. 

Lessons learned from the Mesilla Basin activities could also be used in developing joint work 

plans for the Hueco Bolson and Presidio-Redford Basin in west Texas and the Mimbres Basin in 

southwest New Mexico. 

References

CH2M-Hill Albuquerque, 2002, Groundwater modeling of the Cañutillo wellfield, prepared for El Paso 

Utility Public Service Board.

Creel, B.J., Hawley, J.W., Kennedy, J.F., and Granados-Olivas, A., 2006, Groundwater resources of the 

New Mexico-Texas-Chihuahua border region: New Mexico Journal of Science, v. 44. p. 11–29. 

 

Frenzel, P.F., and Kaehler, C.A., 1990, Geohydrology and simulation of ground-water flow in the 

Mesilla Basin, Dona Ana County, New Mexico, and El Paso, Texas: U.S. Geological Survey OpenFile Report 88–305. 

Hamilton, S.L., and Maddock, Thomas, III, 1993, Application of a ground-water flow model to the 

Mesilla Basin, New Mexico and Texas: Department of Hydrology and Water Resources, University of 

Arizona. 

Hawley, J.W., and Kennedy, J.F., 2004, Creation of a digital hydrogeologic framework model of the 

Mesilla Basin and southern Jornada del Muerto Basin (Report prepared for Lower Rio Grande Water 

Users Organization): New Mexico Water Resources Research Institute, Technical Completion Report 

332, 105 p., with CD ROM including 2005 Addendum extending model into Rincon Valley and 

adjacent areas. 

Heywood, C.E., and Yager, R.M., 2003, Simulated ground-water flow in the Hueco Bolson, an alluvial 

basin aquifer near El Paso, Texas: U.S. Geological Survey Water-Resources Investigations Report 

02–4108. 

Mexican Geological Service, 2011a, Hydrogeological activities in the Conejos-Medanos aquifer, State 

of Chihuahua, Phase I, v. I (report), 109 p. 

 

Mexican Geological Service, 2011b, Hydrogeological activities in the Conejos-Medanos aquifer, State 

of Chihuahua, Phase I, v. II (Appendixes), 355 p. 
30

Schmid, Wolfgang, and Hanson, R.T., 2009, The Farm Process Version 2 (FMP2) for MODFLOW2005 – Modifications and Upgrades to FMP1: U.S. Geological Survey Techniques and Methods 

6–A–32, 102 p. 

S.S. Papadopulos & Associates, 2007, Groundwater flow model for administration and management in 

the Lower Rio Grande Basin: Consultant report prepared for the New Mexico State Engineer, 63 p. 

Weeden, A.C., and Maddock, Thomas, III, 1999, Simulation of groundwater flow in the Rincon Valley 

area and Mesilla Basin, New Mexico and Texas: University of Arizona Research Laboratory for 

Riparian Studies and Department of Hydrology and Water Resources, HWR No. 99–020. 

Project Publications

Kambhammettu, B.V.N.P., Chandramouli, S., and King, J.P., 2011, An improved DEM aggregation 

technique for models with non-uniform resolution: International Journal of Geomatics and 

Geosciences, v. 1, no. 4, p. 962–970. 

Kambhammettu, B.V.N.P., King, J.P., and Allena, Praveena, 2011, Evaluation of mountain-front 

recharge estimation techniques for Southern New Mexico basins: International Journal of Water 

Resources and Environmental Engineering, v. 3, no. 3, p. 66–72. 

Kambhammettu, B.V.N.P., Schmid, Wolfgang, King, J.P., and Creel, B.J., 2012, Effect of elevation 

resolution on evapotranspiration simulations using MODFLOW: Ground Water, v. 50, no. 3, 

p. 367–375. 

Maddock, Thomas, III, Baird, K.J., Hanson, R.T., Schmid, Wolfgang, and Ajami, Hoori, 2012, RIP-ET: 

A riparian evapotranspiration package for MODFLOW-2005: U.S. Geological Survey Techniques 

and Methods 6–A39, 76 p. 

Michelsen, Ari, Alley, William, Sheng, Zhuping, King, J.P., Darr, Michael, Megdal, S.B., and Cafaggi, 

Felix, 2011, Transboundary aquifer assessment program [abst.]: Panel, American Water Resources 

Association 2011 Annual Water Resources Conference, Albuquerque, N.M., November 7–10, 2011. 

Michelsen, Ari, Sheng, Zhuping, Creel, B.J., and Harris, Bill, 2010, Transboundary Aquifer Assessment 

Program: Challenges and opportunities [abst.]: Universities Council on Water Resources/National 

Institutes for Water Resources, Seattle, Wash., July 13–15. 

Texas Water Resources Institute and New Mexico Water Resources Research Institute, 2008, 

U.S.-Mexico Transboundary Aquifer Assessment Program- Arizona (TAAP-A/S): Fact Sheet.

 

Texas Water Resources Institute and New Mexico Water Resources Research Institute, 2009, 

U.S.-Mexico Transboundary Aquifer Assessment Program- Arizona (TAAP-A/S): Fact Sheet.

Texas Water Resources Institute and New Mexico Water Resources Research Institute, 2010, 

U.S.-Mexico Transboundary Aquifer Assessment Program- Arizona (TAAP-A/S): Fact Sheet.

 
31

Texas Water Resources Institute and New Mexico Water Resources Research Institute, 2011, 

U.S.-Mexico Transboundary Aquifer Assessment Program- Arizona (TAAP-A/S): Fact Sheet.