hydrology

Article Groundwater Origin and Dynamics on the Eastern Flank of the Colorado River Delta, Mexico

Hector A. Zamora 1,*, Christopher J. Eastoe 1,† , Jennifer C. McIntosh 2 and Karl W. Flessa 1

1 Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA; [email protected] (C.J.E.); kfl[email protected] (K.W.F.) 2 Department of Hydrology and Atmospheric Sciences, University of Arizona, Tucson, AZ 85721, USA; [email protected] * Correspondence: [email protected] † This author is retired.

Abstract: Isotope data and major ion chemistry were used to identify aquifer recharge mechanisms and geochemical evolution of groundwaters along the US–Mexico border. Local recharge originates as precipitation and occurs during winter through preferential infiltration pathways along the base of the Gila Range. This groundwater is dominated by Na–Cl of meteoric origin and is highly concentrated due to the dissolution of soluble salts accumulated in the near-surface. The hydrochemical evolution of waters in the irrigated floodplain is controlled by Ca–Mg–Cl/Na–Cl-type Colorado River water. However, salinity is increased through evapotranspiration, precipitation of calcite, dissolution of accumulated soil salts, de-dolomitization, and exchange of aqueous Ca2+ for adsorbed Na+. The Na–Cl-dominated local recharge flows southwest from the Gila Range and mixes with the Ca–Mg– 3


Cl/Na–Cl-dominated floodplain waters beneath the Yuma and San Luis Mesas. Low H suggests
 that recharge within the Yuma and San Luis Mesas occurred at least before the 1950s, and 14C data 14 Citation: Zamora, H.A.; Eastoe, C.J.; are consistent with bulk residence times up to 11,500 uncorrected C years before present. Either the McIntosh, J.C.; Flessa, K.W. flow system is not actively recharged, or recharge occurs at a significantly lower rate than what is Groundwater Origin and Dynamics being withdrawn, leading to aquifer overdraft and deterioration. on the Eastern Flank of the Colorado River Delta, Mexico. Hydrology 2021, Keywords: groundwater; environmental isotopes; hydrochemistry; transboundary aquifer; Colorado 8, 80. https://doi.org/10.3390/ River Delta hydrology8020080

Academic Editors: Molla B. Demlie and Tamiru A. Abiye 1. Introduction The Colorado River is strictly managed. Water regulatory practices are implemented to Received: 17 April 2021 provide vital water resources to seven states in the USA and two states in Mexico (Figure1). Accepted: 8 May 2021 Published: 11 May 2021 In the Lower Colorado River Basin, south of the Utah–Arizona border, more than 27 million people depend on the river for sustenance. Nearly 1.2 million ha of farmland are irrigated

Publisher’s Note: MDPI stays neutral with Colorado River water in the fertile and productive fields of the Mexicali and Imperial with regard to jurisdictional claims in Valleys [1]. published maps and institutional affil- The lower Colorado River basin has seen extensive land-use changes in the last century. iations. Pastures and crops have replaced native vegetation, and surface Colorado River water has been diverted for irrigation. These changes led to a massive loss of natural habitat in the Colorado River Delta (termed “Delta” below). The river no longer reaches the lower part of the Delta today, and riparian, wetland, and estuarine habitats occupy less than 5% of their original 780,000 ha extent [2]. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This study aims to establish sources of solutes, sources of aquifer recharge, groundwa- This article is an open access article ter residence time, and geographic variation of major ion chemistry in groundwater on the distributed under the terms and eastern flank of the Colorado River Delta. This is accomplished using environmental iso- 18 2 3 14 conditions of the Creative Commons tope data (δ O, δ H, H, and C) and major ion chemistry. In the over-allocated Colorado Attribution (CC BY) license (https:// River system, distinguishing the different sources of water and salt becomes increasingly creativecommons.org/licenses/by/ important for the long-term management and protection of water resources and the natural 4.0/). and seminatural habitats that depend on them.

Hydrology 2021, 8, 80. https://doi.org/10.3390/hydrology8020080 https://www.mdpi.com/journal/hydrology Hydrology 2021, 8, x FOR PEER REVIEW 2 of 23

Hydrology 2021, 8, 80 rado River system, distinguishing the different sources of water and salt becomes increas-2 of 22 ingly important for the long-term management and protection of water resources and the natural and seminatural habitats that depend on them.

Figure 1. Colorado River Basin ( left)).. Study Study a area:rea: lower lower Colorado Colorado River, River, Colorado River D Deltaelta and major geographical and geological features (right).). The red red-dotted-dotted line shows the limit of the Yuma and San Luis Mesas.Mesas. The Wellton-MohawkWellton-Mohawk Drain (WM Drain) is shown as a solid blue line. OPCNM: Organ Pipe Cactus National Monument. Cross-section A-A’ Drain (WM Drain) is shown as a solid blue line. OPCNM: Organ Pipe Cactus National Monument. Cross-section A-A’ shown as a supplementary file (Figure S1). shown as a supplementary file (Figure S1).

The results of this investigation are used to evaluate the groundwater dynamics and geochemical process of this transboundary aquifer along the US US–Mexico–Mexico border between the states states of of Arizona Arizona and and Sonora. Sonora. Understanding Understanding groundwater groundwater flow flow and andits chemical its chemical evo- lutionevolution are arevital vital for forthe the effective effective management management and and use use of of groundwater groundwater resources resources in in this aquifer vulnerable to overdraft and salinization.

2. Materials Materials and and Methods 2.1. Study Study Area Area 2.1.1. Geography Geography The Colorado Colorado River River D Deltaelta lies lies along along the the western western boundary boundary of the of theSonoran Sonoran Desert Desert and and within the Salton Trough geologic region. The Delta extends from the confluence of within the Salton Trough geologic region. The Delta extends from the confluence of the the Colorado and Gila River near Yuma, Arizona, to the Gulf of California and covers an Colorado and Gila River near Yuma, Arizona, to the Gulf of California and covers an area area of more than 600 km2 (Figure1;[ 3]). The lower Colorado River marks the western of more than 600 km2 (Figure 1; [3]). The lower Colorado River marks the western bound- boundary of this study. The Gran Desierto de Altar is located to the east of the study area ary of this study. The Gran Desierto de Altar is located to the east of the study area and and covers an area of 5500 km2 [4]. The Gila River historically flowed around the Gila covers an area of 5500 km2 [4]. The Gila River historically flowed around the Gila Range, Range, an arid and rugged, northwest-southeast trending range with its highest elevation an arid and rugged, northwest-southeast trending range with its highest elevation at ~960 at ~960 m above sea level (masl; Figure1), before joining the Colorado River. Today, the m above sea level (masl; Figure 1), before joining the Colorado River. Today, the Gila River Gila River rarely reaches the area under normal conditions. rarely reaches the area under normal conditions. 2.1.2. Climate Climate is warm and arid in the area. Records from 1951 to 2010 at Morelos Dam in Baja, California, show maximum temperatures reach 50 ◦C[5]. Annual evaporation rates range between 1.9 and 3.4 m, evapotranspiration is approximately 1.40 m, and annual

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precipitation averages only 63 mm [5]. Precipitation occurs as sporadic events during winter and summer seasons as cyclonic and convective events, respectively [6].

2.1.3. Geology A detailed geological description of the area is provided by [7–11]. In general, the San Luis, Mexicali, and Yuma valleys are part of a graben with an average thickness of 4150 m. The basement consists of igneous and metamorphic rocks of Cretaceous age [7,10]. The valley-fill contains marine sediments deposited during transgressions of the Gulf of California during the Pliocene and Quaternary sedimentary deposits of continental origin [8,11]. Sediment exposed at the surface is of alluvial type (conglomerates, sand, silt, and clays) and represents the most recent deposition cycle from the Colorado River and the Gila River [11]. The study area is bounded by extensive eolian deposits of Pleistocene age to the southeast and igneous and metamorphic rocks of Cretaceous age to the northeast (Gila Range). An escarpment marks the western boundary of the Altar Basin against the Delta. Two faults pass through the study area. The Cerro Prieto fault, which passes southeast into the Gulf of California, is the active southernmost segment of the San Andreas fault system and has strike-slip movement as high as 60 mm/year [12,13]. The Altar fault is an inactive strike-slip fault, running parallel to the Cerro Prieto fault. Both faults dip to the west, have dextral offset and displace the southwestern side downwards.

2.1.4. Colorado and Gila River Discharge Before developing major river engineering projects (the 1930s–1960s), such as Hoover Dam and Glen Canyon Dam, the Colorado River flowed along its natural course and discharged into the Gulf of California. Natural annual flows ranged between 1.6 × 1010 m3 and 1.8 × 1010 m3 at Lee’s Ferry [14]. Peak flows occurred from April to June when late-spring snowmelt from higher elevations entered the area (Figure2). The Gila River contributed an estimated 0.16 × 1010 m3 per year to Colorado River discharge at its confluence near Yuma before upstream diversions dewatered the river [15]. Construction of dams along the Colorado River, particularly the Hoover Dam in 1936, has increased the evaporation of river water and greatly influenced δ18O and δ2H in surface water below Hydrology 2021, 8, x FOR PEER REVIEW 4 of 23 Hoover Dam. Pre-dam and post-dam river water are, consequently, distinctive in isotope composition [16].

FigureFigure 2. 2.Monthly Monthly mean mean discharge discharge in m in3 /s m3 for/s forColorado Colorado River River at Lee’s at Lee’sFerry (USGSFerry ( StationUSGS Station 09380000), and09380000 Gila River), andas Gila combining River as thecombining Gila (USGS the GilaStation (USGS 09474000), Station Verde09474000 (USGS), Verde Station (USGS 09510000) Station and Salt09510000 River) (USGSand Salt Station River ( 09497500).USGS Station All 09497500 data are) available. All data are online available from [online17]. from [17].

The entire flow of the Colorado River is now captured and used before reaching the river’s mouth. South of the USA–Mexico border, no water flows, except during unusually wet years and engineered environmental flows resulting from water treaties between the USA and Mexico [18,19]. Climatic anomalies arise from El Niño Southern Oscillation and affect the entire Colorado River catchment. When upstream storage reservoirs are full, high precipitation during El Niño years can increase river discharge. This was observed at the USA–Mexico border, where daily discharges peaked at 935 m3/s during the mid- 1980s [3].

2.1.5. Hydrogeology Water-bearing units in the area are described in great detail by [9,10]. These are di- vided based on age: Tertiary rocks with poor transmissive properties and Pliocene to Hol- ocene deposits yield a significant amount of water. For this study, we focus on the upper ~300 m of the younger water-bearing sediments where most of the production wells in the study area are found. This part of the aquifer is formed by the upper fine-and medium- grained sediment of the younger alluvium [9,20,21]. Direct infiltration from the Colorado River and overbank flooding were the main source of recharge to the aquifer before major agricultural development [10,22]. Today, the aquifer has an annual recharge of 755 × 106 m3 by infiltration from unlined irrigation canals supplied with Colorado River water and groundwater from the Colorado River Basin upstream of Yuma, Arizona [7,23]. Pre-development, regional groundwater flowed in a northeast-southwest direction from the junction of the Colorado River and Gila River near Yuma, Arizona, to the northern Gulf of California (Figure 3A; [21]). Unlined canals and groundwater pumping have disturbed the source and sink pat- terns of water movement to and from the aquifer within the Delta [10]. Groundwater flow direction has remained constant, but in areas where long-term surface irrigation has oc- curred (e.g., Yuma Valley), groundwater levels are higher now than during pre-develop- ment time (Figure 3B). Several reaches along the river now act as drains for groundwater where groundwater levels are high, but less than 3.5 × 107 m3 (4.6%) of the yearly Colorado River flow discharges as groundwater into the Gulf of California [11,24].

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The entire flow of the Colorado River is now captured and used before reaching the river’s mouth. South of the USA–Mexico border, no water flows, except during unusually wet years and engineered environmental flows resulting from water treaties between the USA and Mexico [18,19]. Climatic anomalies arise from El Niño Southern Oscillation and affect the entire Colorado River catchment. When upstream storage reservoirs are full, high precipitation during El Niño years can increase river discharge. This was observed at the USA–Mexico border, where daily discharges peaked at 935 m3/s during the mid-1980s [3].

2.1.5. Hydrogeology Water-bearing units in the area are described in great detail by [9,10]. These are divided based on age: Tertiary rocks with poor transmissive properties and Pliocene to Holocene deposits yield a significant amount of water. For this study, we focus on the upper ~300 m of the younger water-bearing sediments where most of the production wells in the study area are found. This part of the aquifer is formed by the upper fine-and medium-grained sediment of the younger alluvium [9,20,21]. Direct infiltration from the Colorado River and overbank flooding were the main source of recharge to the aquifer before major agricultural development [10,22]. Today, the aquifer has an annual recharge of 755 × 106 m3 by infiltration from unlined irrigation canals supplied with Colorado River water and groundwater from the Colorado River Basin upstream of Yuma, Arizona [7,23]. Pre-development, regional groundwater flowed in a northeast-southwest direction from the junction of the Colorado River and Gila River near Yuma, Arizona, to the northern Gulf of California (Figure3A; [21]). Unlined canals and groundwater pumping have disturbed the source and sink patterns of water movement to and from the aquifer within the Delta [10]. Groundwater flow direc- tion has remained constant, but in areas where long-term surface irrigation has occurred (e.g., Yuma Valley), groundwater levels are higher now than during pre-development time (Figure3B). Several reaches along the river now act as drains for groundwater where Hydrology 2021, 8, x FOR PEER REVIEWgroundwater levels are high, but less than 3.5 × 107 m3 (4.6%) of the yearly Colorado River5 of 23

flow discharges as groundwater into the Gulf of California [11,24].

Figure 3.3. ((A) GroundwaterGroundwater levels in 1925.1925. (B) Groundwater levels in 2010.2010. Contours represent water table elevation in meters above sea level. Based on data from [[9,11,25]9,11,25]..

2.1.6. Ciénega de Santa Clara The Ciénega de Santa Clara (Ciénega) is a brackish wetland along an old course of the Colorado River in Mexico. The Ciénega lies along a shallow depression on the eastern edge of the Delta and covers an area of 6000 ha dominated by Typha dominguensis (Figure 1; [26]). It is an “off-channel” wetland; its water does not come directly from the Colorado River. The most important source of water for the Ciénega is brackish groundwater (TDS > 2.6 ppt) derived from the Wellton-Mohawk Irrigation Drainage District of Arizona (irri- gated with Colorado River water). Excess agricultural runoff is transported to the Ciénega by a concrete-lined canal, the Wellton-Mohawk Drain, which delivers 1.3 × 108 m3/y. The Riito Drain (Figure 4), which transports wastewater from Mexican agriculture, supplies approximately 1.4 × 107 m3 to the Ciénega [27].

2.2. Field Methods Surface water samples were collected from the Ciénega, Wellton-Mohawk Drain, and Colorado River at Yuma, Arizona during May 2013, October 2013, and July 2014 (Figure 4). These surface water samples were used to establish the evaporation trend of Colorado River water. Groundwater samples were collected from the Minute 242 well field along the Arizona–Sonora border in October 2016. These groundwater samples were used to evaluate potential water sources besides Colorado River water. Additionally, two bulk sediment samples from the San Luis Mesa were analyzed for δ13CCaCO3 to correct 14C ages. Temperature, pH, dissolved O2, and electrical conductivity (EC) levels were meas- ured in the field after each parameter had stabilized. Samples for oxygen, hydrogen, and carbon (DIC) stable isotopes were filtered with a 0.45 μm nylon filter and kept in capped glass vials with no headspace. Unfiltered water samples were collected for 3H and 14C analysis in rinsed 1-L HDPE and amber borosilicate glass bottles, respectively. Samples for ions and alkalinity were filtered with a 0.45 μm nylon filter and kept in HDPE bottles. Cation samples were preserved with concentrated, optima grade HNO3. All samples were kept on ice while in the field and then refrigerated at 4 °C before analysis. Alkalinity was determined by the Gran alkalinity titration method [28] within 12 h of collection. This is expressed as HCO3-, assuming dominance of this anion at the observed pH values and consistent with units used in previous studies.

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2.1.6. Ciénega de Santa Clara The Ciénega de Santa Clara (Ciénega) is a brackish wetland along an old course of the Colorado River in Mexico. The Ciénega lies along a shallow depression on the eastern edge of the Delta and covers an area of 6000 ha dominated by Typha dominguensis (Figure1;[26]) . It is an “off-channel” wetland; its water does not come directly from the Colorado River. The most important source of water for the Ciénega is brackish groundwater (TDS > 2.6 ppt) derived from the Wellton-Mohawk Irrigation Drainage District of Arizona (irrigated with Colorado River water). Excess agricultural runoff is transported to the Ciénega by a concrete-lined canal, the Wellton-Mohawk Drain, which delivers 1.3 × 108 m3/y. The Hydrology 2021, 8, x FOR PEER REVIEWRiito Drain (Figure4), which transports wastewater from Mexican agriculture, supplies6 of 23

approximately 1.4 × 107 m3 to the Ciénega [27].

FigureFigure 4.4. Location of of sampling sampling sites sites in inthe the study study area. area. Symbols Symbols for adjacent for adjacent locations locations overlap overlap in in somesome cases.cases. The red red-dotted-dotted line line shows shows the the western western limit limit of of the the San San Luis Luis and and Yuma Yuma Mesas. Mesas.

2.2.2.3. FieldLaboratory Methods Methods SurfaceValues of water δ18O samplesand δ2H were measured collected fromat the the Environmental Ciénega, Wellton-Mohawk Isotope Laboratory, Drain, De- and Coloradopartment Riverof Geosciences, at Yuma, ArizonaUniversity during of Arizona, May 2013, using October a Finnigan 2013, Delta and-S July mass 2014 spectrom-(Figure4) . Theseeter with surface automated water samplesCO2 equilibration were used and to Cr establish-reduction the attachments. evaporation Analytical trend of Colorado preci- Riversions ( water.1σ) for Groundwaterthese techniques samples are 0.08 were% for collected δ18O and from0.9% thefor δ Minute2H. δ18O 242 and well δ2H fielddata are along thereported Arizona–Sonora in delta notation: border in October 2016. These groundwater samples were used to evaluate potential water sources besides Colorado River water. Additionally, two bulk δ = (R/Rstd − 1) × 1000 (‰) 13 14 sediment samples from the San Luis Mesa were analyzed for δ CCaCO3 to correct C ages. where R is the ratio of the heavier over, the lighter isotope in the sample, and Rstd is the Temperature, pH, dissolved O2, and electrical conductivity (EC) levels were measured inisotope the field ratio after of Vienna each parameter standard hadmean stabilized. ocean water Samples (VSMOW for oxygen,). hydrogen, and carbon 13 (DIC)δ stableCDIC values isotopes were were measured filtered on with a ThermoQuest a 0.45 µm nylon Finnigan filter Delta and Plus kept XL in continuous capped glass- vialsflow withgas-ratio no headspace. mass spectrometer Unfiltered coupled water with samples a Gasbench were collected automated for 3sampler.H and 14 SamplesC analysis were reacted for >1 h with phosphoric acid at room temperature in Exetainer vials previ- ously flushed with He gas. Standardization was based on NBS-19 and NBS-18, and preci- sion was ± 0.30‰ or better (1σ). All δ13C values were expressed in delta notation relative to the Vienna Pee Dee Belemnite (VPDB) standard. Tritium values were measured by liquid scintillation counting on electrolytically en- riched water in a Quantulus 1220 spectrophotometer with a detection limit of 0.7 tritium units (TU) for 8-fold enrichment and 1500 min of counting. Carbon-14 was measured as liberated CO2 reduced to graphite at the NSF-Arizona Accelerator facility. These results are reported as percent modern carbon (pMC) relative to NBS standards oxalic acid I and II.

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in rinsed 1-L HDPE and amber borosilicate glass bottles, respectively. Samples for ions and alkalinity were filtered with a 0.45 µm nylon filter and kept in HDPE bottles. Cation samples were preserved with concentrated, optima grade HNO3. All samples were kept on ice while in the field and then refrigerated at 4 ◦C before analysis. Alkalinity was determined by the Gran alkalinity titration method [28] within 12 h of collection. This is − expressed as HCO3 , assuming dominance of this anion at the observed pH values and consistent with units used in previous studies.

2.3. Laboratory Methods Values of δ18O and δ2H were measured at the Environmental Isotope Laboratory, Department of Geosciences, University of Arizona, using a Finnigan Delta-S mass spec- trometer with automated CO2 equilibration and Cr-reduction attachments. Analytical precisions (1σ) for these techniques are 0.08% for δ18O and 0.9% for δ2H. δ18O and δ2H data are reported in delta notation:

δ = (R/Rstd − 1) × 1000 (‰)

where R is the ratio of the heavier over, the lighter isotope in the sample, and Rstd is the isotope ratio of Vienna standard mean ocean water (VSMOW). 13 δ CDIC values were measured on a ThermoQuest Finnigan Delta Plus XL continuous- flow gas-ratio mass spectrometer coupled with a Gasbench automated sampler. Samples were reacted for >1 h with phosphoric acid at room temperature in Exetainer vials pre- viously flushed with He gas. Standardization was based on NBS-19 and NBS-18, and precision was ± 0.30‰ or better (1σ). All δ13C values were expressed in delta notation relative to the Vienna Pee Dee Belemnite (VPDB) standard. Tritium values were measured by liquid scintillation counting on electrolytically enriched water in a Quantulus 1220 spectrophotometer with a detection limit of 0.7 tritium units (TU) for 8-fold enrichment and 1500 min of counting. Carbon-14 was measured as liberated CO2 reduced to graphite at the NSF-Arizona Accelerator facility. These results are reported as percent modern carbon (pMC) relative to NBS standards oxalic acid I and II. Anion concentrations were determined in the Department of Hydrology and Atmo- spheric Sciences at the University of Arizona using a Dionex ion chromatograph model 3000 with an AS23 analytical column (precision ± 2%). The analyses for cations were performed by the Arizona Laboratory for emerging contaminants (ALEC) at the University of Arizona using a PerkinElmer Elan-II inductively coupled plasma–mass spectrometer (precision ± 2%).

2.4. Data The final dataset for this study (Table S1) contains published and unpublished re- sults for water samples from agricultural wells in the San Luis Valley [29,30], Yuma and San Luis Mesa [10,17,30], and the lower Colorado River floodplain [7,31]. The saturation indices of water samples were calculated using the hydrogeochemical equilibrium model of PHREEQC [32]. Average δ18O, δ2H, and solute chemistry values for the different potential water sources in the area were determined from other existing databases or publications and are used for comparison with individual values in the study area (Table1; Figure5). These endmembers include pre-dam Colorado River water (water recharging before Hoover Dam completion in 1936), post-dam Colorado River water (evaporated, while stored in upstream reservoirs), agricultural discharge, and Gila River. Pre-dam Colorado River water ion concentration and δ18O and δ2H values were approximated using data from USGS station 9380000 at Lee’s Ferry [17]. This station was used because it is located upstream from Lake Mead, where enrichment by evaporation occurs. Colorado River water near Lee’s Ferry is assumed to represent water reaching the Delta before major development along the lower Colorado River. For post-dam Colorado River water δ18O, δ2H, and ion values were calculated using data from USGS station 9522000 at Morelos Dam [17]. Hydrology 2021, 8, x FOR PEER REVIEW 7 of 23

Anion concentrations were determined in the Department of Hydrology and Atmos- pheric Sciences at the University of Arizona using a Dionex ion chromatograph model 3000 with an AS23 analytical column (precision ± 2%). The analyses for cations were per- formed by the Arizona Laboratory for emerging contaminants (ALEC) at the University of Arizona using a PerkinElmer Elan-II inductively coupled plasma–mass spectrometer (precision ± 2%).

2.4. Data The final dataset for this study (Table S1) contains published and unpublished results for water samples from agricultural wells in the San Luis Valley [29,30], Yuma and San Luis Mesa [10,17,30], and the lower Colorado River floodplain [7,31]. The saturation indi- ces of water samples were calculated using the hydrogeochemical equilibrium model of PHREEQC [32]. Average δ18O, δ2H, and solute chemistry values for the different potential water sources in the area were determined from other existing databases or publications and are used for comparison with individual values in the study area (Table 1; Figure 5). These endmembers include pre-dam Colorado River water (water recharging before Hoover Dam completion in 1936), post-dam Colorado River water (evaporated, while stored in upstream reservoirs), agricultural discharge, and Gila River. Pre-dam Colorado River wa- ter ion concentration and δ18O and δ2H values were approximated using data from USGS station 9380000 at Lee’s Ferry [17]. This station was used because it is located upstream from Lake Mead, where enrichment by evaporation occurs. Colorado River water near Hydrology 2021, 8, 80 Lee’s Ferry is assumed to represent water reaching the Delta before major development 7 of 22 along the lower Colorado River. For post-dam Colorado River water δ18O, δ2H, and ion values were calculated using data from USGS station 9522000 at Morelos Dam [17].

18 2 Figure 5.Figure(A) δ 5.18 (OA) andδ O andδ2H δ valuesH values for for thethe water water endmembers endmembers in the in area the [17,33,34] area [17 compared,33,34] comparedto the Global to Meteoric the Global Water Meteoric Line (GMWL; [35]), and the Colorado River evaporation trend (CRET, dashed line). (B) Average values for each endmem- Water Lineber. (GMWL; This figure [35 also]), includes and the winter Colorado rainfall River and the evaporation largest 30% trendof precipitation (CRET, dashedevents at line). Organ ( BPipe) Average Cactus National values for each endmember.Monument This figure[30]. also includes winter rainfall and the largest 30% of precipitation events at Organ Pipe Cactus National Monument [30]. Table 1. Average composition of waters in the study area. Table 1. Average composition of waters in the study area.

Ca2+ Mg2+ Na+ K+ HCO − SO 2− Cl− NO − Br− δ18O δ2H Type Year pH 3 4 3 (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (‰) (‰) Pre-dam 1967–2015 7.8 89 30 95 5 190 290 65 2.16 0.05 −15.0 −115 Colorado River SD 35 12 44 2 35 137 35 5.00 0.04 0.3 2 Post-dam 1980–2015 8.2 89 32 146 5 197 302 149 0.37 0.31 −12.0 −97 Colorado River SD 12 4 29 1 15 49 35 0.17 0.04 0.6 5 Ag. discharge 1961–1995 7.8 271 117 881 9 392 941 1514 2.13 0.62 −10.8 −89 SD 72 37 83 1 45 97 570 0.56 0.01 0.4 4 Gila River 2004–2005 8.0 56 19 342 4 255 262 348 3.64 - −9.9 −71 SD 50 14 352 5 137 257 411 4.92 - 1.3 20 Winter rainfall 1990–2006 −7.2 −47 Largest events 1990–2006 −7.5 −50 SD: standard deviation. δ18O and δ2H relative to V-SMOW (‰). Largest events: The largest 30% of rain events in the period of record.

For agricultural discharge, δ18O, δ2H, and ion values were calculated using data from USGS station 9529300 at the Wellton-Mohawk Drain [17] and [7,33]. Values of δ18O and δ2H, and ion concentrations in the upper Gila River, near Safford, Arizona (Figure1) were calculated using well data from [34]. Multiyear rainfall isotope data collected at Organ Pipe Cactus National Monument (OPCNM; Figure1) in Arizona were used to estimate long-term δ18O and δ2H values of winter precipitation representing local recharge in the area [30]. OPCNM is located 175 km east of the study area at an elevation of 515 masl, which is similar to the average elevations in the Gila Range. Individual and mean δ18O and δ2H values for the different endmembers are shown in Figure5A,B. For rainwater, the means are weighted for precipitation amount.

Evaporated Colorado River Water The (δ18O, δ2H) values of Colorado River water at different degrees of evaporation were modeled using a method described by [36]. Average pre-dam Colorado River water is Hydrology 2021, 8, 80 8 of 22

used as a starting point (δ18O = −15‰ and δ2H = −115‰). A displacement of data to the right of the GMWL reflects evaporative loss. Average humidity is assumed to be 60% to obtain an evaporation slope between 5 and 6, which is characteristic of evaporated Colorado River water in the area [16,37]. This evaporation trend is referred to as the Colorado River evaporation trend (CRET) in several isotope plots (e.g., Figure5). Equilibrium ( α) and kinetic fractionation (∆E) factors for 18O and 2H are calculated using the following equations [38,39]:

3 18 6 2 3 10 lnα Ol−v = 1.137 × (10 /T ) − 0.4156 × (10 /T) − 2.0667 (1)

3 2 6 2 3 10 lnα Hl−v = 24.844 × (10 /T ) − 76.248 × (10 /T) − 52.612 (2) 18 ∆E Ol−v = 14.2 × (1 − h) (3) 2 ∆E Hl−v = 12.5 × (1 − h) (4) In Equations (1) and (2), T is the mean annual temperature (K), and α is the frac- tionation factor. A temperature of 298 Kelvin is assumed for calculation purposes. This temperature is nearly identical to the average temperature at Yuma, Arizona (296 K; [40]). In Equations (3) and (4), h is the relative humidity (0.60).

Hydrology 2021, 8, x FOR PEER REVIEW The enrichment factor Eis calculated using Equation (5): 9 of 23

18 3 E Ol−v = [α − 1] × 10 (5)

The evaporative enrichmentƐ18O forl−v =δ [18α−O1] and× 103 δ2H values can be modeled,(5) according to a RayleighThe evaporative distillation, enrichment by assuming for δ18O and different δ2H values residual can be watermodeled, fractions according (f) to in a the following EquationRayleigh distillation, (6): by assuming different residual water fractions (f) in the following Equation (6): 18 E Ototal × ln(f) = evaporative enrichment (6) 18 Ɛ18Ototal × ln(f) = evaporative18 enrichment (6) E Ototal is the overall enrichment for O in this case. The overall enrichment for evaporationƐ18Ototal is the under overall the enrichment specified for conditions 18O in this case. is +15.06 The overall‰ for enrichment18O and for +84.51 evap-‰ for 2H. The resultoration ofunder Equation the specified (6) is conditions added to is the+15.06‰ average for 18O (δ and18O, +84.51‰δ2H) values for 2H. The of pre-damresult Colorado of Equation (6) is added to the average (δ18O, δ2H) values of pre-dam Colorado River water River water to model the evolution of Colorado River water under different degrees of to model the evolution of Colorado River water under different degrees of evaporation evaporation(Figure 6). (Figure6).

2 18 FigureFigure 6. 6. δδH2 Hvs. vs. δ Oδ 18showingO showing modeled modeled evolution evolution of Colorado of River Colorado water Riverevaporating water under evaporating 60% under 60% relative humidity (mod. evaporated Colorado River). Also shown: data for surface water from the relativeCiénega (labeled humidity CIEN), (mod. average evaporated pre-dam Colorado Colorado River River). water, Also average shown: post-dam data Colorad for surfaceo River water from the Ciwater,énega the (labeled GMWL, and CIEN), the CRET. average Percentages pre-dam indicate Colorado the degree River of water, evaporation average relative post-dam to pre- Colorado River water,dam river the water. GMWL, and the CRET. Percentages indicate the degree of evaporation relative to pre-dam river3. Results water. 3.1. Major Ion Trends The distribution of predominant anions and cations shows that surface Colorado River water evolves from a Ca–HCO3-dominated water type in its headwaters into Na– Ca–Cl-SO4-dominated waters as it travels downstream and reaches the international bor- der with Mexico (Figure 7A). Groundwater samples from the Colorado River floodplain consist of mixed Ca–Mg–Cl/Na–Cl water types. Samples belonging to the Yuma and San Luis Mesas and Gila Range foothills groups show Na+ and Cl- as the predominant ions (Na–Cl-type; Figure 7B). All groundwater samples in the floodplain, Yuma and San Luis Mesas, and Gila Range foothills are undersaturated concerning halite, gypsum, and anhydrite (SI < 0). This allows Na+, Cl−, Ca2+, and SO42− concentrations to increase along the flow paths. Most of the groundwater samples are supersaturated or close to saturation concerning dolomite, calcite, or both (SI between −1 and 1), indicating a strong presence of these two minerals in the aquifer system.

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3. Results 3.1. Major Ion Trends The distribution of predominant anions and cations shows that surface Colorado River water evolves from a Ca–HCO3-dominated water type in its headwaters into Na–Ca–Cl- SO4-dominated waters as it travels downstream and reaches the international border with Mexico (Figure7A). Groundwater samples from the Colorado River floodplain consist of mixed Ca–Mg–Cl/Na–Cl water types. Samples belonging to the Yuma and San Luis Mesas Hydrology 2021, 8, x FOR PEER REVIEW + − 10 of 23 and Gila Range foothills groups show Na and Cl as the predominant ions (Na–Cl-type; Figure7B).

FigureFigure 7. 7. (A(A) )Piper Piper diagram diagram showing showing data data for for surface surface waters waters in in the the Colorado Colorado River River Basin Basin including including data data for for headwaters headwaters (USGS(USGS Station Station 09196500 09196500),), Green Green River, River, Utah Utah (USGS (USGS Station Station 09315000 09315000),), Lee’s Lee’s Ferry Ferry (USGS (USGS Station Station 09380000 09380000),), Hoover Hoover Dam Dam (USGS Station 09421500), US–Mexico border (USGS Station 09522000), and Gila River near Dome, Arizona (USGS Station (USGS Station 09421500), US–Mexico border (USGS Station 09522000), and Gila River near Dome, Arizona (USGS Station 09520500). All USGS data are available online from [17]. (B) Piper diagram showing data for groundwaters in the study 09520500). All USGS data are available online from [17]. (B) Piper diagram showing data for groundwaters in the study area, including Colorado River Floodplain, Yuma and San Luis Mesas, and Gila Range. area, including Colorado River Floodplain, Yuma and San Luis Mesas, and Gila Range. 3.2. Stable Isotopes All groundwater samples in the floodplain, Yuma and San Luis Mesas, and Gila Range 3.2.1.foothills Endmembers are undersaturated and Evaporation concerning Calculation halite, gypsum, and anhydrite (SI < 0). This allows + − 2+ 2− Na Endmember, Cl , Ca , andisotope SO4 compositionsconcentrations were to calculated increase along as means the flow of the paths. data Most shown of the in Figuregroundwater 5A. Pre samples-dam Colorado are supersaturated River water or (δ close18O, δ to2H saturation) values are concerning −15‰ and dolomite, −115‰, calcite,post- damor both Colorado (SI between River water−1 and values 1), indicating are −12‰ a, and strong −97‰, presence agricultural of these discharge two minerals values in are the −1aquifer0.8‰ and system. −89‰, and Gila River water values are −9.9‰ and −71‰ (Table 1; Figure 5B). Rainfall isotope data (δ18O, δ2H) yielded average values of −7.2‰ and −47‰ for winter, 3.2. Stable Isotopes and −7.5‰ and −50‰ for the 30% wettest events (Table 1; Figure 5B). Although the study area3.2.1. is Endmembersclose to the coast, and seawater Evaporation from Calculation the Gulf of California (Figure 8) is not required as an endmemberEndmember for the isotope present compositions dataset. were calculated as means of the data shown in Figure5A. Pre-dam Colorado River water ( δ18O, δ2H) values are −15‰ and −115‰, post-dam Colorado River water values are −12‰, and −97‰, agricultural discharge values are −10.8‰ and −89‰, and Gila River water values are −9.9‰ and −71‰ (Table1; Figure5B) . Rainfall isotope data (δ18O, δ2H) yielded average values of −7.2‰ and −47‰ for winter, and −7.5‰ and −50‰ for the 30% wettest events (Table1; Figure5B) .

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Hydrology 2021, 8, x FOR PEER REVIEW 11 of 23 Although the study area is close to the coast, seawater from the Gulf of California (Figure8) is not required as an endmember for the present dataset.

Figure 8. ((A)) δ18OO and and δδ22HH values values of of surface surface water water and and groundwater groundwater from from the Colorado River floodplain floodplain (CRFP).(CRFP). ((BB)) δδ1818O and δ2HH values values of of waters waters from from the the San San Luis Luis and and Yuma Yuma Mesas, Gila Range relative relative to the the GMWL GMWL and and the the CRET. CRET. Deep well data from [[41].41]. DashedDashed graygray line line shows shows hypothetical hypothetical mixing mixing between between seawater seawater in thein the Gulf Gulf of Californiaof California [42] [42] and and evaporated evapo- rated Colorado River water. Colorado River water.

SurfaceSurface water water samples from the Ciénega Ciénega (labeled(labeled CIEN in Figure6 6)) have have δδ1818O values betweenbetween −1−10.60.6‰‰ andand +6.0‰, +6.0‰, and and δ2HH values values between −−8888‰‰ andand +8‰. +8‰. The highest δδ1818O andand δδ22HH values values are are located located in in the the southern southern part part of of the the Ciénega Ciénega near near the the tidal tidal flats flats ( (sitessites 76 andand 77 77).). All All water water samples samples fall fall to to the the right of the GMWL (Figure 66)).. SomeSome ofof thethe samplessamples inin the the Ciénega Ciénega have have lost lost more more than than 50% 50% of of volume volume by by evaporation evaporation relative relative to the pre pre-dam-dam ColoradoColorado River River endmember endmember (Figure 66).).

3.2.2.3.2.2. Surface Surface and and Groundwater Groundwater Data Data StableStable isotope isotope data data for for the the study study area area are are shown shown in Fig in Figureure 8A,8B.A,B. The The (δ18 (O,δ 18δ2O,H)δ val-2H) uesvalues of Colorado of Colorado River River collected collected at at Yuma, Yuma, Arizona Arizona were were −−111.81.8‰‰ andand −9−955‰,‰ ,respectively respectively 18 2 ((TableTable S1).S1). Wellton-MohawkWellton-Mohawk DrainDrain dischargedischarge (WMD)(WMD) hadhad ((δδ18O,O, δδ2H)H) valuesvalues ofof −−110.60.6‰‰ andand −8−7‰.87‰ Groundwater. Groundwater samples samples from from wells wells in inthe the Colorado Colorado River River floodplain floodplain (CRFP (CRFP),), on 18 2 bothon both sides sides of the of border, the border, have have δ18Oδ valuesO values between between −9.4‰− 9.4and‰ −1and4.7‰,−14.7 and‰ δ,2H and valuesδ H betweenvalues between −75‰ and−75 ‰−112‰,and − and112 ‰plot, and mainly plot mainlyon the onCRET. the CRET.Groundwater Groundwater samples samples from 18 wellsfrom wells in the in Yu thema Yuma and San and Luis San Luis Mesas Mesas (Mesa (Mesa)) have have δ18Oδ valuesO values between between −7.9‰−7.9 and‰ 2 −1and4.9‰,−14.9 and‰ δ,2 andH valuesδ H valuesbetween between −60‰ and−60 −1‰14‰.and These−114 ‰fall. Thesemainly fall along mainly a linear along mix- a inglinear trend mixing between trend the between pre-dam the Colorado pre-dam River Colorado and Riverlocal winter and local precipitation winter precipitation endmem- bers.endmembers. δ18 GroundwaterGroundwater samples f fromrom four wells near the Gila Range have δ18OO values values between − − δ2 − − −77.6‰.6‰ and and −8.7‰,8.7‰, and and δ2H Hvalues values between between −55‰55‰ and and −67‰.67‰.

3.3.3. H and 14C Tritium and 14C activities for Colorado River water at Yuma were ~5 TU in 2017 and ~101 pMC in 2009, respectively (Table S1; [43]). However, values for 3H and 14C were higher during the previous decades when more bomb-pulse 3H and 14C were present in

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3.2.3. H and 14C the atmosphere [44]. Colorado River water measured near the USA–Mexico border con- Tritium and 14C activities for Colorado River water at Yuma were ~5 TU in 2017 and ~101tained pMC 716 in TU 2009, in respectively1967 and 12 (Table–17 TU S1; between [43]). However, 1993 and values 1998 for [136H]. andPost14-bombC were-pulse precip- higheritation during in the the Kofa previous Mountains, decades when 90 km more northeast bomb-pulse of3 HYuma, and 14 Caveraged were present 3.4 inTU the in 2008–2009 atmosphere[44]. Colorado [44]. Colorado River floodplain River water groundwater measured near samples the USA–Mexico range borderbetween contained 5 and 16 TU, and 716San TU Luis in 1967 Mesa and 12–17samples TU betweenrange between 1993 and 1998<0.1 [ 16and]. Post-bomb-pulse 15 TU. The high precipitation values, in10–16 TU, all theoccur Kofa close Mountains, to the 90 Colorado km northeast River of Yuma, (Figure averaged 9); they 3.4 TUcannot in 2008–2009 be explained [44]. Colorado by the recharge of River floodplain groundwater samples range between 5 and 16 TU, and San Luis Mesa river water since 1993 but must include some recharge from the bomb pulse. With one samples range between <0.1 and 15 TU. The high values, 10–16 TU, all occur close to the Coloradoexception, River tritium (Figure is9 );below they cannot detectio be explainedn level in by groundwater the recharge offrom river the water San since Luis and Yuma 1993Mesas, but mustindicating include pre some-bomb recharge recharge. from the bomb pulse. With one exception, tritium is belowThree detection 14C measurements level in groundwater from from the theSan San Luis Luis Mesa and Yumahave Mesas,59, 29, indicating and 26 pMC, corre- pre-bombsponding recharge. to uncorrected 14C ages between 4800 and 11,500 14C years before present.

FigureFigure 9. 9.Tritium Tritium (TU) (TU data) data for groundwater for groundwater samples. samples. Shading indicatesShading topography indicates topography (see Figure1). (see Figure 1). TheThe red-dotted red-dotted line line indicates indicates the extent the extent of the San of the Luis San and Luis Yuma and Mesa. Yuma Mesa.

Three 14C measurements from the San Luis Mesa have 59, 29, and 26 pMC, corre- spondingMost to uncorrectedColorado 14 RiverC ages floodplain between 4800 groundwater and 11,500 14C samples years before consist present. of mixed Ca–Mg– Cl/NaMost–Cl Colorado, but a few River samples floodplain fall exclusively groundwater within samples Na consist–Cl facies. of mixed The Ca–Mg– opposite is true for Cl/Na–Cl,the Yuma but and a few San samples Luis Mesas fall exclusively and Gila within Range Na–Cl foothills facies. where The opposite groundwater is true for samples are theNa Yuma–Cl-dominated, and San Luis but Mesas a few and Gilasamples Range fall foothills within where the groundwaterCa–Mg–Cl/Na samples–Cl mix are (Figure 7B). Na–Cl-dominated,The influence of butSO a42− few-rich samples Colorado fall within River thewater Ca–Mg–Cl/Na–Cl in the floodplain mix is (Figure evident7B). in Figure 10. 2− The influence of SO4 -rich Colorado River water in the floodplain is evident in Figure 10. While most of the groundwaters in the floodplain have SO42− values > 200 mg/L, those in While most of the groundwaters in the floodplain have SO 2− values >200 mg/L, those in 2− 4 the Mesa and Gila Range foothills have2− SO4 values < 200 mg/L. the Mesa and Gila Range foothills have SO4 values <200 mg/L.

HydrologyHydrology 20212021, 8, 8, ,x 80 FOR PEER REVIEW 12 of 22 13 of 23

Figure 10. Cross-plot of Cl− and SO42− concentrations (mg/L) for surface and groundwater in the Figure 10. Cross-plot of Cl− and SO 2− concentrations (mg/L) for surface and groundwater in the 4 − 2− study area. Arrow indicates hypothetical evaporation trajectory− for Cl2−/SO4 = 1:2. study area. Arrow indicates hypothetical evaporation trajectory for Cl /SO4 = 1:2. 4.4. DiscussionDiscussion 4.1.4.1. GeneralGeneral Patterns Patterns Colorado River headwaters are initially Ca–HCO3-dominated due to the dissolution of silicateColorado and carbonate River headwaters minerals. These are watersinitially evolve Ca–HCO into Ca–Mg–Cl/Na–Cl3-dominated due type to the in thedissolution upperof silicate Colorado and carbonate River in part minerals. due to the These interaction waters with evolve the local into geology,Ca–Mg– anthropogenicCl/Na–Cl type in the activitiesupper Colorado (e.g., mining River and in farming), part due and to the evaporative interaction concentration with the local where geology, extensive anthropogenic irriga- tionactivities of land (e.g. occurs, mining (Figure and7A). farming Salts (halite), and and evaporative gypsum) dissolved concentration from the where EagleValley extensive irri- Evaporite,gation of land Paradox occurs Formation, (Figure Mancos 7A). Salts Shale, (halite Chinle and Formation, gypsum and) dissolved their associated from the soils Eagle Val- accountley Evaporite, for approximately Paradox Formation, half of the total Mancos solutes Shale, in this Chinle part of Formation, the river [45 and,46]. their associated soilsAgriculture account for dominates approximat theely floodplain half of the in the total lower solutes Colorado in this River part area.of the Here, river the [45,46]. − + proportionAgriculture of Cl anddominates Na in Coloradothe floodplain River waterin the increases lower Colorado due to irrigation River area. return Here, the flows, marine salt input, and/or halite evaporites in the lower Colorado River basin. proportion of Cl− and Na+ in Colorado River water increases due to irrigation return flows, Evaporation at Lake Mead and mixing with return flow is evident from Figure5A, where post-dammarine salt Colorado input, Riverand/or waters halite plot evaporites to the right in the of the lower GMWL Colorado and overlap River agriculturalbasin. Evaporation dischargeat Lake Mead in some and cases. mixing Nearly with 30% return of the flow total riveris evident surface from discharge Figure has 5A been, where lost topost-dam evapotranspirationColorado River waters when Coloradoplot to the River right water of the enters GMWL Morelos and Dam, overlap as suggested agricultural by the discharge (inδ18 someO, δ2H) cases values. Nearly of post-dam 30% of Colorado the total River river water surface (Figures discharge5 and6 ).has been lost to evapotran- spirationEvaporated when Colorado River River water water would enters fall Morelos along theDam, 1:2 as line suggested in Figure by10, the repre- (δ18O, δ2H) sentingvalues theof post Cl/SO-dam4 mass Colorado ratio in River post-dam water Colorado (Figures River 5 and water. 6). There is an excess of − 2− 2− Cl , relativeEvaporated to SO4 Colorado, in virtually River every water water would sample fall plotted. along Bacterialthe 1:2 line SO4 in reductionFigure 10, repre- could drive water samples to plot to the left of the evaporation line, creating an excess − senting− the Cl/SO4 mass2− ratio in post-dam Colorado River water. There is an excess of Cl , of Cl . However, SO4 concentrations are relatively high, oxic conditions prevail in relative to SO42−, in virtually every2− water sample plotted. Bacterial SO42− reduction could the unconfined aquifer, and SO4 reduction has only been noted in a few wells within thedrive floodplain water samples where organic to plot matter to the is left more of readily the evaporation available [9 ].line, Thus, creating evaporation an excess by of Cl−. 2− − 2− itselfHowever, does not SO explain4 concentrations the observed are relationship relatively between high, oxic Cl conditionsand SO4 ,prevail and there in arethe uncon- − − − additionalfined aquifer, sources and of SO Cl42−. reduction These additional has only sources been ofnoted Cl resultin a few in awells wide within range ofthe Cl floodplain concentrations,where organic whichmatter tend is more to be lowerreadily near available the Colorado [9]. Thus, River evaporation floodplain than by itself in the does not Mesa (Figure 11A). − 2− explain the observed relationship between Cl and SO4 , and there are additional sources of Cl−. These additional sources of Cl− result in a wide range of Cl− concentrations, which tend to be lower near the Colorado River floodplain than in the Mesa (Figure 11A). The (δ18O, δ2H) values for groundwater in the study area are also variable. Figure 11B shows the differences in δ2H in groundwaters. Within the floodplain, the observed range of δ2H corresponds to mixtures of average pre-dam river water with δ2H values near

Hydrology 2021, 8, x FOR PEER REVIEW 14 of 23

−114‰ and post-dam river water with average values near −97‰ (compare Figure 8A).

Hydrology 2021, 8, 80 Mixtures with a high proportion of pre-dam water (<−105‰) dominate groundwater13 of 22 be- neath a broad area of the floodplain in Mexico. The area is poorly constrained to the south, and this pattern may extend further south than indicated in Figure 11B.

FigureFigure 11. 11.(A).( AContour). Contour map map showing showing spatial spatial patterns patterns for forCl−. Cl (B−).( ContourB) Contour map mapshowing showing spatial spatial patterns patterns for δ for2H. δBlack2H. - filledBlack-filled circles represent circles represent control controlpoints. points.SLRC: San SLRC: Luis San Rio Luis Colorado, Rio Colorado, Mexico. Mexico. WMD: WMD: Wellton Wellton-Mohawk-Mohawk Drain. Drain.

18 2 TheThe (δ (δ18O,O, δ2δHH)) values values of for groundwaters groundwater in derived the study from area local are alsoprecipitation variable. Figure (−7.6‰ 11 Band 2 −5shows1%; [30] the) differencesare slightly in lowerδ H in than groundwaters. the average Within winter the precipitation floodplain, the (δ18 observedO and δ2 rangeH) values of δ2 δ2 − (−7.2‰H corresponds and −47‰ to), mixturesbut are consistent of average with pre-dam the isotope river water composition with Hvalues of the nearlargest114 30%‰ of and post-dam river water with average values near −97‰ (compare Figure8A). Mixtures rain events (−7.5‰ and −50‰, Figure 5B; [30,47–49]). The extensive alluvial fans observed with a high proportion of pre-dam water (<−105‰) dominate groundwater beneath a at the base of the Gila Range suggest that mountain system recharge and focused recharge broad area of the floodplain in Mexico. The area is poorly constrained to the south, and in ephemeral streams are likely to occur at the mountain front, as in other semi-arid basins this pattern may extend further south than indicated in Figure 11B. in southernThe ( δArizona18O, δ2H) [50 values–52]. This of groundwaters occurs during derived winterfrom when local precipitation precipitation exceeds (−7.6 evap-‰ otranspiration.and −51%; [30 Groundwater]) are slightly lowerflows thanfrom thethese average recharge winter zones precipitation in the mountain (δ18O andfrontδ 2ofH) the Gilavalues Range (−7.2 west‰ andinto− the47 ‰Yuma), but and are San consistent Luis Mesas. with the Mountain isotope composition system recharge of the is largest present in30% at least of rain one events of the ( −samples7.5‰ and located−50‰, at Figurethe base5B; of [30 the,47 –Gila49]). Range The extensive (Figure alluvial8B, −7.6‰ fans and 55‰observed). at the base of the Gila Range suggest that mountain system recharge and focused rechargeClearly, in ephemeralmany such streams samples are are likely Colorado to occur River at the water mountain or mixtures front, asthat in otherare predomi- semi- nantlyarid basinslocal recharge in southern (Figure Arizona 8B). Infiltration [50–52]. This of occursriver water during beneath winter the when mesas precipitation is physically difficultexceeds to evapotranspiration. occur as far east as Groundwater the pediment flows of the from Gila these Range recharge (Figure zones 3). in Three the mountain floodplain groundwaterfront of the Gila samples Range clearly west into contain the Yuma mixtures and San of Luislocal Mesas.recharge Mountain with river system water recharge (Figure 8Ais). present Precise in estimation at least one of of ratios the samples in each located case is at problematic the base of thebecause Gila Rangeof the (Figuredifficulty8B, of − specifying7.6‰ and a river 55‰). water end member on the river evaporation trend, which intersects the Clearly, many such samples are Colorado River water or mixtures that are predomi- mixing trend at a small angle. nantly local recharge (Figure8B). Infiltration of river water beneath the mesas is physically The groundwater levels (Figure 3), location of the samples (Figure 4) and chemical difficult to occur as far east as the pediment of the Gila Range (Figure3). Three floodplain and isotopic composition (Figures 7 and 8) suggest that Na–Cl-dominated groundwaters groundwater samples clearly contain mixtures of local recharge with river water (Figure from8A). the Precise Yuma estimation and San ofLuis ratios Mesas in eachand casethe Gila is problematic Range (mountain because system of the difficulty recharge of) are specifying a river water end member on the river evaporation trend, which intersects the mixing trend at a small angle.

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The groundwater levels (Figure3), location of the samples (Figure4) and chemical and isotopic composition (Figures7 and8) suggest that Na–Cl-dominated groundwaters from the Yuma and San Luis Mesas and the Gila Range (mountain system recharge) are moving westward, and mixing with Ca–Mg–Cl/Na–Cl waters from the floodplain (Colorado River). This idea is illustrated in Figure 11, which shows higher Cl− and δ2H over the eastern side of the study, relative to groundwaters in the floodplain and intermediate values between them. Previous studies in the area [7,31] suggested that the Na–Cl-dominated waters along the border represent Gila River water. The presence of Gila River water cannot be discounted from isotope data alone; however, water with higher (δ18O and δ2H) values than the floodplain waters also occurs at the mountain front, where recharge from the Gila River is not possible. Therefore, such water is attributed here to local recharge. Before major development, Gila River water in Yuma likely had (δ18O and δ2H) values consistent with the evaporation of high-elevation precipitation in the headwaters originat- ing on the GMWL at δ18O = −12 to −10‰ (Figure5A). There is an evaporation trend in the Gila River samples that overlaps with the Gila Range data (compare Figures5A and8B ). However, this is likely influenced by modern irrigation and the infiltration and percola- tion of evaporated agricultural return, which did not occur before major development in the floodplain. Evaporation of Gila River surface water in pre-development times certainly occurred, as it occurs in the river upstream of dams [53]. Historical hydrochemical data along the lower Gila River are scant, and recent data show that today groundwater up to 90 km upstream from the Colorado and Gila River confluence is dominated by Colorado River chemistry [33]. Peak flows in the Colorado River occurred from April to June when late- spring snowmelt arrived in the area and replenished the aquifer. Historical Colorado River streamflow was at least two orders of magnitude larger than the Gila River during high flow season. It is very likely that the two rivers mixed, even before their confluence, resulting in waters dominated by Colorado River chemistry, and a pure Gila River endmember would be hard to find west of the Gila Range. Independently of the origin of groundwaters in the eastern side of the study area, the low 3H levels indicate that recharge within the Yuma and San Luis Mesas occurred at least before the 19500s, before the detonation of thermonuclear devices for most groundwater samples (Figure9), and the 14C data are consistent with bulk residence times of thousands of (uncorrected 14C) years before present (between 4800 and 11,500). Combining old water and limited modern recharge across the Mesa suggests that the aquifer is vulnerable to overdraft.

4.2. Source of Solutes 4.2.1. Na+ and Cl− Most waters from the Colorado River floodplain, Yuma and San Luis Mesas, and Gila Range foothills have a Na/Cl equivalent ratio close to the trends corresponding to halite dissolution and seawater dilution (Figure 12A). A Na/Cl equivalent ratio higher than one indicates the release of Na+ from silicate weathering reactions [54] in the Delta sediments. Halite beds likely exist in the Delta due to marine transgression/regression cycles and seawater evaporation, but within the study area, there is no evidence of them in the available well-log data [9]. Partial dissolution of evaporite deposits explains high salinity in groundwaters in the western part of the Colorado River Delta [55]. There, halite and sylvite associated with lacustrine clayey sediments have been identified by X-ray diffraction and severely affect Cl− concentrations in groundwater. These clays are also likely found in the study area. HydrologyHydrology 20212021, 8,, x8 ,FOR 80 PEER REVIEW 15 of16 22 of 23

FigureFigure 12. 12. (A()A Na) Na+ vs.+ vs. Cl Cl− (meq/L).− (meq/L). Lines Lines show show 1:1 1:1 halite halite dissolution dissolution and seawaterseawater dilution dilution trajectories. trajectories. (B )( ClB)− Cl(mg/L)− (mg/L) vs. vs. Br−Br (mg/L).− (mg/L). Inset Inset shows shows samples samples with with low low concentrations. concentrations. (C (C) Cl) Cl−/Br−/Br− (mass)− (mass) vs. vs.Cl−Cl (mg/L)− (mg/L) with with seawater/Colorado seawater/Colorado River waterRiver mixing water mixingtrend. The trend. horizontal The horizontal line shows line shows the theseawater seawater (SW) (SW) Cl/Br Cl/Br ratio ratio [56]. [56]. Graphs Graphs include datadata for for floodplain floodplain (CRFP),(CRFP), Mesa, Mesa, Gila Gila Range, Range, surface surface water water from from the the Ciénega Ciénega (CIEN), local rainfall,rainfall, and and Colorado Colorado River. River. Colorado Colorado River River data data obtained from USGS Station 09404200. See text for further explanation. obtained from USGS Station 09404200. See text for further explanation.

GroundwaterGroundwater levels levels in in the the sampled areasareas areare several several meters meters higher higher than than the the high high tide tide levelslevels in in the the nearby nearby coastline coastline of of the GulfGulf ofof CaliforniaCalifornia (Figures (Figures3 and3 and4). Isotope4). Isotope data data for for thethe Mesa Mesa could could be be interpreted interpreted as as indicating mixingmixing of of Colorado Colorado River River water water and and seawater seawater (Figure(Figure 8B8B).). Based Based on the elevationelevationof of the the water water table table and and the the location location of of the the groundwater groundwater samples,samples, this this is isphysically physically impossible. impossible. However, However, this this is a is possibility a possibility for forgroundwater groundwater sam- plessamples obtained obtained from fromdeeper deeper wells wells (>1200 (>1200 m) at m) Riito at Riito (Figure (Figure 8B;8 B;[41] [41).]). SinceSince no no halite halite-bearing-bearing strata are known withinwithin the the study study area, area, and and no no evidence evidence exists exists for seawater intrusion (excluding deep wells), the possible sources of Cl− are (1) Salt- for seawater intrusion (excluding deep wells), the possible sources of Cl− are (1) Salt-bear- bearing clays, (2) irrigation water, (3) precipitation, and (4) dry deposition and eventual ing clays, (2) irrigation water, (3) precipitation, and (4) dry deposition and eventual dis- dissolution of marine-derived salts. The ions Cl− and Br− provide a useful tracer combi- solution of marine-derived salts. The ions Cl- and Br- provide a useful tracer combination nation to identify the source of salinity in groundwater. Bromide is rejected during the toprocess identify of the halite source precipitation, of salinity and in the groundwater. Cl/Br massratio Bromide of solid is NaClrejected is usually during 2–3 the orders process ofof halite magnitude precipitation, higher and than the in theCl/Br original mass ratio waters of (~5000;solid Na [57Cl]). is usually The Cl/Br 2–3 mass orders ratio of mag- of nitudeseawater higher is about than 290in the and original is preserved waters in precipitation(~5000; [57]). occurringThe Cl/Br near mass the ratio sea [of58 seawater]. is about 290 and is preserved in precipitation occurring near the sea [58]. The Cl/Br mass ratio of post dam Colorado River water upstream of Lake Mead (USGS Station 09404200) appears consistent with a trend line resulting from halite disso- lution. This line plots very close to the Cl− axis because of the low Br− content in the mineral (Figure 12B, inset). Closer inspection of Cl/Br data in the study area (Figure 12C) provides

Hydrology 2021, 8, 80 16 of 22

The Cl/Br mass ratio of post dam Colorado River water upstream of Lake Mead (USGS Station 09404200) appears consistent with a trend line resulting from halite dissolution. This line plots very close to the Cl− axis because of the low Br− content in the mineral (Figure 12B, inset). Closer inspection of Cl/Br data in the study area (Figure 12C) provides an alternative explanation of Br− content in the river water. Figure 12C indicates large ranges in both Cl− (1–60 mg/L) and Cl/Br (1000–3000) in river water. The figure shows a mixing line for seawater with a river water composition chosen as 10 mg/L Cl−, and a Cl/Br = 2000. Other mixing lines are possible for alternative choices of river water composition. The range of Cl− could be explained in part by changes in the dilution of salt input from upstream evaporites. However, the prominent linear data array to the right of the mixing line is better explained by very small additions of sea salt to river water. The Cl/Br mass ratio of local precipitation follows a trend line resulting from the dilution of seawater (Figure 12B). The Cl/Br mass ratio in local precipitation ranges between 150 and 274, similar to the marine Cl/Br mass ratio and is consistent with marine-derived aerosols (Figure 12C). A single sample from the Gila Range and a few samples from the floodplain and the Yuma and San Luis Mesas plot near the marine Cl/Br mass ratio (Figure 12C). Most of the samples have intermediate Cl/Br equivalent ratios. This indicates mixing between Colorado River water having irrigation and halite-derived Cl−, and local recharge having Na+ and Cl− originating from seawater aerosols. Groundwaters within the floodplain and the Yuma and San Luis Mesas have Cl− concentrations between 132 and 1000 mg/L (Table S1). It is important to emphasize that some of the variability in Cl− concentration is likely explained by the spatial and temporal distribution of the sample collection. Water samples were obtained from wells with depths between 40 and 242 m from the surface. Shallower wells are more likely to be disturbed by anthropogenic activity, such as irrigation. The historical data used in this study are for samples collected between 1962 and 2016. Older samples could reflect a chemical composition more closely related to pre-dam Colorado River water with evaporation and less anthropogenic sources of solutes, and newer samples could be more similar to post-dam Colorado River water.

2+ 2+ 2− − 4.2.2. Ca , Mg , SO4 , and HCO3 The dissolution of calcite, dolomite, and gypsum results in waters dominated by Ca2+, 2+ 2− − Mg , SO4 , and HCO3 [59]. There is an approximate 1:1 relationship for groundwaters 2+ 2+ 2− − in the study area with a slight deficiency of (Ca + Mg ) relative to (SO4 + HCO3 ), particularly in groundwater samples from the floodplain (Figure 13). The excess negative charge is balanced by Na+ likely derived from old groundwater discharging into the river through the exchange of Ca2+ or Mg2+ for Na+ with clay minerals. Cation exchange also explains the excess Na+ relative to Cl− observed in Figure 12A and causes floodplain groundwaters to plot above the 1:1 halite dissolution trend. The highest Cl/SO4 mass ratio in the Gila Range samples (~6) approaches the ratio in seawater (~7.4, Figure 14). This further supports the idea that local recharge contains marine salts transported to the mountains either as marine aerosol or dust. As locally recharged groundwaters having a high Cl/SO4 molar ratio move westward, they mix 2− with an SO4 -dominated endmember (Colorado River water), as illustrated by the Mesa samples in Figure 14A,B (dashed line). The Mg/Ca mass ratio for the Gila Range samples varies between 0 and 0.6 (Figure 14B). Samples 43 and 82, both located in the Mesa, have the lowest δ18O and δ2H values of all the samples (−14.9 and −114, and −14.8‰ and −111, respectively), and characterize pre-dam Colorado River water. We assume that the Mg/Ca mass ratio range (0.2–0.5) of these samples represents pre-dam Colorado River water. Within the floodplain, Mg/Ca molar ratios range between 0.2 and ~3. Some degree of evaporation is observed in floodplain samples (Figures8A and 14A), but the Mg/Ca mass ratios would remain constant if this were the only process occurring in the floodplain and would plot in the dashed circle in Figure 14B. Three additional processes are believed to affect Colorado Hydrology 2021, 8, 80 17 of 22

River floodplain groundwaters 1) precipitation of solid phases, such as calcium carbonate, Hydrology 2021, 8, x FOR PEER REVIEW 2+ 2+ 18 of 23+ 2) de-dolomitization of Mg-bearing carbonates, and 3) exchange of Ca or Mg for Na Hydrology 2021, 8, x FOR PEER REVIEW 18 of 23 in the vadose zone, as previously discussed.

FigureFigure 13. CaCa + + Mg Mg vs. vs. SO4 SO4 ++ HCO3 HCO3 ((meq/L)meq/L) for for groundwater groundwater from from Yuma Yuma–San–San Luis Luis Mesas Mesas ( (MESA),MESA), GilaFigureGila Range, 13. Ca and + Mg Colorado vs. SO4 River + HCO3 floodplain floodplain (meq/L )( (CRFP). CRFPfor groundwater). The solid linefrom represents Yuma–San 1:1 Luis plotting Mesas location. (MESA ), Gila Range, and Colorado River floodplain (CRFP). The solid line represents 1:1 plotting location.

Figure 14. (A) Cl− (mg/L) vs. Cl−/SO42- (mass ratio) values for floodplain (CRFP), Yuma–San Luis Mesas (Mesa), and Gila − − − 2- 2− Range.Figure 14.The (( Adashed) ClCl (mg/L)(mg/L) line represents vs. vs. Cl Cl/SO /SOa4 hypothetical (mass4 (mass ratio) ratio)mixing values values trend.for floodplain for The floodplain dotted (CRFP), line (CRFP), represents Yuma Yuma–San–San the Luis potential LuisMesas Mesas (Mesa),evaporation (Mesa), and Gila andtra- jectGilaRange.ory. Range. The(B) Mg/Ca dashed The dashed vs. line Cl represents− line/SO4 represents2- (mass a hypotheticalratio) a hypothetical values mixingfor floodplain mixing trend. trend. The(CRFP), Thedotted dottedMesa, line and linerepresents Gila represents Range the potential thesamples. potential evaporationThe evaporationdashed linetra- jectory. (B) Mg/Ca vs. Cl−/SO−42- (mass2− ratio) values for floodplain (CRFP), Mesa, and Gila Range samples. The dashed line representstrajectory. (potentialB) Mg/Ca evaporation vs. Cl /SO trajectories.4 (mass ratio) Solid values blue lines for floodplain show ratios (CRFP), of seawater Mesa, andin A Gila and RangeB [56]. samples. The dashed linerepresents represents potential potential evaporation evaporation trajectories. trajectories. Solid Solid blue blue lines lines show show ratios ratios of seawater of seawater in A in and A andB [56]. B [ 56]. The Mg/Ca mass ratio for the Gila Range samples varies between 0 and 0.6 (Figure 14B).The Samples Mg/Ca 43 massand 82, ratio both for located the Gila in theRange Mesa, samples have thevaries lowest between δ18O and0 and δ2H 0.6 values (Figure of all14B the). Samples samples 43 (−1 and4.9 and82, both −114, located and −1 in4.8‰ the Mesa,and −1 have11, respectively the lowest )δ, 18andO and characterize δ2H values pre of- damall the Colorado samples River(−14.9 water. and −1 We14, assumeand −14.8‰ that theand Mg/Ca −111, respectively mass ratio range), and (characterize0.2–0.5) of these pre- samplesdam Colorado represents River pre water.-dam We Colorado assume River that the water. Mg/Ca Within mass the ratio floodplain, range (0.2 Mg/Ca–0.5) of molar these ratiossamples range represents between pre 0.2-dam and Colorado ~3. Some Riverdegree water. of evaporation Within the isfloodplain, observed Mg/Cain floodplain molar samplesratios range (Figures between 8A and 0.2 14Aand )~3., but Some the Mg/Cadegree massof evaporation ratios would is observedremain constant in floodplain if this weresamples the (onlyFigures process 8A and occurring 14A), but in thethe floodplainMg/Ca mass and ratios would would plot remainin the dashed constant circle if this in Figurewere the 14B. only Three process additional occurring processes in the floodplainare believed and to would affect plotColorado in the River dashed floodplain circle in Figure 14B. Three additional processes are believed to affect Colorado River floodplain

Hydrology 2021, 8, 80 18 of 22

4.3. Hydrochemical Evolution Features of the regional flow system, the relations between major solutes, and stable isotope data suggest that the following set of reactions is responsible for the hydrochemical evolution of groundwater in the study area:

2+ 2− Ca + CO3 ←→ CaCO3 (7)

2+ 2− CaSO4 → Ca +SO4 (8) 2+ 2+ 2− Ca(Mg)CO3 → Ca + Mg + CO3 (9) Ca2+ + 2Na-X = Ca-X + 2Na+ (10) In reaction (10), X represents an ion exchange site occupied by two monovalent cations or one divalent cation. The evolution of groundwaters in the study area is described in the following para- graphs. Recharge having ionic ratios similar to those of seawater enters the aquifer along the Gila Range. Most rainwater is concentrated by evaporation and transpiration by water- efficient native vegetation, leading to the accumulation of meteoric salts near the surface. These readily soluble salts are dissolved during the most intense and infrequent events, + 2+ − 2− and contribute with Na , Ca , Cl , and SO4 to groundwater when excess precipita- tion reaches the aquifer. Average local recharge in the region plots near the GMWL. This suggests that infiltration occurs during winter as the mountain system recharge when evaporation is low and through preferential pathways along the major washes draining the Gila Range. The concentration of range-front groundwater is remarkably higher than the rainfall it was derived from, as observed in the Gila Range samples, and is dominated by Na–Cl. This groundwater flows towards the southwest and mixes with Ca–Mg–Cl/Na–Cl Colorado River water along the Yuma and San Luis Mesas (Figure8). Mineral–water equilibria suggest that dissolution–precipitation of calcite and dolomite, dissolution of halite and gypsum, and exchange of aqueous Ca2+ for adsorbed Na+ control the concentrations of solutes in the floodplain. Groundwater pumping draws sulfate-rich groundwater used for flood irrigation in the Yuma and San Luis Valley. Soil water is subjected to evapotranspiration, and Ca2+ and dissolved inorganic carbon are removed by precipitation of calcium carbonate. Precipitation of calcium carbonate allows the further dissolution of gypsum by the common ion effect. In the special case where groundwater is in equilibrium with calcite and dolomite, the dissolution of dolomite (de-dolomitization) increases Mg2+ concentrations, as observed in Figure 14B[60]. Montmorillonite is the most abundant clay in the study area and has considerable capacity for cation exchange [9]. As soil water moves through the soil, Na+ is released for Ca2+ during the cation exchange process. This affects the Mg/Ca ratio in floodplain 2+ 2+ 2− samples (Figure 14) and explains the deficit of Ca + Mg relative to SO4 and HCO3 (Figure, balanced by the excess Na+ observed in Figure 12A. The groundwater produced by this set of reactions is enriched in readily soluble salts left behind by evapotranspiration of irrigation water and contributes to the salinization of the aquifer when excess irrigation infiltrates and reaches the water table [60]. Once in the aquifer, the enriched solution mixes with Ca–Mg–Cl/Na–Cl groundwater and Na–Cl groundwaters derived from local recharge (Figure 15). Hydrology 2021, 8, 80 19 of 22 Hydrology 2021, 8, x FOR PEER REVIEW 20 of 23

FigureFigure 15.15. ConceptualConceptual modelmodel ofof thethe studystudy area.area.

5. Conclusions 18 2 Stable isotopesisotopes ((δδ18O and δ2H) distinguish four potential water endmembers in thethe Colorado River Delta:Delta: post post-dam-dam river water, pre-dampre-dam river water, Gila RiverRiver water,water, andand locallocal recharge.recharge. Evaporation effects areare prominentprominent inin thethe dataset;dataset; ColoradoColorado RiverRiver samplessamples formform aa singlesingle evaporationevaporation trendtrend ofof slopeslope 5.8.5.8. GroundwaterGroundwater fromfrom thethe DeltaDelta floodplainfloodplain andand waterwater fromfrom thethe CiCiénegaénega de de Santa Santa Clara Clara plot plot on on the the river river evaporation evaporation trend. trend. Seawater Seawater cannot can- intrudenot intrude on the on shallowthe shallow aquifers aquifers examined examined in this in this study. study. InIn thethe GilaGila Range,Range, locallocal mountainmountain system rechargerecharge results fromfrom thethe largestlargest 30%30% ofof winterwinter rainfallrainfall events. events. Recharge Recharge occurs occurs through through preferential preferential infiltration infiltration pathways pathways along along the majorthe major washes washes draining draining the Gila the Range. Gila Range. Water Water from smaller from smaller rainfall eventsrainfall is events lost to is evapora- lost to tionevaporation and transpiration, and transpiration, which causes which the cause accumulations the accumulation of meteoric of meteoric salts with salts seawater with sea- ion ratioswater nearion ratios the surface. near the Accumulated surface. Accumulated salts are dissolved salts are duringdissolved the during large and the infrequent large and precipitationinfrequent precipitation events, yielding events, infiltration yielding more infiltration concentrated more thanconcentrated rainwater; than these rainwater; solutions infiltratethese solutions into the infiltrate water table. into the Solutes water are table. dominated Solutes by are Na–Cl dominated and contribute by Na–Cl Na and+, Ca con-2+, − + 2−2+ − 2- Cltribute, and Na SO, 4Ca to, Cl the, and aquifer. SO4 to the aquifer. InIn thethe irrigatedirrigated floodplainfloodplain ofof thethe ColoradoColorado RiverRiver Delta,Delta, hydrochemicalhydrochemical evolutionevolution isis mostly controlled byby thethe original Ca–Mg–Cl/Na–Cl-typeCa–Mg–Cl/Na–Cl-type Colorado River water,water, withwith small (<1%)(<1%) additionsadditions ofof marine marine salt. salt. Mineral Mineral saturation saturation states, states, ionic ionic relations, relations, and and stable stable isotopes iso- indicatetopes indicate that salinity that salinity is augmented is augmented by evapotranspiration, by evapotranspiration, precipitation precipitation of calcite thatof calcite leads tothat the leads dissolution to the dissolution of gypsum byof gypsum the common by the ion common effect, dissolution ion effect, of dissolution accumulated of soilaccumu- salts, de-dolomitization,lated soil salts, de-dolomitization, and exchange of and aqueous exchange Ca2+ offor aqueous adsorbed Ca2+ Na for+. Pre-damadsorbed ColoradoNa+. Pre- Riverdam Colorado water is commonRiver water in floodplain is common groundwater. in floodplain groundwater. 18 2 InIn thethe YumaYuma andand SanSan LuisLuis Mesas,Mesas, valuesvalues ofofδ δ18OO andand δδ2HH indicateindicate mixingmixing betweenbetween locallocal rechargerecharge atat thethe GilaGila Range Range and and Colorado Colorado River River water. water. Na–Cl-dominated Na–Cl-dominated groundwater groundwa- flowster flows southwest southwest from from the the Gila Gila Range Range and and mixes mixes with with the the Ca–Mg–Cl/Na–Cl-dominated Ca–Mg–Cl/Na–Cl-dominated 3 floodplainfloodplain waters. Low Low 3HH indicate indicatess that that groundwater groundwater within within the theYuma Yuma and and San SanLuis Luis Me- sas infiltrated before the 1950′s, and 14C data are consistent with bulk residence times of thousands of years (4800 and 11,500 uncorrected 14C years before present).

Hydrology 2021, 8, 80 20 of 22

Mesas infiltrated before the 19500s, and 14C data are consistent with bulk residence times of thousands of years (4800 and 11,500 uncorrected 14C years before present).

Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/hydrology8020080/s1, Figure S1: General geology and cross-section A–A’. Table S1: Isotopic and chemical composition of waters in the study area. Author Contributions: Conceptualization, H.A.Z. and C.J.E.; methodology, H.A.Z., C.J.E. and J.C.M.; validation, H.A.Z., C.J.E. and J.C.M.; formal analysis, H.A.Z., C.J.E. and J.C.M.; investigation, H.A.Z.; resources, K.W.F., J.C.M.; writing—original draft preparation, H.A.Z.; writing—review and edit- ing, C.J.E., J.C.M. and K.W.F.; visualization, H.A.Z., C.J.E.; supervision, K.W.F. and J.C.M.; project administration, H.A.Z. and K.W.F. All authors have read and agreed to the published version of the manuscript. Funding: This research was supported by a National Science Foundation Graduate Research Fellow- ship Grant (DGE-1143953), a University of Arizona Water Sustainability Program Fellowship, and a University of Arizona Geosciences Department R. Wilson Thompson Scholarship (H.A.Z.). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data used in this study are available at https://waterdata.usgs.gov/ nwis (accessed on 28 April 2021) and other sources provided in Table S1. Acknowledgments: The authors would like to thank two anonymous reviewers for their suggestions and comments to improve this manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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