Investigation of the Origin of Hueco Bolson and Mesilla Basin Aquifers (US and Mexico) with Isotopic Data Analysis

: An important tool to identify the origin of a groundwater resource is the use of isotopic signatures. Isotopic signatures give us the age of water and provide information as to the water’s origin, potential transit at geologic structures, source of salinization, and possible recharge points. The purpose of this study was to collect and analyze well samples to evaluate isotopic tracers ( δ 18 O and tritium) in the transboundary Conejos-M é danos/Mesilla aquifer located between the US and Mexico. This new analyzed information was compared with the isotopic information available in the US Mesilla and US-MX Hueco basins generated by previous works, which described the common origin of the Hueco Bolson and Mesilla Basins aquifers. This study used isotopic analysis to validate the theory of the original formation and interconnectivity of both transboundary basins. This research presents new data of δ 18 O and tritium, and a comparison with previous published data from other workers, versus the known global meteoric water line (GMWL) and the Rio Grande evaporation line (RGEL). Results show that the groundwater at the transboundary aquifer features an evaporated isotopic signal, which is consistent with referenced published data that discusses the geologic history of aquifer formations at the studied area. This study is important because isotopic studies from the area were nonexistent and because isotopic data can explain recharge scenarios that relate to groundwater quality.


Introduction
In the Paso del Norte (PdN) transboundary aquifers region, located between the United States and Mexico, where New Mexico, Texas, and Chihuahua meet, the climate is semiarid. Water is increasingly scarce due to surface supply reductions caused by drought and climate change, increased demands from growing regional populations, and municipal and industrial (M&I) expansion affecting availability for environmental demands. Based on these reductions, there is an urgent need for better understanding and management of the quantity and quality of the region's scarce water resources. In this binational region, groundwater is the main source for agriculture and M&I water demands; therefore, understanding the origin of groundwater recharge is critical for better management and long-term sustainability of the basin's groundwater [1,2]. Estimation of groundwater recharge can be made via different methods, such as the general water balance approach, field measurement, or isotopic studies. The evaluation is more accurate when isotope and geochemistry methods are combined [3,4]. Isotope and geochemistry methods are One of the most important rivers in the US is the Rio Grande, or the Rio Bravo as it is called in Mexico ( Figure 1). The Rio Grande watershed has an area of approximately 924,300 miles 2 (2,394,000 km 2 ) and includes regions in both the US and Mexico [14]. With a length of about 1900 miles (3060 Km), it is the 20th longest river in the world, the 5th longest river in North America, and is the 2nd longest American river after the Mississippi [15]. The Rio Grande begins in the San Juan Mountains of southern Colorado, which are part of the Rocky Mountains, and flows through New Mexico and Texas. In the south, the Rio Grande marks the borderline between the US and Mexico [16]. In Mexico, the river runs through Chihuahua, Coahuila, Nuevo Leon, and Tamaulipas, finally ending in the Gulf of Mexico. The Rio Grande has two international dams, Falcon and La Amistad, that are managed by the International Boundary and Water Commission/Comisión Internacional de Limites y Agua (IBWC/CILA) [14]. Figure 1 shows the entire watershed of the Rio Grande from Colorado to the Gulf of Mexico.
From the above-mentioned studies, the ones using environmental tracers such as δ 18 O and tritium as well as and basic physicochemical parameters were selected for our analysis. These studies also provided the spatial distribution that enabled us to cover the area between the Hueco Bolson and Conejos-Médanos/Mesilla aquifers.

Study Area
Of the various aquifers along the Rio Grande, this study focuses on one of the most important transboundary regions between the United States and Mexico: the cross-border area of Juárez, Chihuahua in Mexico and Las Cruces, NM and El Paso, TX in the US. In this Paso del Norte or PdN transboundary region, groundwater uses are mainly supported by two transboundary aquifers: the Hueco Bolson and the Conejos Médanos/Mesilla Basin aquifers ( Figure 1). Several communities along the US-Mexico border in New Mexico, Texas and Chihuahua depend on these aquifers for domestic, agricultural, and industrial water use [30]. In this study, special attention was given to the Mexican side of the Conejos-Médanos Basin aquifer where isotopic studies that could explain recharge scenarios in the area and their relationship with groundwater quality were nonexistent.
From the above-mentioned studies, the ones using environmental tracers such as δ 18 O and tritium as well as and basic physicochemical parameters were selected for our analysis. These studies also provided the spatial distribution that enabled us to cover the area between the Hueco Bolson and Conejos-Médanos/Mesilla aquifers.

Study Area
Of the various aquifers along the Rio Grande, this study focuses on one of the most important transboundary regions between the United States and Mexico: the cross-border area of Juárez, Chihuahua in Mexico and Las Cruces, NM and El Paso, TX in the US. In this Paso del Norte or PdN transboundary region, groundwater uses are mainly supported by two transboundary aquifers: the Hueco Bolson and the Conejos Médanos/Mesilla Basin aquifers ( Figure 1). Several communities along the US-Mexico border in New Mexico, Texas and Chihuahua depend on these aquifers for domestic, agricultural, and industrial water use [30]. In this study, special attention was given to the Mexican side of the Conejos-Médanos Basin aquifer where isotopic studies that could explain recharge scenarios in the area and their relationship with groundwater quality were nonexistent. Cliett (1969) [31] mentioned that the geology of the Conejos-Médanos Basin aquifer is comparable to the Hueco Bolson aquifer, both having similar depositional environments on the geological time scale of the aquifers. Despite these similarities, they differ in their lithology and groundwater qualities, with differing sediments from contemporary basin fill within the surface area of the aquifer. Additionally, Cliett (1969) [31] defined that the two sediment units are hydraulically connected, meeting the aquifer at an estimated average depth of 152.4 m (500 ft). Regarding water levels, in the case of the shallow Hueco Bolson aquifer, along the agricultural zone of the Valle de Juárez, static levels were on average 12.19 m (40 ft) and superficially at 3 m (10 ft). Hawley et al. (2009) [32] developed a hydrogeological model based on reports and peer-reviewed research to promote the exchange of information to provide a better understanding of water problems and possible alternative solutions to address them. His group's hydrogeological model includes the area of the Mesilla aquifer, a section of the Rio Grande in north-central Chihuahua, Mexico, and parts adjacent to the south of the Jornada del Muerto Basin, where the contact between the strata is shown as well as the basin's sedimentary fill. The basement that represents the bedrock and the tectonic characteristics of the area are reflected not only in the composition of the sedimentary fill, but also in the groundwater flow and chemistry according to its time of residence. The source of sediment fill in this aquifer was the surrounding mountains, consisting largely of Paleozoic sedimentary rocks inclined on a base of Precambrian rocks; these mountains also contain Tertiary volcanic rocks [31].
Appendix A (see Figure A1) shows the sedimentary Santa Fe Group with the evolution and tectonic faults of the basins in the southern region of the Rio Grande. In the past 25 million years, this region has had a profound effect on the distribution of the groupings in the lithofacies (strata) of the Santa Fe Group [33]. Hawley and Lozinsky (1992) [34] subdivided the Santa Fe Group into three stratigraphic units: lower, middle, and upper. These units are defined based on the general lithological character, the depositional environments of the fill, and the characteristics related to the post-depositional history.
Hawley and Swanson (2022 in revision) [35], show that the hydrogeological framework controls on groundwater flow and chemistry in the transboundary-aquifers system west of the lower Mesilla Valley (MeV) and PdN transboundary aquifers systems in this areaare comprised of: 1) thick Santa Fe Group (SFG) rift-basin fill (as much as 600 m), and 2) the thin (≤20 m) alluvial aquifers of the inner-river valley. They also recognized that at least the upper part of the SFG aquifer system was present in Chihuahua, located as far south as the Federal Highway 2 corridor west of the Juarez and Sapello mountain ranges in Mexico. In regard to groundwater quality in the transboundary Mesilla/Conejos-Médanos Basin aquifer, Hawley and Swanson (2022 in revision) [35] address that the ongoing research has demonstrated that very large quantities of fresh to slightly saline water are stored in the basin-fill aquifer system, where most groundwater in storage is at least 11ka and was recharged during the last glacial/pluvial stage of the Late Pleistocene Epoch (~29 to 11 ka).

Materials and Methods
The Conejos-Médanos Basin data were collected from the JMAS wells on the Mexican side of the Mesilla Basin aquifer. We collected sixteen samples (Figure 2a   This study offers a significant contribution as it completes the characterization of the Conejos-Médanos/Mesilla Basin aquifer isotopic system by providing results from the Mexican side of the aquifer to the already existing data from the US side. To complete the system analysis in this region, we compared our results with similar previous research on the US side of the Mesilla Basin aquifer [11] and a study of the Hueco Bolson aquifer between the US and Mexican sides [12].

Mesilla Basin Aquifer Data
In 2010, Teeple (2017) [11] gathered 44 isotopic samples (Table 1) from four hydrologic units in the Mesilla Basin aquifer on the US side. He used the subdivision of the groundwater flow system outlined by Hawley and Lozinsky (1992) [34] to divide the study area. Subdivisions made by them were four hydrological units (Table 1) including the Rio Grande Alluvium, which is from a quaternary system and is part of the Santa Fe Group. The Santa Fe Group is a Tertiary system divided into three hydrogeologic units, the Upper, Middle, and Lower Santa Fe Group. The southern boundary in the study area of Teeple (2017) [11] was the border between the US and Mexico.  This study offers a significant contribution as it completes the characterization of the Conejos-Médanos/Mesilla Basin aquifer isotopic system by providing results from the Mexican side of the aquifer to the already existing data from the US side. To complete the system analysis in this region, we compared our results with similar previous research on the US side of the Mesilla Basin aquifer [11] and a study of the Hueco Bolson aquifer between the US and Mexican sides [12].

Mesilla Basin Aquifer Data
In 2010, Teeple (2017) [11] gathered 44 isotopic samples (Table 1) from four hydrologic units in the Mesilla Basin aquifer on the US side. He used the subdivision of the groundwater flow system outlined by Hawley and Lozinsky (1992) [34] to divide the study area. Subdivisions made by them were four hydrological units (Table 1) including the Rio Grande Alluvium, which is from a quaternary system and is part of the Santa Fe Group. The Santa Fe Group is a Tertiary system divided into three hydrogeologic units, the Upper, Middle, and Lower Santa Fe Group. The southern boundary in the study area of Teeple (2017) [11] was the border between the US and Mexico. This study offers a significant contribution as it completes the characterization of the Conejos-Médanos/Mesilla Basin aquifer isotopic system by providing results from the Mexican side of the aquifer to the already existing data from the US side. To complete the system analysis in this region, we compared our results with similar previous research on the US side of the Mesilla Basin aquifer [11] and a study of the Hueco Bolson aquifer between the US and Mexican sides [12].

Mesilla Basin Aquifer Data
In 2010, Teeple (2017) [11] gathered 44 isotopic samples (Table 1) from four hydrologic units in the Mesilla Basin aquifer on the US side. He used the subdivision of the groundwater flow system outlined by Hawley and Lozinsky (1992) [34] to divide the study area. Subdivisions made by them were four hydrological units (Table 1) including the Rio Grande Alluvium, which is from a quaternary system and is part of the Santa Fe Group. The Santa Fe Group is a Tertiary system divided into three hydrogeologic units, the Upper, Middle, and Lower Santa Fe Group. The southern boundary in the study area of Teeple (2017) [11] was the border between the US and Mexico.
The aquifer was divided into four hydrogeological units based on the terrain stratigraphy and groundwater flow of the Mesilla aquifer as shown in Table 1.  [11].

Area Samples
Rio Tritium results shown by Teeple (2017) [11] were analyzed at the Menlo Park Tritium Laboratory in Menlo Park, CA under the procedures of Östlund and Werner (1962) [36] and Thatcher et al. (1977) [37].
The analyses for stable isotope ratios of δD and δ 18 O in Teeple (2017) [11] were conducted at the USGS Stable Isotope Laboratory in Reston, Va. Under the described methods in Révész and Coplen (2008b) [38].
This study was carried out on the US side of the Mesilla aquifer in cooperation with the USGS, IBWC, NM WRRI, NMSU, Texas AgriLife Research, TWRI, and Texas A&M. The results from the 44 samples in the Teeple (2017) [11] study were predominantly Na-HCO3 or a Na-SO4-HCO3 geochemistry water groups. For tritium, the results indicate negative values, which means there was no tritium content because of the decay. Teeple (2017) [11] mentioned that results show groundwater flows are generally from the north to south-southeast and that there is a pattern of groundwater discharging in the PdN.

Hueco Bolson Aquifer Data
Previous studies of the Hueco Bolson aquifer on the Mexican side indicate an increasing trend of calcium and sulfate ions with total dissolved solids (TDS) of more than 750 mg/L. This shows a deterioration in water quality during the 1965-1999 period [39]. Eastoe et al. (2007) [12] conducted an analysis of the isotopic concentration in the Hueco Bolson. They made a subdivision of hydrologic units ( Table 2). This subdivision encompasses the Hueco Bolson Aquifer in both the US and Mexico.  [12] gathered 75 samples of groundwater and precipitation. Groundwater was sampled from public and private wells; precipitation samples were from the Juarez region. Stable oxygen and hydrogen isotopes were measured with a gas source isotope radio-frequency mass spectrometer (Finnigan). The delta value was standardized with the Vienna Standard Mean Ocean Water (VSMOW). Liquid scintillation spectrophotometry was used for tritium analysis. The stable and unstable isotope analysis was carried out in the laboratory at the University of Arizona.
Results from stable isotope data showed four types of groundwater recharge. The authors identified two sources of recharge from the Rio Grande and another two sources of recharge from local precipitation.
Previous studies used to perform this assessment were selected as they have published the same type of analysis and data samples in different locations. Table 3 shows the data from the sources referred to in this study by the author. Appendix B (see Table A1) contains a record of all the data used to perform the analysis. "ID" means the identification of the sample in this study; "Source" is the name of the well sampled; "Date" refers to the year when the sample was taken; "Latitude and longitude" mean the sample coordinates; "δ 18 O and T" refer to the isotopic values obtained for oxygen, hydrogen, and tritium, respectively; and "Group," to the group previously named by the authors. Additionally, from Eastoe et al.  [11]. In this study, the Conejos Médanos Basin is labeled (CM).

Results
Hydrogeochemical results show groundwater ions are predominantly Cl+SO 4 and HCO 3, throughout the area. There is a mixture of waters that have the main components Na + , Cl − and SO 4 − ions. Due to the type of sediment fill deposit around the Conejos Médanos aquifer, the presence of these ions throughout the aquifer was expected. Geochemically, this reflects the rock interaction that predominates in this area and reveals current rock deterioration through the mineralization of the waters throughout the region of the Conejos Médanos aquifer. Figure 4 shows the Mesilla and Hueco aquifers and geographical locations of the samples collected by this study, Teeple (2017) [11], and Eastoe et al. (2007) [12].

Tritium
The tritium results obtained in this study (Figure 4, green points) in the Conejos Médanos Basin varied from −0.70 to 0.58 Tritium Units (TU), which is a non-significant tritium content because the absence of tritium or values below <0.5 TU indicate that the age of waters is not greater than 50 years. This is an important finding because it indicates that the water present in this zone is not of recent origin, which demonstrates that there is no recharge in this zone. Furthermore, this study does not report any significant tritium concentrations in the Conejos Médanos Aquifer.
The Mesilla Basin aquifer results obtained by Teeple (2017) [11] indicate the presence of pre-boom waters, which refers to water recharged prior to 1950. Teeple (2017) [11] found high concentrations of tritium in two samples collected from wells in the Rio Grande Alluvium; the values were 4.6 TU (T Q18) and 7.5 TU (T Q26). In the Hueco Bolson, the highest concentrations followed the same path as the Mesilla Basin aquifer [12].

Tritium
The tritium results obtained in this study (Figure 4, green points) in the Conejos Médanos Basin varied from −0.70 to 0.58 Tritium Units (TU), which is a non-significant tritium content because the absence of tritium or values below <0.5 TU indicate that the age of waters is not greater than 50 years. This is an important finding because it indicates that the water present in this zone is not of recent origin, which demonstrates that there is no recharge in this zone. Furthermore, this study does not report any significant tritium concentrations in the Conejos Médanos Aquifer.
The Mesilla Basin aquifer results obtained by Teeple (2017) [11] indicate the presence of pre-boom waters, which refers to water recharged prior to 1950. Teeple (2017) [11] found high concentrations of tritium in two samples collected from wells in the Rio Grande Alluvium; the values were 4.6 TU (T Q18) and 7.5 TU (T Q26). In the Hueco Bolson, the highest concentrations followed the same path as the Mesilla Basin aquifer [12].   [12]. The yellow points with black borders show the values with more than 2 TU from Teeple (2017) [11]. The orange points indicate values lower than 2 TU from Eastoe et al. (2007) [12]. The yellow points indicate values lower than 2 TU from Teeple (2017) [11]. The green points indicate values lower than 2 TU from this study. The Mesilla Basin aquifer is featured in blue, and the Hueco Bolson in light blue. The line in dark blue shows the Rio Grande mainstream. Figure 6 shows a compilation of the sample points. The samples are grouped into numbers and letters. The letters are given by the author and apply only to the samples taken by Eastoe et al. (2007) [12]. The data gathered from Eastoe et al. (2007) [12] are featured in orange squares (Group A), circles (Group B), and diamonds (Group C); each shape represents a different group given by the author. The data from this study are shown by green circles; and the data by Teeple (2017) [11] in yellow circles. The values of all points were compared with the GMWL and the RGEL to determine the changes in the  [12]. The yellow points with black borders show the values with more than 2 TU from Teeple (2017) [11]. The orange points indicate values lower than 2 TU from Eastoe et al. (2007) [12]. The yellow points indicate values lower than 2 TU from Teeple (2017) [11]. The green points indicate values lower than 2 TU from this study. The Mesilla Basin aquifer is featured in blue, and the Hueco Bolson in light blue. The line in dark blue shows the Rio Grande mainstream. Figure 6 shows a compilation of the sample points. The samples are grouped into numbers and letters. The letters are given by the author and apply only to the samples taken by Eastoe et al. (2007) [12]. The data gathered from Eastoe et al. (2007) [12] are featured in orange squares (Group A), circles (Group B), and diamonds (Group C); each shape represents a different group given by the author. The data from this study are shown by green circles; and the data by Teeple (2017) [11] in yellow circles. The values of all points were compared with the GMWL and the RGEL to determine the changes in the water's isotopic composition, produced by different processes. A total of three groups were obtained.

Oxygen 18 (δ 18 O)
Group 1 is in the GMWL and is made up of samples from group C. Some of these were taken by Eastoe et al. (2007) [12] from the Hueco Bolson (orange diamonds), while five samples came from the Teeple (2017) [11] study (yellow circles). Group C comes from the Franklin and Organ Mountains. Eastoe et al. (2007) [12] mentioned that similar water could be originating in the Juarez Mountains (Sierra de Juárez). On the other hand, the five samples from Teeple (2017) [11] (yellow points) are TQ12, TQ14, TQ16, TQ30, and TQ32 (See Appendix B). These samples were taken in the Mesilla Basin near the Rio Grande Alluvium, which means that water from the river is present in these locations. In Group 1, waters are located in or near the GMWL because no current depletion can be seen in the isotopes.
Group 2 results feature 14 samples close to the line while the rest are slightly above the line. Of those first fourteen samples, three (orange squares) are E1, E2, and E3 (See Appendix B); they are part of Group A and were taken by Eastoe et al. (2007) [12] in the Hueco Bolson aquifer in Chihuahua, near the Rio Grande. These three samples have an isotopic composition of δ 18 O, which varies slightly between −8.6 and −9.4. Another nine samples (yellow points) were TQ00, TQ03, TQ09, TQ13, TQ18, TQ23, TQ24, TQ25, and TQ36 (See Appendix B); they were taken by Teeple (2017) [11] and show an isotopic composition of δ 18 O with a variation of −7.74 to −8.97. The last of the fourteen samples found in RGEL were taken by this study in the Conejos-Médanos set of wells of the JMAS; these featured an isotopic composition of δ 18 O and a variation of −8.83. The rest of the Group 2 samples that are slightly above the RGEL were taken by this study and Teeple (2017) [11] in the Mesilla/Conejos-Médanos Basin.
The results of stable δ 18 O isotopes in this study are not near the GMWL, but they are near the RGEL. According to Teeple (2017) [11], and Witcher et al. (2004) [24], these results could indicate that groundwater has a Rio Grande isotopic signature from the ancestral Rio Grande and this could be a sign of evaporated waters. In addition, they show that recharge sources include precipitation, bedrock fissure water, and irrigation return water. Finally, they also point to water evaporation.
Group 3 is made of three samples which are in or near the RGEL. This group is formed by three samples from Group A taken by Eastoe et al. (2007) [12] in the Hueco Bolson aquifer in Chihuahua near the Rio Grande. The group is made up of Group B (orange circles), taken by Eastoe et al. (2007) [12] and consisting of samples collected beneath the urban area of Juárez City and the Rio Grande floodplain in El Paso. The geographical area in which the samples were collected is a semi-arid area where evaporation processes occur; this phenomenon could have affected the process. This dataset falls below the GMWL, indicating that water has evaporated. Group 3 is also formed by samples taken by Teeple (2017) [11].
The study by Teeple (2017) [11] reports that values of less than −80.0 and −10.5 δ 18 O/δD (‰) have an apparent age of less than 10,000 carbon-14 years before present (1950). Samples from this age are found near the Rio Grande Alluvion. Values greater than −80.0 and −10.5 δ 18 O/δD (‰) have an age greater than 10,000 carbon-14 years before present (1950). Samples of this age are found in the southeast of the Mesilla Basin aquifer, near the Hueco Bolson and the Juarez Mountains. Such a group of results is consistent with results from this study in the Conejos-Médanos region and with those of Group C, from Eastoe et al. (2007) [12], which are marked as Group 3 in Figure 6. The study by Teeple (2017) [11] reports that values of less than −80.0 and −10.5 δ 18 O/δD (‰) have an apparent age of less than 10,000 carbon-14 years before present (1950). Samples from this age are found near the Rio Grande Alluvion. Values greater than −80.0 and −10.5 δ 18 O/δD (‰) have an age greater than 10,000 carbon-14 years before present (1950). Samples of this age are found in the southeast of the Mesilla Basin aquifer, near the Hueco Bolson and the Juarez Mountains. Such a group of results is consistent with results from this study in the Conejos-Médanos region and with those of Group C, from Eastoe et al. (2007) [12], which are marked as Group 3 in Figure 6.

Conclusions
According to the age determined by the results of the isotopic concentration and the δ 18 O/δD of the water, Group 2 is formed by old water. Occasionally an addition of 18 O is caused by dissolution processes, and this can increase with geothermal activity; having this geothermal change could have caused a movement to the right of the GMWL. This, in Figure 6, indicates that the "X" axis, which is 18 O, moved to the right, achieving a greater concentration of 18 O. On the contrary, the "Y" axis, which represents a 16 O concentration, decreased. This change to a concentration greater than 18 O and lower than 16 O results in an isotopically heavier δ 18 O signature but without any change in the δ 2 H signature [2,24]. Most of the groundwater samples that are plotted along the displaced GMWL represent isotopically lighter water, with δD values of less than −80.00 per thousand and δ 18 O values of less than −10.50 per thousand [40]. This isotopic signature indicates that the samples in Group 2 probably underwent water recharge during the relatively humid and cool Pleistocene climate [40].  [12] and Teeple (2017) [11] was compared to the global meteoric water line (GMWL) and Rio Grande evaporation line (RGEL). The graph was divided into three groups, these groups considered all the samples in Appendix B. Group 1) is formed by water samples from the Mesilla and Hueco basins taken by Teeple (2017) [11] and Eastoe et al. (2007) [12]. Group 2) consists of samples from the Hueco Bolson and the Mesilla/Conejos-Médanos Basin aquifers, and they are samples taken by this study, Teeple (2017) [11] and Eastoe et al. (2007) [12]. Group 3) contains samples from the Mesilla Basin aquifer and the Bolson del Hueco; the samples were taken by Teeple (2017) [11] and Eastoe et al. (2007) [12].

Conclusions
According to the age determined by the results of the isotopic concentration and the δ 18 O/δD of the water, Group 2 is formed by old water. Occasionally an addition of 18 O is caused by dissolution processes, and this can increase with geothermal activity; having this geothermal change could have caused a movement to the right of the GMWL. This, in Figure 6, indicates that the "X" axis, which is 18 O, moved to the right, achieving a greater concentration of 18 O. On the contrary, the "Y" axis, which represents a 16 O concentration, decreased. This change to a concentration greater than 18 O and lower than 16 O results in an isotopically heavier δ 18 O signature but without any change in the δ 2 H signature [2,24]. Most of the groundwater samples that are plotted along the displaced GMWL represent isotopically lighter water, with δD values of less than −80.00 per thousand and δ 18 O values of less than −10.50 per thousand [40]. This isotopic signature indicates that the samples in Group 2 probably underwent water recharge during the relatively humid and cool Pleistocene climate [40].
According to Witcher et al. (2004) [24] and Bumgarner (2012) [40], the GMWL in the studied area has been displaced and represents ancient groundwater and geothermal groundwater, from which 18 O of the rocks have been obtained. This was due to an exchange processes that typically occurs with the water-rock interaction and probable hydrothermal alteration. Such an alteration occurs when the oxygen present in the groundwater is exchanged due to the composition of the rock, temperature, texture, and length of contact [24].
The compilation of isotopic data provided by this article is important as it allows for the comparison of water samples from different locations in the US-Mexico borderland area of the Hueco Bolson and Mesilla Basin aquifers. The locations of the samples collected contribute to understanding the water origin of the studied area. Hawley and Kottlowski (1969) [13] established that the Rio Grande flowed across the western area of the Juarez Mountains and that water from the Rio Grande drained into the Cabeza de Baca Ancient Lake, going through the sedimentary deposits which are presently part of the Mesilla Basin aquifer [3]. However, with the formation of the Juarez Mountains in the Quaternary period, the Rio Grande changed its course, carving its way through the El Paso Canyon over the course of recent geological times, flowing between the Franklin Mountains and the Juarez Mountains through the canyon that formed between the neighboring mountains [13].
As different authors mention, a primary source of recharge into the Mesilla Basin aquifer system is the Rio Grande Alluvium in the Mesilla Valley because of the seepage losses from the riverbed. From previous and new data evaluated, we conclude that the Conejos-Médanos Basin aquifer has the same source of water as the Hueco Bolson does from Group A of Eastoe et al. (2007) [12]. The Group A samples were taken near the Rio Grande at the foot mountain in the Juarez Mountains. Moreover, as was expected, the Group 1 samples collected by Teeple (2017) [11] at the south of the Mesilla Valley to the Conejos Médanos Basin aquifer signal the presence of the same type of water in this area.
In conclusion, the samples collected and analyzed by this study complete the description of the Hueco Bolson and the Mesilla/Conejos-Médanos Basin at the US-Mexico transboundary area. According to previous study results shown for Group 2, a stable isotope δ 18 O concentration falls below the GMWL in the evaporated zone, which indicates that these are old waters that have undergone evaporation, horizontal infiltration, or dissolution processes. Moreover, groundwater values indicate that groundwater recharge sources include precipitation, bedrock fissure water, or both. Furthermore, results are consistent with findings by Eastoe et al. (2007) [12], Teeple (2017) [11], Hawley and Kottlowski (1969) [13], Witcher et al. (2004) [24], and Bumgarner (2017) [40], whose findings indicate that the groundwater is not recent and that it was recharged thousands of years ago when the climate was more humid, which could be the cause for the same isotopic content in the Hueco Bolson and Conejos-Médanos/Mesilla Basin aquifers near the Juarez Mountains. Funding: The author's work has been supported by funding through the U.S. Geological Survey Award #G17AC00441 to New Mexico State University, entitled "Transboundary Aquifer Assessment Program". Informed Consent Statement: Informed consent was obtained from all subjects involved in the study.

Data Availability Statement:
The date presented in this study are available on request from the corresponding author.  [12], T by Teeple (2017) [11], TS by This study.