geosciences

Article Hydrology of Mountain Blocks in Arizona and New Mexico as Revealed by Isotopes in Groundwater and Precipitation

Christopher J. Eastoe 1,* and William E. Wright 2

1 Department of Geosciences (retired), University of Arizona, Tucson, AZ 85721, USA 2 Laboratory of Tree Ring Research, University of Arizona, Tucson, AZ 85721, USA; [email protected] * Correspondence: [email protected]



Received: 20 September 2019; Accepted: 25 October 2019; Published: 28 October 2019


Abstract: Mountain-block groundwater in the Southern Basin-and-Range Province shows a variety of patterns of δ18O and δ2H that indicate multiple recharge mechanisms. At 2420 m above sea level (masl) in Tucson Basin, seasonal amount-weighted means of δ18O and δ2H for summer are 8.3, − 53%, and for winter, 10.8 and 70%, respectively. Elevation-effect coefficients for δ18O and − − − δ2H are as follows: summer, 1.6 and 7.7 % per km and winter, 1.1 and 8.9 % per km. Little − − − − altitude effect exists in 25% of seasons studied. At 2420 masl, amount-weighted monthly averages of δ18O and δ2H decrease in summer but increase in winter as precipitation intensity increases. In snow-banks, δ18O and δ2H commonly plots close to the winter local meteoric water line (LMWL). Four principal patterns of (δ18O, δ2H) data have been identified: (1) data plotting along LMWLs for all precipitation at >1800 masl; (2) data plotting along modified LMWLs for the wettest 30% of months at <1700 masl; (3) evaporation trends at all elevations; (4) other patterns, including those affected by ancient groundwater. Young, tritiated groundwater predominates in studied mountain blocks. Ancient groundwater forms separate systems and mixes with young groundwater. Recharge mechanisms reflect a complex interplay of precipitation season, altitude, precipitation intensity, groundwater age and geology. Tucson Basin alluvium receives mountain-front recharge containing 50%–90% winter precipitation.

Keywords: Arizona; New Mexico; hydrology; mountain block; recharge; stable isotopes; tritium; carbon-14

1. Introduction In the Basin and Range Province [1] of Southwestern North America, mountain blocks play an important role in regional hydrology. Water from the highest mountain ranges is conveyed to adjacent basin-fill aquifers either as surface flow or as sub-surface mountain-block recharge [2]. Since the 1880s, the alluvial aquifers have been a crucial water source for agriculture, mining and urban centers in the region. Groundwater hydrology studies in mountain ranges are potentially difficult because of the challenges of data collection in remote and inaccessible terrain with few sampling points [2].

Geosciences 2019, 9, 461; doi:10.3390/geosciences9110461 www.mdpi.com/journal/geosciences Geosciences 2019, 9, 461 2 of 22

Studies of mountain-system hydrology have been conducted at a variety of spatial scales. Smaller-scale studies have focused on water budgets of individual hillslopes or small watersheds, using parameters such as water chemistry, temperature and isotopes to examine the distribution of available water between runoff, soil water and bedrock groundwater [3–8]. At a larger scale, isotope and noble gas ratio data have identified groundwater moving from a mountain block into basin alluvium in Salt Lake Valley, Utah, USA [9,10]. Stable isotope studies have addressed the residence Geosciencestime 2019 of, 9, mountain-block x FOR PEER REVIEW groundwater of Table Mountain, South Africa, and the Cascade Range, Oregon, 2 of 22 USA [11,12]. A potentialA potential approach approach to mountain-block to mountain-block hydrology hydrology consists consists in comparingin comparing isotope isotope data of data of groundwater from springs and wells with isotopes in mountain precipitation, taking into account groundwater from springs and wells with isotopes in mountain precipitation, taking into account the the altitude and seasonal effects, with a view to constraining recharge mechanisms and flow paths. altitude andSuch seasonal studies have effects, been undertaken with a view in theto constr Basin-and-Rangeaining recharge Province mechanisms [13–17]. Figure 1and is a flow conceptual paths. Such studies havesketch been of a mountain undertaken block, showingin the Basin-and-Ra possibilities fornge water Province movement [13–17]. that might Figure be evaluated 1 is a using conceptual sketch ofstable a mountain isotope data. block, showing possibilities for water movement that might be evaluated using stable isotope data.

Figure 1.Figure Conceptual 1. Conceptual diagram diagram for forprocesses processes of of mounta mountain-blockin-block hydrology hydrology in the in Basin-and-Range the Basin-and-Range Province.Province. A = recharge A = recharge of high-altitude of high-altitude precipitation, precipitation, fracture flow flow to allto lowerall lower elevations; elevations; B = recharge B = recharge of lower-altitude precipitation; C = high-altitude recharge subject to cyclic infiltration, discharge, of lower-altitude precipitation; C = high-altitude recharge subject to cyclic infiltration, discharge, evaporation and infiltration; D = mountain-block recharge to basin alluvium; E = mountain-front evaporationrecharge and to infiltration; basin alluvium. D = mountain-block recharge to basin alluvium; E = mountain-front recharge to basin alluvium. In this article, stable oxygen and hydrogen isotope data from environmental waters are reviewed In thisin an article, assessment stable of groundwateroxygen and characteristics hydrogen isot in sixope mountain data from blocks environmental (Santa Catalina, Rincon, waters Tucson, are reviewed Santa Rita, Galiuro, and Sacramento) of the Basin and Range Province in Southern Arizona and New in an assessmentMexico, and of in groundwater the Mogollon Highlands characteristics at Payson, in Arizona six mountain (Figure2). blocks Data for (Santa the Huachuca Catalina, and Rincon, Tucson, SantaChiricahua Rita, Mountains Galiuro, (Mts.)and Sacramento) (Figures S1 and of S2), the which Basin illustrate and Rang less clearlye Province the phenomena in Southern to be Arizona and Newdescribed Mexico, in theand other in locations,the Mogollon are reviewed Highlands in Supplementary at Payson, Information. Arizona The (Figure aims of 2). the Data study for the Huachucaare: and (1) detailedChiricahua evaluation Mountains of isotope (Mts.) character (Figures of high-altitude S1 and precipitationS2), which inillustrate the Santa Catalinaless clearly the phenomena(Mts); to (2) be comparison described of in isotope the other data for locations, groundwater are in reviewed multiple mountain in Supplementary blocks; (3) interpretation Information. The of stable O and H isotope data in the context of isotope altitude effects, local geology and groundwater aims of theages; study (4) interpretation are: (1) detailed of isotope evaluation data patterns of inisotope terms of character recharge mechanisms; of high-altitude and (5) re-evaluationprecipitation in the Santa Catalinaof seasonality (Mts); of (2) mountain-front comparison recharge of isotope to the data alluvial for aquifer groundwater in Tucson Basin.in multiple mountain blocks; (3) interpretation of stable O and H isotope data in the context of isotope altitude effects, local geology and groundwater ages; (4) interpretation of isotope data patterns in terms of recharge mechanisms; and (5) re-evaluation of seasonality of mountain-front recharge to the alluvial aquifer in Tucson Basin.

Geosciences 2019, 9, x FOR PEER REVIEW 2 of 22

A potential approach to mountain-block hydrology consists in comparing isotope data of groundwater from springs and wells with isotopes in mountain precipitation, taking into account the altitude and seasonal effects, with a view to constraining recharge mechanisms and flow paths. Such studies have been undertaken in the Basin-and-Range Province [13–17]. Figure 1 is a conceptual sketch of a mountain block, showing possibilities for water movement that might be evaluated using stable isotope data.

Figure 1. Conceptual diagram for processes of mountain-block hydrology in the Basin-and-Range Province. A = recharge of high-altitude precipitation, fracture flow to all lower elevations; B = recharge of lower-altitude precipitation; C = high-altitude recharge subject to cyclic infiltration, discharge, evaporation and infiltration; D = mountain-block recharge to basin alluvium; E = mountain-front recharge to basin alluvium. In this article, stable oxygen and hydrogen isotope data from environmental waters are reviewed in an assessment of groundwater characteristics in six mountain blocks (Santa Catalina, Rincon, Tucson, Santa Rita, Galiuro, and Sacramento) of the Basin and Range Province in Southern Arizona and New Mexico, and in the Mogollon Highlands at Payson, Arizona (Figure 2). Data for the Huachuca and Chiricahua Mountains (Mts.) (Figures S1 and S2), which illustrate less clearly the phenomena to be described in the other locations, are reviewed in Supplementary Information. The aims of the study are: (1) detailed evaluation of isotope character of high-altitude precipitation in the Santa Catalina (Mts); (2) comparison of isotope data for groundwater in multiple mountain blocks; (3) interpretation of stable O and H isotope data in the context of isotope altitude effects, local geology Geosciencesand groundwater2019, 9, 461 ages; (4) interpretation of isotope data patterns in terms of recharge mechanisms;3 of 22 and (5) re-evaluation of seasonality of mountain-front recharge to the alluvial aquifer in Tucson Basin.

Figure 2. Map of Arizona and part of New Mexico, showing locations of mountain ranges described in this study. C = Chiricahua Mts.; G = Galiuro Mts.; H = Huachuca Mts.; MH = Mogollon Highlands; R = Rincon Mts.;S = Sacramento Mts., SC = Santa Catalina Mts.; SR = Santa Rita Mts.; T = Tucson Mts.

2. Materials and Methods

2.1. Study Area The mountain blocks considered here are hard-rock ranges of the Basin-and-Range province, plus part of the southern rim of the Colorado Plateau. The Basin-and-Range Province formed by Neogene tectonic extension, part of an extended episode of taphrogenesis reflecting the interaction of mid-ocean ridges and triple junctions with trenches along the western edge of the continent [18]. Extension split a hard-rock plateau into a series of mountain ranges separated by basins filled with alluvium, a process that began 15 Ma ago in Southeastern Arizona [18]. The ranges are fault-bounded horsts or tilted blocks of consolidated Early Proterozoic to Neogene rocks [19]. The complex interior structure of the ranges reflects evolution from a Late Cretaceous compressive stress regime to a Neogene extensional regime [19]. The region has an arid to semiarid climate, except at the crests of the higher mountain ranges. Annual average precipitation ranges from 75 to over 1000 mm annually, with the higher totals occurring at the range crests [20]. Potential evaporation greatly exceeds rainfall [20,21]. Rainfall typically occurs in two seasons or modes: in summer, localized, potentially intense convective precipitation results from the North American monsoon, and in winter, widespread orographic precipitation originates from Pacific frontal systems. These precipitation modes are consistently separated in the spring by a hot period with little or no rain during May and June. Dry, lower mountain slopes typically support grasses, forbs, succulents and shrubs, succeeded upwards by stunted evergreen oaks and conifers. Well-watered range-crests support forests of conifers and deciduous trees. Most woodland parts of the study areas have little human settlement and are protected by National Forest or National Park designation; in such areas, fire has been suppressed for many years. Since the onset of long-term drought in the late 1990s, the vegetation and surface hydrology of large areas have been greatly affected by forest fires. Higher-density human settlement in these mountain ranges is present locally in the Southern Sacramento, Santa Catalina and Tucson Mts. and the Mogollon Highlands. When sampling for this study began in the 1990s, springs of modest discharge were widely distributed in most of the ranges considered here, except for the Tucson Mts. Attempts at re-sampling Geosciences 2019, 9, 461 4 of 22 since 2012 have shown that many springs no longer discharge in the Santa Catalina and Galiuro Mts. The change in hydrology reflects drought conditions that have prevailed since about 1998 [22]. The hydrogeology of each range is strongly influenced by its lithologies and structures. In the following summary, representative references are given for the geology. Detailed geological maps and cross-sections can be found in those references.

2.2. Santa Catalina and Rincon Mts These ranges form the northern and northeastern boundary of Tucson Basin, and constitute a single structural block of the Basin-and-Range Province. In the Santa Catalina Mts. (SCM), the highest peak is Mt. Lemmon, 2791 masl. Steep slopes and V-shaped drainages are characteristic. In the Rincon Mts. (RM), the highest peak is Mica Mountain, 2641 masl. The central massifs of both ranges consist mainly of granitoid intrusions, including thick sheets of Paleogene equigranular peraluminous leucogranite and associated pegmatite intruding tilted Proterozoic and Paleozoic strata to the northeast, and intensely deformed Proterozoic granite to the southwest [23,24]. The SCM-RM block formed the lower plate of a Neogene low-angle detachment fault marked by a band of impermeable mylonite [25]. A pronounced set of sub-horizontal leucogranite sheets on Mica Mountain (RM) creates a stepped topography that is absent in the SCM. Previous stable isotope studies in the SCM addressed the origin of spring water [26], the origin of groundwater associated with the detachment fault [27], the nature of precipitation at a station at 2420 masl [28] and relationships among precipitation, critical zone groundwater, soil water and stream flow [6,8,29,30].

2.3. Tucson Mts The Tucson Mountains form a terrain of steep slopes and sharp peaks interspersed with broad valleys and pediments, and bounding Tucson Basin to the west. Wassen Peak, 1428 masl, is the highest point. Late Cretaceous–Paleogene granitoids intrude coeval felsic volcanic rocks and Permian to Jurassic sedimentary rocks [24]. Fracture-hosted groundwater is pumped from as much as 170 m below valley-floors and pediments; water levels are declining rapidly. Detailed isotope (stable O, H in water, stable S, O in sulfate, 3H, 14C, 87Sr/86Sr) and geochemical data were presented for fracture-hosted 2 groundwater from a 9 km area near the Yuma Mine, 32.19◦ N, 111.73◦ W[31].

2.4. Santa Rita Mts In the steep-sided massif of Mt. Wrightson (2882 masl), Triassic and Jurassic granitoids intrude Triassic felsic volcanic rocks with interbedded eolian sandstone. Paleozoic limestone and Mesozoic clastic sedimentary rocks underlie the eastern slope of the massif [32]. Published groundwater isotope data (stable O, H and tritium) are available for the Rosemont area at 1500–1700 masl on the northern ridge of the range [33], where complex faulting juxtaposes Proterozoic granitoid, Paleozoic marine shelf strata and Mesozoic clastic sedimentary rocks [32,34,35].

2.5. Galiuro Mts Steep slopes and V-shaped topography are characteristic. The highest point is Bassett Peak, 2332 masl. Late Paleogene felsic and intermediate volcanic rocks unconformably overlie discontinuous outcrops of Late Cretaceous–Paleogene andesite and granitoids [36]. At the south end of the range, a group of hot springs (Hooker and nearby hot springs) discharges at 53 ◦C. Stable O, H isotope data were measured for a few springs at the north end of the range [37].

2.6. Mogollon Highlands Paleozoic strata, predominantly limestone, sandstone and siltstone, unconformably overlie Proterozoic granitoids [38]. Highest elevations, 2300–2400 masl, occur at the Mogollon Rim (the Geosciences 2019, 9, 461 5 of 22 southern edge of the Colorado Plateau) where the strata are flat-lying. South of the Mogollon Rim, at elevations of 1500–1700 masl, the Paleozoic strata and Proterozoic granitoids are juxtaposed in complex fashion by normal faults. Available reports address the hydrogeology of the Mogollon Highlands [39] and isotopes in groundwater [40].

2.7. Sacramento Mts The highest point in the range is Cathey Peak, 2923 masl. Landforms at high elevation include V-shaped canyons, broad valleys with thick soil, and local karst features. Paleozoic strata, predominantly limestone, are bounded by steep normal faults to the west, and dip eastward. Small springs discharge where shale beds form aquitards, but regional hydrology is dominated by long-distance karst permeability [16]. Groundwater isotope (stable O, H in water, 3H, 14C) and geochemical data amassed during wet years, 2006–2009, [16] were compared with isotope data collected in 2003, following a dry period [17].

2.8. Recharge Seasonality The difference in stable isotope composition between summer and winter rain in Tucson Basin has been used with isotope mass-balance to determine that winter recharge is dominant in the basin aquifer [41–44]. Both O and H isotopes should be used in the mass-balance approach [45] because of an isotope effect related to precipitation intensity like that in tropical regions [46]. Two principal recharge mechanisms involving local precipitation (not river water of remote derivation) operate in alluvial basins of Arizona [45]. Basins in Northwestern Arizona receive recharge from winter precipitation, with strong evaporation effects. Basins in Southeastern and Central Arizona receive both summer and winter recharge, but only from precipitation falling in the wettest months. The seasonality of recharge in mountain blocks may not correspond to that in adjacent basins, and may vary with altitude [47].

2.9. Altitude Effects The altitude effect on stable isotopes in precipitation [48] consists in decreases in δ18O and δ2H with increasing sampling elevation. The altitude-δ18O relationship has been used to determine altitudes of recharge [49,50] and paleoelevations in ancient mountain ranges [51,52]. For worldwide stations at 18 latitudes <70◦ and altitudes <5000 m, the relationship between δ O and altitude is linear, and the range of isotopic lapse rate for δ18O is 1.0 to 5.1%/km with an average of 2.8 1.3 (1σ)%/km [53]. − − − ± Snow sublimation may be superimposed on isotope altitude effects, decreasing lapse rates of δ18O and leading to underestimates of paleo-elevation [54]. Isotope altitude effects are commonly absent on mountain ranges in continental interiors and in snow [55]. Altitude effects in interior ranges of Arizona and New Mexico appear to be complex [45]. Altitude effects are present in summer but not in winter on a north-facing transect near Prescott, Arizona, outside the present study area; on a neighboring south-facing transect, altitude effects are present in both seasons, but are imperfectly correlated with altitude [56]. On the east-facing slope of the Sacramento Mts., New Mexico, normal altitude effects were observed in summer, but reverse effects (δ18O increasing with altitude) occurred in two winters of three studied [16]. In Tucson Basin, data collected at 2420 masl between 1995 and 2004 at Palisades Ranger Station (PRS) in the Santa Catalina Mts. [28] (Supplementary Table S1) have been compared with a 32-year dataset collected at 747 masl at the University of Arizona (UA) station in Tucson [22,28]. PRS has an average precipitation of 750 mm/year; precipitation at PRS is more frequent than at UA, where average precipitation is 360 mm/yr. The long-term estimates of altitude coefficients (Figure3a) are based on all precipitation events for each station. Geosciences 2019, 9, x FOR PEER REVIEW 6 of 22

3. Results Previously unpublished isotope data are listed with sample location data in Supplementary Tables S1–S3. Sources of published groundwater data are cited in each section, below.

3.1. Precipitation, Santa Catalina Mts. Figure 3a shows all available data for PRS [28]. Gaps exist, and many data represent composites of rain events over multiple weeks (Table S1). Data for summer (defined here as June to October) and winter (November to May) span broad, largely identical ranges; most data plot on a linear trend above the global meteoric water line (GMWL) [58] between δ18O values of −19‰ and −3‰. Summer data predominate at δ18O > −5‰. Seasonal local meteoric water lines (LMWL), weighted for precipitationGeosciences 2019, 9amount, 461 [59] differ in slope and intercept (Figure 3a). Long-term amount-weighted6 of 22 averages (Figure 3b) are (δ18O, δ2H) = (−10.8‰, −70‰) for winter and (−8.3‰, −53‰) for summer.

a 40 b Summer: y = 7.0 x + 4.2 -40 0 Winter: y = 8.3x + 20.1 S -40 -60 W H, ‰ H, ‰ H, 2 2 δ δ -80 Summer -80 -120 Winter GMWL -160 -100 -25-20-15-10-50 5 -12 -10 -8 -6 18 δ18O, ‰ δ O, ‰ Ppt. means all data Winter means Lapse rates, ‰ per km d c 8 Summer δ18O δ2H -40 Summer means GMWL Winter Winter -1.1 -8.9 6 Summer -1.6 -7.7 -60 O PRS ‰ PRS O 18 H, ‰ H, -80 4 2 δ -100

O UA - UA O δ 2 Other snow banks: y = 9.0x + 26.3 18 δ -120 0 -16 -14 -12 -10 -8 -6 -12 -10 -8 -6 -4 -2 0 18 18 δ O, ‰ δ O, ‰ UA Snow bank 2007 Other snow banks Snowmelt GMWL Winter LMWL Linear (Other snow banks)

Figure 3. Isotope data for precipitation in the Santa Catalina Mts. (a). δ2H vs. δ18O for individual Figure 3. Isotope data for precipitation in the Santa Catalina Mts. (a). δ2H vs. δ18O for individual measurements. Values of δ18O are amount-weighted means. (b). δ2H vs. δ18O for seasonal measurements. Values of δ18O are amount-weighted means. (b). δ2H vs. δ18O for seasonal amount- amount-weighted mean data. (c). Altitude effects measured between the University of Arizona weighted mean data. (c). Altitude effects measured between the University of Arizona (UA, 747 masl) (UA, 747 masl) and Palisades Ranger Station (PRS, 2420 masl) over entire winter and summer seasons, and Palisades Ranger Station (PRS, 2420 masl) over entire winter and summer seasons, 1996–2003. 1996–2003. (d). δ2H vs. δ18O for snow-banks. (d). δ2H vs. δ18O for snow-banks. 2.10. Sampling and Analyses 3.2. Altitude Effects at Annual Time Scale Spring water was sampled as close as possible to the discharge point. Water supply wells in 18 continualDifferences use were in sampledseasonal withoutamount-weighted further purging. mean Samplesδ O between were storedPRS and in robust,UA are tightly distributed sealed evenlyplastic bottles.between Measurements 0, signifying no of altitudeδ18O and effect,δ2H wereand 4.9, made corresponding on an automated to −2.9 gas-source ‰/km (Figure isotope 3c). ratio In Southernmass spectrometer Arizona, estimation (Finnigan Deltaof average S) at the isotope Environmental altitude effects Isotope clearly Laboratory requires of data the Universityfor multiple of years. Available data indicate little to no altitude effect (< 1‰ difference in Figure 3c) in four of sixteen Arizona, Tucson, Arizona. For H isotopes, samples were reacted at 750 ◦C with Cr metal using a individualFinnigan H seasons./Device coupled to the mass spectrometer. For O isotopes, samples were equilibrated with CO2 at approximately 15 ◦C in an automated equilibration device coupled to the mass spectrometer. 3.3. Isotopes in Snow International reference materials VSMOW and SLAP were used for standardization [57]. On the basis of repeatedWinter internalprecipitation standards, above analytical 2000 masl precision in the Sa (1ntaσ) wasCatalina 0.9% Mts.or better is commonly for δ2H andsnow, 0.08 locally % or 18 formingbetter for snow-banksδ O. Prior tothat 1992, may water persist samples until werethe spri preparedng equinox. manually Vertical by reduction sections to through H2 gas usinga snow- Zn bankmetal on for Mt. H isotopes, Bigelow and Road CO 2(32.4158°equilibration N, 110.730 for O isotopes.2° W) were Analytical sampled precision between (1σ )2007 was poorer,and 2012 1.5% to investigateto 2% for δ the2H, occurrence and 0.15% offor isotopeδ18O. shifts Tritium due was to sublimation measured on or 0.18 other L samplessnow-metamorphic following electrolytic processes enrichment, using Quantulus 1220 spectrophotometers with a detection limit of 0.6 tritium units

(TU). Calibration was based on National Institute of Standards and Technology standard reference materials 4361 B and C. Carbon-14 measurements were made on 50 L water samples at the Radiocarbon Laboratory, University of Arizona. Dissolved inorganic carbon was precipitated as BaCO3, processed to benzene, and measured using Quantulus 1220 liquid-scintillation spectrophotometers. The detection limit for full-size samples was 0.8 percent modern carbon (pMC) or lower. Calibration was based on international standard Oxalic Acid I. In this article, the presence of finite tritium in a groundwater sample is used as evidence that the sample contains at least some recharge from precipitation that fell since about 1953. Finite tritium does not preclude the presence of older recharge mixed with the younger water. Geosciences 2019, 9, 461 7 of 22

3. Results Previously unpublished isotope data are listed with sample location data in Supplementary Tables S1–S3. Sources of published groundwater data are cited in each section, below.

3.1. Precipitation, Santa Catalina Mts Figure3a shows all available data for PRS [ 28]. Gaps exist, and many data represent composites of rain events over multiple weeks (Table S1). Data for summer (defined here as June to October) and winter (November to May) span broad, largely identical ranges; most data plot on a linear trend above the global meteoric water line (GMWL) [58] between δ18O values of 19% and 3%. Summer data − − predominate at δ18O > 5%. Seasonal local meteoric water lines (LMWL), weighted for precipitation − amount [59] differ in slope and intercept (Figure3a). Long-term amount-weighted averages (Figure3b) are (δ18O, δ2H) = ( 10.8%, 70%) for winter and ( 8.3%, 53%) for summer. − − − − 3.2. Altitude Effects at Annual Time Scale Differences in seasonal amount-weighted mean δ18O between PRS and UA are distributed evenly between 0, signifying no altitude effect, and 4.9, corresponding to 2.9 %/km (Figure3c). − In Southern Arizona, estimation of average isotope altitude effects clearly requires data for multiple years. Available data indicate little to no altitude effect (< 1% difference in Figure3c) in four of sixteen individual seasons.

3.3. Isotopes in Snow Winter precipitation above 2000 masl in the Santa Catalina Mts. is commonly snow, locally forming snow-banks that may persist until the spring equinox. Vertical sections through a snow-bank on Mt. Bigelow Road (32.4158◦ N, 110.7302◦ W) were sampled between 2007 and 2012 to investigate the occurrence of isotope shifts due to sublimation or other snow-metamorphic processes that are known elsewhere in the Basin and Range Province [54,60]. Except in 2007, the data form a scattered linear trend (Figure3d), essentially conforming to the winter LMWL. In 2007, much of the snow plotted far above the GMWL, reflecting snow-bank isotope fractionation opposite in sense to that observed in Southeast California and Nevada [54]. Three meltwaters plot with snow-bank data, and a fourth plots to the right of the GMWL, as predicted for snow that has partly sublimed [14]. Large isotope fractionation within snow-banks does not occur in most years in the Santa Catalina Mts. When present, it may result from sublimation and recondensation of vapor along a thermal gradient [61]. Snow metamorphism studies in Alaska [61] and the Southwest USA [54,60] do not show the large isotope shifts to the left of the GMWL that were identified in this study (Figure3d).

3.4. Precipitation Intensity Effects Recharge in subtropical and tropical regions has the isotope composition of precipitation falling in the wettest months [46]. This effect is also present in Southern Arizona, where an isotope intensity effect exists in the UA precipitation isotope dataset, and recharge appears to be dominated by precipitation falling during approximately the wettest 30% of months (Figure4a) [ 45]. The PRS precipitation dataset is not ideal for such analysis, because of composite samples that are not easily arranged into calendar-month brackets, and because of the relatively small number of monthly data (30 for winter, 37 for summer, compared with 159 and 134, respectively, at UA). Where necessary, data were combined into approximate month-long non-calendrical brackets. Limitations notwithstanding, the wettest summer months (plotted at x 60 in Figure4b) at PRS have lower δ18O and δ2H than the long-term ≥ amount-weighted averages (plotted at x = 0). In winter, the opposite occurs: the wettest months have higher values of δ18O and δ2H than the long-term averages. Means representing the wettest 30% of months at PRS are nearly collinear with the LMWL as defined by long-term means of all data (Figure5a). Geosciences 2019, 9, x FOR PEER REVIEW 7 of 22

that are known elsewhere in the Basin and Range Province [54,60]. Except in 2007, the data form a scattered linear trend (Figure 3d), essentially conforming to the winter LMWL. In 2007, much of the snow plotted far above the GMWL, reflecting snow-bank isotope fractionation opposite in sense to that observed in Southeast California and Nevada [54]. Three meltwaters plot with snow-bank data, and a fourth plots to the right of the GMWL, as predicted for snow that has partly sublimed [14]. Large isotope fractionation within snow-banks does not occur in most years in the Santa Catalina Mts. When present, it may result from sublimation and recondensation of vapor along a thermal gradient [61]. Snow metamorphism studies in Alaska [61] and the Southwest USA [54,60] do not show the large isotope shifts to the left of the GMWL that were identified in this study (Figure 3d).

3.4. Precipitation Intensity Effects Recharge in subtropical and tropical regions has the isotope composition of precipitation falling in the wettest months [46]. This effect is also present in Southern Arizona, where an isotope intensity effect exists in the UA precipitation isotope dataset, and recharge appears to be dominated by precipitation falling during approximately the wettest 30% of months (Figure 4a) [45]. The PRS precipitation dataset is not ideal for such analysis, because of composite samples that are not easily arranged into calendar-month brackets, and because of the relatively small number of monthly data (30 for winter, 37 for summer, compared with 159 and 134, respectively, at UA). Where necessary, data were combined into approximate month-long non-calendrical brackets. Limitations notwithstanding, the wettest summer months (plotted at x ≥ 60 in Figure 4b) at PRS have lower δ18O and δ2H than the long-term amount-weighted averages (plotted at x = 0). In winter, the opposite occurs: the wettest months have higher values of δ18O and δ2H than the long-term averages. Means representing the wettest 30% of months at PRS are nearly collinear with the LMWL as defined by Geoscienceslong-term2019 means, 9, 461 of all data (Figure 5a). 8 of 22

(a)

(b)

Figure 4. Effect of rainfall intensity on δD and δ18O in precipitation; data from the (a) University of

Arizona (UA) and (b) Palisades Ranger Station (PRS) in the Santa Catalina Mts. Data from [22,28]. 18 AFigure value of4. 70Effect (for instance)of rainfall on intensity the x-axis on corresponds δD and δ O to in cumulative precipitation; amount-weighted data from the data(a) University for the 70th of toArizona 100th percentile (UA) and of (b months,) Palisades arranged Ranger in Station order of(PRS) monthly in the precipitation Santa Catalina amount–i.e., Mts. Data tofrom the [22,28]. wettest A Geosciences 2019, 9, x FOR PEER REVIEW 8 thof 22 30%value of months.of 70 (for instance) on the x-axis corresponds to cumulative amount-weighted data for the 70 to 100th percentile of months, arranged in order of monthly precipitation amount–i.e., to the wettest 30% of months.

Therefore, isotope data cannot reveal a relationship between precipitation intensity and recharge at the elevation of PRS; such relationships rely on a distinction between the LMWL (long-term means) and the line including wettest-month means [45,46]. Recharge may nonetheless occur mainly during the wettest summer months at high altitudes. In winter, this is less likely because snow-banks and their meltwater can represent all winter precipitation events. The isotope composition of high- altitude winter recharge is, therefore, better estimated by the weighted mean for all winter events.

2 18 Figure 5. ((a).). δδ2HH vs. vs. δδ18OO in in groundwater groundwater (GW) (GW) from from the the Sant Santaa Catalina Catalina Mts., Mts., grouped grouped according according to toaltitude altitude of discharge. of discharge. Also Also show shownn are arelong-term long-term amount-weighted amount-weighted mean mean data datafor precipitation for precipitation at 2420 at 2420masl masl[28] averaging [28] averaging over overall data all dataand over and overthe wettest the wettest 30% of 30% months, of months, and for and precipitation for precipitation at 1400 at 1400masl masl[22,28], [22 ,averaging28], averaging over over the wettest the wettest 30% 30% of mo of months.nths. The The snowbank snowbank line line is is from from Figure Figure 33d.d. (bb).). 18 δ18OO in in high-altitude high-altitude groundwater from the Santa Catalina Mts.Mts. as a function of altitude. Sloping lines 18 indicate long-termlong-term δδ18O lapse-rates. Blue Blue dashed lines ma markrk altitudes 2300 and 1400 masl. Green and blue rectangles are discusseddiscussed inin thethe text.text.

3.5. Groundwater, Santa Catalina Mts. Figure 5a shows data for the main range of the Santa Catalina Mts., sampled in 1994–1996 [26], omitting data for samples that had apparently evaporated between the discharge and sampling points. Most samples plot close to the GMWL. Springs discharging at elevations above 2300 masl have a restricted range of isotope composition, plotting near or just below the amount-weighted mean for winter precipitation. Springs discharging below 2300 masl have a much broader range that extends from mean summer precipitation to values far below mean winter precipitation. For springs discharging below 1400 masl, the range of isotope composition is more restricted and possibly related (with one exception) to mean precipitation for the wettest months near 1400 masl. Plotting δ18O as a function of discharge altitude (Figure 5b), it is possible to constrain recharge seasonality. Reasoning that recharge must occur at elevations above the discharge points, winter recharge is predominant for samples plotting within the green rectangle, regardless of recharge altitude. Similarly, samples plotting in the blue rectangle have approximately equal summer and winter recharge, or predominant summer recharge. Points to the left of the green rectangle can only be explained by recharge of water from unusually low-δ18O events, recorded at PRS in both summer and winter (Supplementary Table S1).

Geosciences 2019, 9, 461 9 of 22

Therefore, isotope data cannot reveal a relationship between precipitation intensity and recharge at the elevation of PRS; such relationships rely on a distinction between the LMWL (long-term means) and the line including wettest-month means [45,46]. Recharge may nonetheless occur mainly during the wettest summer months at high altitudes. In winter, this is less likely because snow-banks and their meltwater can represent all winter precipitation events. The isotope composition of high-altitude winter recharge is, therefore, better estimated by the weighted mean for all winter events.

3.5. Groundwater, Santa Catalina Mts Figure5a shows data for the main range of the Santa Catalina Mts., sampled in 1994–1996 [ 26], omitting data for samples that had apparently evaporated between the discharge and sampling points. Most samples plot close to the GMWL. Springs discharging at elevations above 2300 masl have a restricted range of isotope composition, plotting near or just below the amount-weighted mean for winter precipitation. Springs discharging below 2300 masl have a much broader range that extends from mean summer precipitation to values far below mean winter precipitation. For springs discharging below 1400 masl, the range of isotope composition is more restricted and possibly related (with one exception) to mean precipitation for the wettest months near 1400 masl. Plotting δ18O as a function of discharge altitude (Figure5b), it is possible to constrain recharge seasonality. Reasoning that recharge must occur at elevations above the discharge points, winter recharge is predominant for samples plotting within the green rectangle, regardless of recharge altitude. Similarly, samples plotting in the blue rectangle have approximately equal summer and winter recharge, or predominant summer recharge. Points to the left of the green rectangle can only be explained by recharge of water from unusually low-δ18O events, recorded at PRS in both summer and winter (Supplementary Table S1). All spring discharge in the main massif of the range contained tritium, and, therefore, contains some post-1953 recharge [26]. Groundwater flow appears to occur on a time scale of decades, most likely through shallow fracture systems. The drying of many springs since the beginning of drought conditions in about 1998 is additional evidence of the short time scale of flow. Winter recharge at the range crest could supply discharge to springs with data plotting in the green rectangle (Figure5b). All springs considered could potentially have catchments extending to the range crest. However, more than half of the springs below 2300 masl were supplied with water of other isotope composition. Recharge was, therefore, occurring at lower elevations, with a wide range of combinations of winter and summer precipitation. No explanation is possible at present for apparent preferential recharge after unusually low-δ18O events; in winter, such events are not the largest precipitation events (Figure4b; Supplementary Table S1).

3.6. Springs, Rincon Mts Two sets of samples are considered (Figure6). Spring water was collected in 2010 on Mica Mountain, the main massif of the range, by National Parks Service staff [62], mostly from small pools into the bottom of which spring water was seeping; some of the samples showed strong evidence of evaporation and others may be slightly evaporated. A second set of spring and well water samples was collected close to outcrop of the Catalina detachment fault by one of us (CJE), Joy Gillick, and Alex Leonard between 1996 and 2014. Data for springs discharging above 2180 masl form an array between mean winter and summer precipitation at PRS, and show no evidence of evaporation, with one exception. Springs discharging below 2180 masl on Mica Mountain form an evaporation trend of slope near 3.5, the trend apparently also including two of the springs discharging above 2180 masl, and the mean isotope composition of the wettest 30% of summer months. Two intensely evaporated samples with (δ18O, δ2H) = (+7.3, 2%) and (+15.0, +23%) are omitted from the plot, but lie on the evaporation trend and are included − in the regression calculation. Most of the spring waters contained finite tritium when sampled in 2010. Geosciences 2019, 9, x FOR PEER REVIEW 9 of 22

All spring discharge in the main massif of the range contained tritium, and, therefore, contains some post-1953 recharge [26]. Groundwater flow appears to occur on a time scale of decades, most likely through shallow fracture systems. The drying of many springs since the beginning of drought conditions in about 1998 is additional evidence of the short time scale of flow. Winter recharge at the range crest could supply discharge to springs with data plotting in the green rectangle (Figure 5b). All springs considered could potentially have catchments extending to the range crest. However, more than half of the springs below 2300 masl were supplied with water of other isotope composition. Recharge was, therefore, occurring at lower elevations, with a wide range of combinations of winter and summer precipitation. No explanation is possible at present for apparent preferential recharge after unusually low-δ18O events; in winter, such events are not the largest precipitation events (Figure 4b; Supplementary Table S1).

3.6. Springs, Rincon Mts. Two sets of samples are considered (Figure 6). Spring water was collected in 2010 on Mica Mountain, the main massif of the range, by National Parks Service staff [62], mostly from small pools into the bottom of which spring water was seeping; some of the samples showed strong evidence of evaporation and others may be slightly evaporated. A second set of spring and well water samples was collected close to outcrop of the Catalina detachment fault by one of us (CJE), Joy Gillick, and Geosciences 2019, 9, 461 10 of 22 Alex Leonard between 1996 and 2014.

Figure 6. δ2H vs. δ18O in groundwater from the Rincon Mts. Also shown are long-term amount-weighted Figure 6. δ2H vs. δ18O in groundwater from the Rincon Mts. Also shown are long-term amount- mean data for precipitation at 2420 masl (from Figure5). Data for ancient water are for Southeastern weighted mean data for precipitation at 2420 masl (from Figure 5). Data for ancient water are for Arizona groundwater containing < 8 pMC. [44,63,64]. W = winter, S = summer. Southeastern Arizona groundwater containing < 8 pMC. [44,63,64]. W = winter, S = summer. Groundwater drawn from wells in hard rock and from Agua Caliente spring near the Catalina detachmentData for fault springs at the discharging base of the above Rincon 2180 Mts. masl plots form near an (δ 18arrayO, δ 2betweenH) = ( 10, mean76% winter). A and similar summer set of − − precipitationdata, predating at automatedPRS, and show analytical no evidence techniques, of ev wasaporation, collected with in the one early except 1980sion. [27 Springs]. The groundwater discharging belowis of sodium-sulfate 2180 masl on type;Mica sulfate-oxygenMountain form isotopes an evaporation indicate trend that the of sulfateslope near source 3.5, is the oxidized trend apparently sulfide [65]. alsoTritium including is below two detection, of the springs and discharging14C content abov rangese 2180 from masl, 16 toand 47 the percent mean modernisotope composition carbon (pMC). of 18 2 theCorrected wettest dates 30% areof summer not offered, months. because Two sulfide intensely oxidation evaporated has probably samples enhanced with (δ theO, dissolution δ H) = (+7.3, of −accessory2‰) and carbonate(+15.0, +23‰) of unknown are omittedδ13 Cfrom value the from plot, the bu aquifer.t lie on the evaporation trend and are included in theThree regression recharge calculation. mechanisms Most are of presentthe spring in thewaters Rincon contained Mts. finite (1) At tritium high-altitudes, when sampled various in 2010.combinations of winter and summer recharge plot on the high-altitude LMWL. Most groundwater of this kindGroundwater bears detectable drawn tritiumfrom wells (Supplementary in hard rock Tableand from S2) and Agua appears Caliente to flowspring with near short the residence Catalina 18 2 detachmenttime through fault shallow at the fractures.base of the (2) Rincon High-altitude Mts. plots recharge near (δ thatO, δ isH) either = (−10, equal −76‰). parts A high-altitudesimilar set of data,winter predating and summer automated precipitation analytical (considering techniques seasonal, was amount-weighted collected in the means early for 1980s all precipitation [27]. The data) or entirely precipitation from the wettest 30% of summer months. This kind of water appears to recharge and discharge cyclically in stream beds, undergoing evaporation in the process. (3) Recharge of slightly evaporated water with (δ18O, δ2H) lower than the values for present-day weighted-mean winter precipitation at PRS. The groundwater is of long residence time, probably thousands of years, apparently flowing through a deep fracture system to an aquifer confined beneath mylonite of the Catalina detachment fault. Such groundwater may be a mixture between ancient precipitation (represented in Figure6 by eleven data points for Southeastern Arizona groundwater containing <8 pMC [44,63,64] with more recent high-altitude recharge. Note that the data from [64] include four samples without 14C data, but from the same deep, saline aquifer as dated samples.

3.7. Groundwater, Tucson Mts The Tucson Mts. are of lower elevation and have lower precipitation than the other ranges surrounding Tucson Basin. Samples were taken from domestic wells and from one spring, all in hard rock. Values of (δ18O, δ2H) (Supplementary Table S2) form a linear array approximately parallel to and to the right of the GMWL (Figure7). Samples that contain finite tritium plot at the upper end of the trend, while the oldest sample, containing 7.8 pMC, plots at the lower end. This suggests a relationship between stable O and H isotopes and groundwater residence time, although the age distribution (indicated approximately by uncorrected pMC) is complex in the middle of the array. Geosciences 2019, 9, x FOR PEER REVIEW 10 of 22

groundwater is of sodium-sulfate type; sulfate-oxygen isotopes indicate that the sulfate source is oxidized sulfide [65]. Tritium is below detection, and 14C content ranges from 16 to 47 percent modern carbon (pMC). Corrected dates are not offered, because sulfide oxidation has probably enhanced the dissolution of accessory carbonate of unknown δ13C value from the aquifer. Three recharge mechanisms are present in the Rincon Mts. (1) At high-altitudes, various combinations of winter and summer recharge plot on the high-altitude LMWL. Most groundwater of this kind bears detectable tritium (Supplementary Table S2) and appears to flow with short residence time through shallow fractures. (2) High-altitude recharge that is either equal parts high-altitude winter and summer precipitation (considering seasonal amount-weighted means for all precipitation data) or entirely precipitation from the wettest 30% of summer months. This kind of water appears to recharge and discharge cyclically in stream beds, undergoing evaporation in the process. (3) Recharge of slightly evaporated water with (δ18O, δ2H) lower than the values for present-day weighted-mean winter precipitation at PRS. The groundwater is of long residence time, probably thousands of years, apparently flowing through a deep fracture system to an aquifer confined beneath mylonite of the Catalina detachment fault. Such groundwater may be a mixture between ancient precipitation (represented in Figure 6 by eleven data points for Southeastern Arizona groundwater containing <8 pMC [44,63,64] with more recent high-altitude recharge. Note that the data from [64] include four samples without 14C data, but from the same deep, saline aquifer as dated samples.

3.7. Groundwater, Tucson Mts. The Tucson Mts. are of lower elevation and have lower precipitation than the other ranges surrounding Tucson Basin. Samples were taken from domestic wells and from one spring, all in hard rock. Values of (δ18O, δ2H) (Supplementary Table S2) form a linear array approximately parallel to and to the right of the GMWL (Figure 7). Samples that contain finite tritium plot at the upper end of the trend, while the oldest sample, containing 7.8 pMC, plots at the lower end. This suggests a Geosciencesrelationship2019 ,between9, 461 stable O and H isotopes and groundwater residence time, although the11 ofage 22 distribution (indicated approximately by uncorrected pMC) is complex in the middle of the array. Values ofof ((δδ1818O, δ2H)H) at at the the upper upper end end of of the the trend trend fit fit a a modified modified LMWL for the wettest 30% of months at 10001000 masl, masl, rather rather than than the the LMWL LMWL for allfor data all (bothdata lines(both interpolated lines interpolated between between UA and PRSUA altitudes).and PRS Aaltitudes). precipitation A precipitation intensity e ffintensitect operatesy effect on operates recent recharge, on recent which recharge, arises which equally arises from equally winter from and summerwinter and precipitation, summer precipitation, or predominantly or predominantly from summer from precipitation. summer precipitation. The lower The end lower of the end array of indicatesthe array ancientindicates recharge ancient atrecharge a time of at lowera time ( δof18 lowerO, δ2H) (δ18 inO, average δ2H) in precipitation. average precipitation.

Figure 7. δ2H vs. δ18O in groundwater (GW) from the Tucson Mts. Also shown are long-term amount-weightedFigure 7. δ2H vs. δ18 meanO in datagroundwater for precipitation (GW) from for Tucsonthe Tucson Basin Mts. (Figures Also 3shown and4), are adjusted long-term for altitude.amount- weighted mean data for precipitation for Tucson Basin (Figures 3, 4), adjusted for altitude. Data GeosciencesData 2019 sources:, 9, x FOR this PEER study REVIEW and [31 ], shown as elliptical field. W = winter, S = summer. 11 of 22 sources: this study and [31], shown as elliptical field. W = winter, S = summer. 3.8. Groundwater, Santa Rita Mts 3.8. Groundwater, Santa Rita Mts. Data for springs higher than 1800 masl on the Mt. Wrightson massif (Figure8) plot on the Data for springs higher than 1800 masl on the Mt. Wrightson massif (Figure 8) plot on the LMWL LMWL for precipitation at PRS, between summer and winter means, with no indication of evaporation. for precipitation at PRS, between summer and winter means, with no indication of evaporation. The The range of δ18O is similar over about 800 m of altitude. All samples in which tritium was measured range of δ18O is similar over about 800 m of altitude. All samples in which tritium was measured contain finite tritium (Supplementary Table S2). In this case, all recharge may occur at the range crest, contain finite tritium (Supplementary Table S2). In this case, all recharge may occur at the range crest, with approximately equal winter and summer contributions. with approximately equal winter and summer contributions.

Figure 8. δ2H vs. δ18O in groundwater (GW) from the Santa Rita Mts. Also shown are long-term Figureamount-weighted 8. δ2H vs. δ mean18O in data groundwater for precipitation (GW) for from Tucson the BasinSanta (FiguresRita Mts.3 and Also4); “30%shown wettest” are long-term denotes amount-weightedmean data for the mean wettest data 30% for of precipitation summer and for winter Tucson months, Basin adjusted (Figures for 3, altitude.4); “30% wettest” Lines A anddenotes B are meansuggested data for evaporation the wettest trends 30% of of summer slope 4. and Data winter sources: months, this study, adjusted [33]. for W altitude.= winter, Lines S = summer. A and B are suggested evaporation trends of slope 4. Data sources: this study, [33]. W = winter, S = summer Data from Rosemont, for wells and springs below 1600 masl, plot either along a line passing throughData weighted-meanfrom Rosemont, averages for wells of and (δ18 springsO, δ2H) below for the 1600 wettest masl, 30% plot of monthseither along at 1600 a line masl passing (which throughis close weighted-mean to collinear with averages the GMWL), of (δ18 orO, toδ2H) the for right the wettest of that line.30% of Most months data at form 1600 amasl cluster (which with is a closecorresponding to collinear evaporation with the GMWL), trend (line or Ato inthe Figure right8 ,of slope that 4),line. and Most are consistentdata form witha cluster about-equal with a corresponding evaporation trend (line A in Figure 8a, slope 4), and are consistent with about-equal contributions of summer and winter recharge. A few data points suggest predominant winter recharge but may also reflect mixing with ancient recharge. The field of data is delimited downward by an evaporation trend (line B), which appears to be related to the data for ancient water in Figure 6. No 14C measurements are available for this data set.

3.9. Groundwater, Galiuro Mts. Most samples taken in 1994 plot along an evaporation trend of slope near 4 (Figure 9). Care was taken to avoid water that might have evaporated since discharge. A few samples, including Hooker Hot Springs, plot along the GMWL. Samples taken in 1982 [37] plot close to the evaporation trend. Samples taken in 2012 at Upper and Lower Ash Springs plot closer to the GMWL. Other springs that might have been re-sampled proved to be dry in 2012. Most samples taken in 1994 contained finite tritium, between 1 and 6 TU, at the time of sampling. A sample from Hooker Hot Spring contained tritium below detection, and 17 pMC.

Geosciences 2019, 9, 461 12 of 22 contributions of summer and winter recharge. A few data points suggest predominant winter recharge but may also reflect mixing with ancient recharge. The field of data is delimited downward by an evaporation trend (line B), which appears to be related to the data for ancient water in Figure6. No 14C measurements are available for this data set.

3.9. Groundwater, Galiuro Mts Most samples taken in 1994 plot along an evaporation trend of slope near 4 (Figure9). Care was taken to avoid water that might have evaporated since discharge. A few samples, including Hooker Hot Springs, plot along the GMWL. Samples taken in 1982 [37] plot close to the evaporation trend. Samples taken in 2012 at Upper and Lower Ash Springs plot closer to the GMWL. Other springs that might have been re-sampled proved to be dry in 2012. Most samples taken in 1994 contained finite tritium,Geosciences between 2019, 9, x 1FOR and PEER 6 TU, REVIEW at the time of sampling. A sample from Hooker Hot Spring contained12 of 22 tritium below detection, and 17 pMC.

Figure 9. δ2H vs. δ18O in groundwater (GW) from the Galiuro Mts. Also shown are long-term amount-weightedFigure 9. δ2H vs. δ18 meanO in groundwater data for precipitation (GW) from based the Galiuro on data Mts. for Tucson Also shown Basin, are all long-term events (Figures amount-3 andweighted4), adjusted mean for data altitude. for precipitation NAWP = Northern based on Arizona data winterfor Tucson precipitation, Basin, all independent events (Figures of altitude 3, 4), eadjustedffect [45,56 for] Thealtitude. dashed NAWP line is = aNort suggestedhern Arizona evaporation winter trend precipitation, of slope 4. independent Data sources: of this altitude study; effect [37]. W[45,56]= winter, The Sdashed= summer. line is a suggested evaporation trend of slope 4. Data sources: this study; [37]. W = winter, S = summer. The data are consistent with two main recharge mechanisms in the range. Cold groundwater of decadalThe residence data are time,consistent evaporated with two prior main to infiltration, recharge mechanisms originated asin rechargethe range. plotting Cold groundwater below present of meandecadal winter residence precipitation time, evaporated at 2000 masl prior (based to infiltra ontion, Tucson originated Basin data) as recharge but resembling plotting below mean present winter precipitationmean winter that precipitation has no altitude at 2000 effect masl in Northern(based on Arizona Tucson [45Basin,56]. data) By 2012, but thisresembling recharge mean regime winter may noprecipitation longer have that operated has no in altitude the catchments effect in of Northe Upperrn and Arizona Lower [45,56]. Ash Springs. By 201 At2, this Hooker recharge Hot Springs regime atmay the no south longer end have of the operated range, hot in groundwaterthe catchments of of long Upper residence and Lower time, probablyAsh Springs. thousands At Hooker of years, Hot wasSprings not evaporatedat the south prior end toof rechargethe range, and hot flows ground throughwater aof deeper long residence fracture system. time, probably thousands of years, was not evaporated prior to recharge and flows through a deeper fracture system. 3.10. Groundwater Mogollon Highlands 3.10.Groundwater Groundwater Mogollon from an Highlands. area between the lower flanks of the Mogollon Rim and the Town of PaysonGroundwater yielded (δ18O, fromδ2H) an data area forming between a linear the lower evaporation flanks trendof the of Mogollon slope 5 (Figure Rim and10), andthe showTown no of 18 relationshipPayson yielded to altitude (δ18O, [δ402H)]. Samplesdata forming with δa linearO > evaporation10% are affected trend by of rechargeslope 5 (Figure of treated 10), water and fromshow − theno Paysonrelationship Town to Lake. altitude The [40]. data Samples trend is closelywith δ18 similarO > −10‰ to trends are affected for alluvial by recharge basins in of Northwestern treated water Arizonafrom the and Payson for major Town rivers Lake. that The rise alongdata thetrend Mogollon is closely Rim. similar Such trendsto trends originate for alluvial in regional basins mean in winterNorthwestern precipitation Arizona that and has for no altitudemajor rivers effect that in Northern rise along Arizona the Mogollon [45,56]. Rim. Such trends originate in regional mean winter precipitation that has no altitude effect in Northern Arizona [45,56].

Figure 10. δ2H vs. δ18O in groundwater (GW) from the Mogollon Highlands. Also shown are seasonal amount-weighted mean data for precipitation. NAWP = Northern Arizona winter precipitation, independent of altitude effect [45,56] Data source [40].

Geosciences 2019, 9, x FOR PEER REVIEW 12 of 22

Figure 9. δ2H vs. δ18O in groundwater (GW) from the Galiuro Mts. Also shown are long-term amount- weighted mean data for precipitation based on data for Tucson Basin, all events (Figures 3, 4), adjusted for altitude. NAWP = Northern Arizona winter precipitation, independent of altitude effect [45,56] The dashed line is a suggested evaporation trend of slope 4. Data sources: this study; [37]. W = winter, S = summer.

The data are consistent with two main recharge mechanisms in the range. Cold groundwater of decadal residence time, evaporated prior to infiltration, originated as recharge plotting below present mean winter precipitation at 2000 masl (based on Tucson Basin data) but resembling mean winter precipitation that has no altitude effect in Northern Arizona [45,56]. By 2012, this recharge regime may no longer have operated in the catchments of Upper and Lower Ash Springs. At Hooker Hot Springs at the south end of the range, hot groundwater of long residence time, probably thousands of years, was not evaporated prior to recharge and flows through a deeper fracture system.

3.10. Groundwater Mogollon Highlands. Groundwater from an area between the lower flanks of the Mogollon Rim and the Town of Payson yielded (δ18O, δ2H) data forming a linear evaporation trend of slope 5 (Figure 10), and show no relationship to altitude [40]. Samples with δ18O > −10‰ are affected by recharge of treated water from the Payson Town Lake. The data trend is closely similar to trends for alluvial basins in GeosciencesNorthwestern2019, 9 ,Arizona 461 and for major rivers that rise along the Mogollon Rim. Such trends originate13 of 22 in regional mean winter precipitation that has no altitude effect in Northern Arizona [45,56].

Figure 10. δ2HH vs. vs. δδ1818OO in in groundwater groundwater (GW) (GW) from from the the Mogollo Mogollonn Highlands. Highlands. Also Also shown shown are seasonal Geosciences 2019, 9, x FOR PEER REVIEW 13 of 22 amount-weighted mean data forfor precipitation.precipitation. NA NAWPWP = Northern Arizona winter precipitation, 3.11. Groundwater,independent ofof Sacramento altitudealtitude eeffectffect Mts. [[45,56]45 , 56] DataData sourcesource [[40].40]. 3.11.The Groundwater, data (Figure Sacramento 11) represent Mts springs and precipitation sampled in 2006–2009, which were wetterThe than data average (Figure years 11) represent [16], and springs springs and and precipitation wells sampled sampled in 2003, in 2006–2009, following whichseveral were dry wetteryears [17].than All average samples years are [from16], andcarbonate-dominated springs and wells strata. sampled in 2003, following several dry years [17]. All samples are from carbonate-dominated strata.

-30 S, 1810 masl -40 -50 S, 2805 masl 1750- 2400 masl

-60 2400 - 2800 masl

H, ‰ W, 2805 masl

2 -70 δ -80

-90 SMT, slope = 5.6 -100 -14 -13 -12 -11 -10 -9 -8 -7 δ18O, ‰ Groundwater 2003 Springs 2006-2009 GMWL Seasonal Precip. Linear (Groundwater 2003)

2 18 Figure 11. δ H vs. δ O in groundwater (GW) from the Sacramento Mts., differentiated by time of sampling and by altitude (2006 2009 samples only). Also shown are seasonal amount-weighted − Figuremean data11. δ for2H vs. precipitation δ18O in groundwater in summer (GW) 2007, andfrom winter the Sacramen 2007–2008,to Mts., collection differentiated altitudes by indicated. time of samplingSMT = Sacramento and by altitude Mts. (2006 trend,−2009 a suggested samples only). evaporation Also shown trend. are Data seasonal sources: amount-weighted groundwater [ 16mean,17]; dataprecipitation for precipitation [16]. W =inwinter, summer S = 2007,summer. and winter 2007–2008, collection altitudes indicated. SMT = Sacramento Mts. trend, a suggested evaporation trend. Data sources: groundwater [16,17]; precipitationData collected [16]. in W 2003 = winter, from S springs= summer. and wells at 1750 to 2550 masl form a linear trend (SMT) of slope 5.6, consistent with evaporated winter precipitation (measured later, in 2007 and 2008) as the principalData collected source of in recharge. 2003 from The springs slope and is high wells for at an 1750 evaporation to 2550 masl trend form and a maylinear be trend influenced (SMT) byof slopesummer 5.6, recharge.consistent Summer with evaporated recharge iswinter clearly prec presentipitation during (measured 2006–2009, later, when in 2007 the and data 2008) for springs as the principaldischarging source between of recharge. 1750 and The 2400 slope masl is formhigh for an arrayan evaporation stretching trend between and themay SMT be influenced and summer by summerprecipitation. recharge. Discharge Summer from recharge springs is betweenclearly presen 2400t and during 2800 2006–2009, masl form when a second the data array for stretching springs dischargingbetween the between SMT and 1750 a mixture and 2400 of masl summer form and an array winter stretching precipitation. between The the groundwater SMT and summer system precipitation.is dynamic, susceptible Discharge from to change springs in isotopebetween content 2400 and at annual2800 masl time form scales. a second Winter array recharge stretching with betweenevaporation the dominatesSMT and a at mixture all altitudes of summer above 1750 and maslwint iner dryprecipitation. years, but inThe years groundwater when above-average system is dynamic,summer rainfallsusceptible occurs, to summerchange rechargein isotope appears content in groundwater.at annual time scales. Winter recharge with evaporation dominates at all altitudes above 1750 masl in dry years, but in years when above-average summer rainfall occurs, summer recharge appears in groundwater.

3.12. Seasonality of Mountain-front Recharge, Tucson Basin. Water from the Santa Catalina and Rincon Mts. is supplied to Tucson basin by three major catchments: Rillito Creek, Cañada del Oro and Rincon Creek (Figure 12a). Mountain-front recharge occurs at distances up to 10 km from range fronts [44]. Recharge occurs mainly during the wettest 30% of months [45].

Geosciences 2019, 9, 461 14 of 22

3.12. Seasonality of Mountain-front Recharge, Tucson Basin Water from the Santa Catalina and Rincon Mts. is supplied to Tucson basin by three major catchments: Rillito Creek, Cañada del Oro and Rincon Creek (Figure 12a). Mountain-front recharge occurs at distances up to 10 km from range fronts [44]. Recharge occurs mainly during the wettest 30% ofGeosciences months 2019 [45]., 9, x FOR PEER REVIEW 14 of 22

Figure 12. a Figure 12.( ().a). Map Map ofof TucsonTucson Basin,Basin, after [44], [44], showing showing principal principal drainages, drainages, major major recharge recharge zones zones as as indicatedindicated by by tritium, tritium, and and zones zones with with 50% 50% winter winter recharge. recharge. ((bb).). SketchSketch diagramdiagram of the conical model model of a mountainof a mountain range range divided divided into into three three layers. layers.

AssumptionsAssumptions for for the the re-evaluationre-evaluation are: (1) (1) that that precipitation precipitation amount amount and and isotope isotope lapse lapse rates rates are are linearlinear functions functions of of altitude, altitude, established established byby data for for the the UA UA and and PRS PRS sites, sites, and and (2) (2) that that runoff runo fromff from any any altitudealtitude has has the the same same likelihood likelihood ofof reachingreaching the ba basinsin floor. floor. Therefore, Therefore, the the bulk bulk isotope isotope content content of of runorunoffff from from the the mountain mountain block block isis thethe isotopeisotope conten contentt of of precipitation precipitation at at some some altitude altitude A, A,above above and and belowbelow which which the the volume volume of of precipitationprecipitation is the same. Altitude Altitude A A is is determined determined by by considering considering each each mountainmountain range range as as a a cone cone divided divided intointo n equally spaced spaced layers layers (Figure (Figure 12b). 12b). The The results results for fora pyramid, a pyramid, oror for for intersecting intersecting cones cones and and pyramidspyramids are identical to to those those for for the the cone, cone, and and do do not not depend depend on onapical apical angle.angle. Each Each layer layer has has an an interpolatedinterpolated annual precipitation precipitation amount amount and and an an annular annular footprint footprint area; area; the the volumevolume of of precipitation precipitation for the the layer layer is is the the product product annularannular footprint footprint x precipitationx precipitation (mm). (mm) Volumes. Volumes are aresummed summed for for all alllayers layers above above and andbelow below a chosen a chosen layer. layer.For the For Santa the Catalina Santa Catalina Mts., using Mts., 100 using layers 100 layersfrom from1000 to 1000 2800 to masl, 2800 altitude masl, altitude A is about A is 1660 about masl 1660 (Supplementary masl (Supplementary Table 4). Interpolated, Table S4). Interpolated, amount- 18 2 amount-weighted,weighted, seasonal seasonal values of values(δ O, δ ofH) ( δin18 precipitationO, δ2H) in precipitation at 1660 masl are at compared 1660 masl with are comparedgroundwater with groundwaterisotope data isotopefor the parts data of for Tucson the parts Basin of Tucsonrecharged Basin from recharged mountain from runoff mountain (Figure 13). runo ff (Figure 13). The groundwater data do not conform to the LMWL defined by the seasonal means of all W WETTEST 30% OF MONTHS 50% AT 1660 masl S precipitation data atCAÑADA 1660 DEL masl ORO (Figure5b), but instead fall-30 on a modified LMWL corresponding to the wettest 30% of months at 1660 masl. The modified LMWL-40 was calculatedS by applying the long-term -50 isotope lapse rates for Tucson Basin (Figure3a) to the amount-weightedW mean isotope compositions -60 RINCON CREEK H, ‰ for the wettest 30% of months at the UA site (Figure2 -704). In order to avoid scatter of isotope data δ in younger, less integrated groundwater, only samples-80 containingPrecip 1660 masl all lessPrecip than 1660 1.5masl 30% TU were used [44]. GMWL Rillito RILLITO CREEK -90 In areas recharged from Rincon Creek and Cañada del Oro, winterRincon rechargeCanada del makes Oro up 75% to 90% of -100 1 measurement -12-11-10-9-8-7-6-5 the total. In areas recharged from Rillito Creek, winter recharge constitutesδ18O, ‰ 50% to 75% of the total. Water with about 50% winter recharge (samples in Figure 13 with δ18O 9.3%) occurs in discrete ≤ − areas at some distance-10.5 from -10.0 recharge sources -9.5 (Figure -9.0 12a). -8.5 -8.0 δ18O, ‰

Figure 13. Frequency histograms of δ18O in groundwater recharged from three major streams in Tucson Basin, compared with the δ18O range of amount-weighted seasonal mean precipitation for the wettest 30% of months at an attitude of 1660 masl (blue bar). Only samples containing less than 1.5 TU [44] are plotted. W = winter, S = summer. Inset: δ2H vs. δ18O for the groundwater samples plotted in the histograms, after [44,45], with amount-weighted seasonal means for precipitation at 1660 masl. Green symbols indicate means for all data, and red symbols for the wettest 30% of months.

The groundwater data do not conform to the LMWL defined by the seasonal means of all precipitation data at 1660 masl (Figure 5b), but instead fall on a modified LMWL corresponding to

Geosciences 2019, 9, x FOR PEER REVIEW 14 of 22

Figure 12. (a). Map of Tucson Basin, after [44], showing principal drainages, major recharge zones as indicated by tritium, and zones with 50% winter recharge. (b). Sketch diagram of the conical model of a mountain range divided into three layers.

Assumptions for the re-evaluation are: (1) that precipitation amount and isotope lapse rates are linear functions of altitude, established by data for the UA and PRS sites, and (2) that runoff from any altitude has the same likelihood of reaching the basin floor. Therefore, the bulk isotope content of runoff from the mountain block is the isotope content of precipitation at some altitude A, above and below which the volume of precipitation is the same. Altitude A is determined by considering each mountain range as a cone divided into n equally spaced layers (Figure 12b). The results for a pyramid, or for intersecting cones and pyramids are identical to those for the cone, and do not depend on apical angle. Each layer has an interpolated annual precipitation amount and an annular footprint area; the volume of precipitation for the layer is the product annular footprint x precipitation (mm). Volumes are summed for all layers above and below a chosen layer. For the Santa Catalina Mts., using 100 layers from 1000 to 2800 masl, altitude A is about 1660 masl (Supplementary Table 4). Interpolated, amount- weighted,Geosciences 2019 seasonal, 9, 461 values of (δ18O, δ2H) in precipitation at 1660 masl are compared with groundwater15 of 22 isotope data for the parts of Tucson Basin recharged from mountain runoff (Figure 13).

W WETTEST 30% OF MONTHS 50% AT 1660 masl S CAÑADA DEL ORO -30 -40 S -50 W -60 RINCON CREEK H, ‰ 2 -70 δ

-80 Precip 1660 masl all Precip 1660 masl 30% GMWL Rillito RILLITO CREEK -90 Rincon Canada del Oro -100 1 measurement -12-11-10-9-8-7-6-5 δ18O, ‰

-10.5 -10.0 -9.5 -9.0 -8.5 -8.0 δ18O, ‰ Figure 13. Frequency histograms of δ18O in groundwater recharged from three major streams in Tucson Figure 13. Frequency histograms of δ18O in groundwater recharged from three major streams in Basin, compared with the δ18O range of amount-weighted seasonal mean precipitation for the wettest Tucson Basin, compared with the δ18O range of amount-weighted seasonal mean precipitation for the 30% of months at an attitude of 1660 masl (blue bar). Only samples containing less than 1.5 TU [44] wettest 30% of months at an attitude of 1660 masl (blue bar). Only samples containing less than 1.5 are plotted. W = winter, S = summer. Inset: δ2H vs. δ18O for the groundwater samples plotted in the TU [44] are plotted. W = winter, S = summer. Inset: δ2H vs. δ18O for the groundwater samples plotted histograms, after [44,45], with amount-weighted seasonal means for precipitation at 1660 masl. Green in the histograms, after [44,45], with amount-weighted seasonal means for precipitation at 1660 masl. symbols indicate means for all data, and red symbols for the wettest 30% of months. Green symbols indicate means for all data, and red symbols for the wettest 30% of months. 4. Discussion The groundwater data do not conform to the LMWL defined by the seasonal means of all precipitation4.1. Residence data Times at 1660 masl (Figure 5b), but instead fall on a modified LMWL corresponding to

Where tritium data are available, they indicate the prominence of young groundwater in each of the mountain blocks. Residence times of multiple decades are likely. Ancient groundwater is recognized in certain blocks, either from 14C data or from distinctive stable O and H isotope data. Available 14C data without correction suggest residence times of thousands of years. The shortest residence times are found in the carbonate-dominated aquifers of the Sacramento Mountains, where groundwater isotopes change at a time-scale of 5 years or less. Contrasting systems of young and ancient groundwater have been distinguished in the Rincon and Galiuro Mts, and possibly in the Rosemont area of the Santa Rita Mts. Except in the Galiuro Mts, there is isotope evidence of mixing between old and young water. Preservation of ancient water in a mountain block is likely to be related to the depth and connectivity of fracture systems. In general, the depth of penetration of groundwater in mountain block aquifers is uncertain. Studies from regions of active tectonics have demonstrated groundwater flow in fractured rock to depths reaching 300 m below the surface [66] and references therein. In the Mont Blanc massif between France and Italy, 1950s or younger recharge has penetrated to a depth exceeding 2000 m on the basis of tritium measurements of water dripping into a highway tunnel [67]. Measurements of 36Cl and tritium in a tunnel within the unsaturated zone at Yucca Mountain, Nevada, USA indicated post-bomb groundwater infiltration along fractures to depths as great as 500 m [68]. The presence of warm water seems consistent with deep flow in the Galiuro Mts. In the Tucson Mts., ancient water is produced from fractured rock up to 170 m below the surface. In the Rosemont area, close juxtaposition of wells producing young and ancient groundwater [33] suggests limited connectivity between sets of fractures. Climate is also implicated in the persistence of ancient groundwater. O and H isotope data like those for ancient water in this study are interpreted as evidence that ancient water was recharged in the late Pleistocene, at a time of cooler and wetter climate in the Southwest USA [69]. Recharge was more abundant than at present under such conditions. In the low-elevation Tucson Mts., present rainfall may be insufficient to bring about recharge, leading to the dominance of ancient groundwater in much of that mountain block. Geosciences 2019, 9, 461 16 of 22

4.2. Multiple Recharge Mechanisms The patterns of stable isotope data described above result from multiple recharge mechanisms affecting young groundwater, in combination with mixing between young and ancient groundwater. The data patterns may be grouped as follows. (i) Data plotting on a modified present-day LMWL defined by seasonal amount-weighted means for the wettest months; (ii) data falling along the present-day LMWL drawn through amount-weighted means for all summer and winter precipitation, and plotting between the means; (iii) data falling along evaporation trends; (iv) data with no relationship to present-day LMWLs or evaporation trends. Group (i) matches Pattern 2 of groundwater in alluvial basins [45], and process B (Figure1). It occurs at Rosemont in the Santa Rita Mts., and in recent recharge in the Tucson Mts., in both cases at altitudes below 1700 masl. This data pattern arises when recharge occurs during the wettest 30% (approximately) of months and reflects the loss of surface water to evaporation during drier months. At Rosemont, equal amounts of summer and winter recharge are typical (Figure8), and in the Tucson Mts., more summer than winter recharge (Figure7). Mixing of recent recharge with ancient groundwater is indicated in both cases. Groundwater from springs discharging at elevations below 1400 masl in the Santa Catalina Mts. also largely fits this pattern, relative to mean isotope compositions for precipitation at 1400 masl (Figure5a). Group (ii) is characteristic of groundwater from elevations above 1800 masl in the Santa Rita and Rincon Mts. (Figures6 and8), and matches process A (Figure1). Precipitation from wettest months at such altitudes is collinear with the LMWL for all precipitation (Figure6). Both summer and winter recharge are indicated in both ranges, but the proportions remain uncertain because of the collinearity of the LMWL and the modified LMWL, and the possibility of summer recharge mainly during the wettest months. Group (iii) corresponds to Pattern 1 recharge in alluvial basins of Central and Northwestern Arizona [45] and process C (Figure1). The pattern reflects recharge of winter precipitation only, typically evaporated, in a region with no altitude dependence of O and H isotopes in winter precipitation. Group (iii) is observed in the Mogollon Highlands (Figure 10) and the Galiuro Mts. until 1996, but possibly not since 2012 (Figure9). In both cases, the evaporation trend passes through the point ( δ18O, δ2H) = ( 11.2, 77%), corresponding to mean winter precipitation in the Pattern 1 region [45]. The persistent − − evaporation trend in the Sacramento Mts. [17] also originates in winter precipitation, in this case with (δ18O, δ2H) near ( 13.5, 87%); after wet summers, however, isotope compositions move towards the − − LMWL (Figure 11). The evaporation trend at Rosemont in the Santa Rita Mts. originates in a mixture of summer and winter recharge (Figure8). The evaporation trend in the Rincon Mts. originates mainly as high-elevation summer precipitation (Figure7). In both cases, the isotope data are consistent with multiple cycles of infiltration, discharge into streambeds, evaporation and renewed infiltration. Group (iv) comprises other patterns, and includes ancient groundwaters, as established by 14C data in the Rincon, Tucson and Galiuro Mts. (Figures6,7 and9; Supplementary Table S2). Transmission of such water into basin alluvium (process D of Figure1) has been documented near the Rincon Mts. [65]. In the Santa Catalina Mts., Group (iv) also includes spring waters from 1400 to 2300 masl (Figure5a), many of which cannot be derived from long-term mean winter precipitation, regardless of the altitude of recharge (Figure5b). Precipitation from certain events or seasons (Figure3b,c) might account for the very low values of (δ18O, δ2H) in springs between 1400 and 2300 masl, but an explanation of that nature is problematic because it requires preferential infiltration of water of extreme isotope composition. A similar infiltration phenomenon was identified, but not satisfactorily explained, in soil water in the Marshall Gulch catchment, at 2300 to 2600 masl in the Santa Catalina Mts. [70]. Infiltration may be favored by progressive wetting of soil during a precipitation season, later precipitation events being more likely to survive evapotranspiration and lead to recharge. However, precipitation events with low (δ18O, δ2H) values occur randomly in both summer or winter rainy seasons in the Santa Catalina Mts. (Supplementary Table S2). Infiltration may also be favored by precipitation intensity (Figure4b), Geosciences 2019, 9, 461 17 of 22 as explained above. The highest precipitation intensity in summer corresponds with bulk monthly values of (δ18O, δ2H) as low as ( 10.3, 67%) in rainwater, insufficient to explain the observed values − − in spring water. In winter, precipitation intensity fails as an explanation because highest precipitation intensity corresponds with monthly (δ18O, δ2H) values that are higher than the long-term winter mean.

4.3. Seasonality of Recharge As in surrounding alluvial basins [45], recharge in mountain blocks is locally dominated by winter precipitation, by summer precipitation, or by subequal mixtures. A particular recharge seasonality may be characteristic of an entire mountain range (e.g., Mogollon Highlands, Galiuro Mts. in the 1990s) or part of a range (Rincon Mts). Alternatively, recharge seasonality may depend upon altitude (Santa Rita Mts., Santa Catalina Mts.). Altitude-dependence may reflect the tendency of winter snow to accumulate in banks and melt in bulk at winter’s end at high altitudes, whereas snow from individual events melts separately throughout winter at lower levels. Such an effect may account for the greater importance of winter infiltration above 2400 masl in the Sacramento Mts. (Figure 11). It may also explain the relative isotope uniformity of spring water (near mean winter precipitation) above 2300 masl in the Santa Catalina Mts., contrasting with the broad range of δ18O at 1400–2300 masl (Figure5a), but leaves unexplained the implied preferential recharge from low δ18O events. − 4.4. Influence of Geology All ranges studied consist predominantly of silicate rocks, except for the carbonate-dominated Sacramento Mts. Unhindered infiltration into the highly permeable limestones of the Sacramento Mts. is reflected in the short-term changes in isotope data in that range. Thick soil profiles in typical high-elevation valleys appear to play a role in generating the dry-period evaporation trend in groundwater of the range [17]. A contrast in permeability is present at the crest and on the north slope of the Santa Catalina Mts, where tilted strata overlie less fractured, less permeable granite. Most of the springs of the range discharge from these strata [26]. Whether this geological environment is implicated in the unusual distribution of groundwater isotopes with altitude (Figure5b) is not known. The very different isotope patterns in groundwater from the Santa Catalina and Rincon Mts. (Figures5a and6) occur in proximate ranges with closely similar lithology. The greater influence of summer recharge in the Rincon Mts. may arise from structural geology, the stepped topography governed by intrusion style at the range crest enhancing retention of summer runoff more effectively than the V-shaped canyons of the Santa Catalina Mts. In the Tucson Mts., the Galiuro Mts., and the Mt Wrightson massif of the Santa Rita Mts., groundwater is hosted by fractured volcanic rock. In these cases, geology does not appear to impose a single pattern on isotope data in groundwater, and altitude, as discussed above, may be the primary control. The evaporation trend of groundwater in the Galiuro Mts. is like that in the Mogollon Highlands. Both mountain blocks are contiguous with, and behave as part of, a regional zone in North-Central Arizona in which winter recharge is dominant, and no isotope altitude effect is present [45].

4.5. Future Research The complexity of (δ18O, δ2H) data patterns and recharge mechanisms in the mountain blocks reviewed here stands in sharp contrast to the simpler zonation of neighboring alluvial basins [45]. From the discussion above, it emerges that hydrologic processes in the mountain blocks reflect a complicated interplay of factors including precipitation seasonality, altitude, precipitation intensity, groundwater age and geology. The recharge mechanisms described in this article operate within a small number of mountain blocks that lie within a small portion of the North American Cordillera. In that area, the isotopic distinction between winter and summer precipitation provides a useful means of evaluating groundwater isotope data. Detailed studies of groundwater in other mountain blocks, both in the region of subequal winter and summer precipitation and beyond, will add to the Geosciences 2019, 9, 461 18 of 22 understanding of mountain block hydrogeology. It is impossible to predict, given the complexity of results from the present study, the recharge mechanisms that will operate in neighboring mountain blocks, let alone in more distant examples. Detailed study of each mountain block will be required. Such research will benefit greatly from additional detailed information about isotope lapse rates in precipitation. The two detailed datasets [28,31] for sites in Arizona reveal intricacies that can be resolved only through long-term commitment to the gathering of data.

5. Conclusions 1. In 1995–2004, precipitation at 2420 masl at PRS in the Santa Catalina Mts. had seasonal amount-weighted averages of (δ18O, δ2H) and LMWLs as follows: in summer ( 8.3, 53%), and − − δ2H = 7.0δ18O + 4.2; in winter, ( 10.8, 70%) and δ2H = 8.3δ18O + 20.1. Wetter months have − − lower (δ18O, δ2H) in summer, but higher (δ18O, δ2H) in winter. 2. In 2008–2013, snow-bank samples defined a line δ2H = 9.0δ18O + 26.3; in 2007, isotope metamorphism gave rise to (δ18O, δ2H) values deviating strongly from this line. 3. Long-term coefficients for isotope altitude effect (δ18O, δ2H) between Tucson and PRS are ( 1.6, − 7.7 % per 1000 m) in summer, and ( 1.1, 8.9 % per 1000 m) in winter. Altitude effect was − − − absent in 25% of seasons examined. 4. Groundwater containing finite tritium predominates in all ranges studied except the Tucson Mts., and is interpreted as young groundwater, or mixtures containing young groundwater. Ancient groundwater, indicated by 14C and distinctive (δ18O, δ2H) data, is present in the Galiuro, Tucson, Rincon and Santa Rita Mts. 5. Multiple recharge mechanisms of young groundwater are indicated by patterns of (δ18O, δ2H) data. The patterns are influenced by altitude, geology and mixing with ancient groundwater. 6. Regional, zoned recharge mechanisms of neighboring alluvial basins [45] are present in the silicate mountain blocks. Winter-only recharge with evaporation occurs in the Mogollon Highlands and the Galiuro Mts. Infiltration of winter and summer infiltration from the wettest months, in various proportions, occurs below 1700 masl in the Santa Rita and Tucson Mts, and below 1400 masl in the Santa Catalina Mts. 7. Additional recharge mechanisms include: (1) infiltration of both winter and summer precipitation in various proportions at high elevations in the Rincon and Santa Rita Mts., yielding (δ18O, δ2H) data plotting along high-altitude LMWLs; (2) Infiltration of high-elevation summer-dominant precipitation, with strong evaporation, in the Rincon Mountains; (3) recharge of water with (δ18O, δ2H) values below those of long-term mean winter precipitation above 1400 masl in the Santa Catalina Mts.; and (4) alternation of winter recharge with evaporation (dry years) and recharge of mixed winter and summer precipitation without evaporation (wet years) in the carbonate rocks of the Sacramento Mts. 8. A complex interplay of isotope effects related to altitude, precipitation seasonality, precipitation intensity and groundwater age is responsible for the multiple patterns of isotope data. 9. Assuming that mountain runoff from any altitude has an equal likelihood of reaching the basin floor, mountain-front recharge to alluvium in Tucson Basin consists of 50%–90% winter runoff from surrounding mountain blocks.

Supplementary Materials: The following previously unpublished data are available online at http://www. mdpi.com/2076-3263/9/11/461/s1: Isotopes in precipitation at Palisades Ranger Station, Santa Catalina Mts. in Supplementary Table S1; Isotopes in groundwater, with sample location data, in Supplementary Table S2; Isotopes in snowbanks on Mt. Bigelow, Santa Catalina Mts. in Supplementary Table S3. Supplementary Table S4 shows the calculation of average isotope composition in runoff from a conical mountain. Other Supplementary Material gives isotope data for springs in the Huachuca and Chiricahua Mts. Author Contributions: Conceptualization, C.J.E.; methodology, validation, laboratory analyses C.J.E., W.E.W.; writing—original draft preparation, C.J.E., writing—review and editing, C.J.E., W.E.W.; visualization C.J.E. Geosciences 2019, 9, 461 19 of 22

Funding: Funding for isotope analyses listed in Supplementary Tables S1–S3 was provided by the Environmental Isotope Laboratory at the University of Arizona, except for analyses of groundwater in the Rincon Mts., which were funded by the U.S. National Parks Service. Acknowledgments: The authors acknowledge the National Park Service for permission to use data for the Rincon Mts., and numerous private owners for access to their property. The authors are grateful to two anonymous reviewers and the editors for suggestions that improved the quality of the manuscript. Conflicts of Interest: The authors know of no conflicts of interest related to this work.

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