Geologic origins of salinization in a semi-arid river: The role of sedimentary basin brines James F. Hogan* Department of Hydrology and Water Resources, University of Arizona, Tucson, Arizona 85721, USA, and SAHRA—Center for Sustainability of Semi-Arid Hydrology and Riparian Areas, University of Arizona, Tucson, Arizona 85721, USA Fred M. Phillips Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology, Suzanne K. Mills Socorro, New Mexico 87801, USA, and SAHRA—Center for Sustainability of Semi-Arid Hydrology Jan M.H. Hendrickx and Riparian Areas, University of Arizona, Tucson, Arizona, 85721, USA Joaquin Ruiz Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA John T. Chesley Yemane Asmerom Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA ABSTRACT Semi-arid and arid rivers typically exhibit increasing salinity levels downstream, a trend often attributed to irrigated agriculture, primarily due to evapotranspiration. In contrast, the results of our investigations in one salinized river suggest that geological sources of salt added by groundwater discharge are more important than agricultural effects. We performed detailed synoptic sampling of the Upper Rio Grande– Rio Bravo, an arid-climate river with significant irrigated agriculture, and identified a series of salinity increases localized at the distal ends of sedimentary basins. Using Cl/Br, Ca/Sr, 87Sr/ 86Sr, and 36Cl/Cl ratios and δ234U values as environmental tracers, we show that these increases result from localized discharge of high-salinity groundwater of a sedimentary brine source. These groundwater fluxes, while very small (<1 m3s–1), are the dominant solute input and, combined with downstream evapotranspirative concentration, result in salinization. Furthermore, 36Cl/Cl ratios and δ234U values for these brines are close to secular equilibrium, indicating brine ages on the order of millions of years. The recognition of a substantial geologic salinity source for the Rio Grande implies that alternative salinity management solutions, such as interception of saline groundwater, might be more effective in reducing salinity than changes in agricultural practices. Keywords: salinity, brines, sedimentary basins, Rio Grande, isotope geochemistry. INTRODUCTION Salinization of rivers presents a major challenge to the sustainability of the world’s water resources and agriculture (Postel, 1999). Currently 2.3 billion people live in water-stressed river basins (e.g., the Indus, Nile, Rio Grande, Tigris-Euphrates, and Orange), mostly in semi-arid and arid regions (Johnson et al., 2001), where in addition to water scarcity, the buildup of salinity limits both domestic and agricultural use of water (Ghassemi et al., 1995). River salinization results in great economic damage through direct reduction of crop productivity, long-term damage to agricultural soils, the necessity of treatment for municipal and industrial uses, and damage to pipes and fixtures (Ghassemi et al., 1995; Postel, 1999). River salinization has been intensively studied for more than 50 yr (Ghassemi et al., 1995). Salinization results primarily from two factors, (1) solute addition (atmospheric deposition, mineral weathering, inflow of subsurface saline waters of connate, diagenetic, or geothermal origin, and anthropogenic sources), and (2) the concentrating processes of open-water evaporation and riparian and agricultural transpiration. Most frequently, the effects of irrigated agriculture are held responsible, either through evapotranspirative concentration, mobilization of salt stored in the vadose zone due to clearing of native vegetation and/or drainage of irrigation *E-mail: [email protected] waters, or displacement of shallow saline groundwater (e.g., Allison et al., 1990; Herczeg et al., 1993; Ghassemi et al., 1995; Postel, 1999). Typically, salinization is treated locally (e.g., at the scale of an irrigation district) at the downstream end of the river; however, the actual sources of salinity may be upstream. In these upstream regions, natural solute inputs of geologic origin that may appear insignificant may be concentrated downstream through evaporation and transpiration. Thus in order to develop effective policy and management, salinity issues must be evaluated at the river-basin scale and must consider geologic as well as anthropogenic solute sources and the processes that may concentrate these sources. RIO GRANDE SALINIZATION In this study we focus on the upper 1200 km of the Rio Grande– Rio Bravo (United States and Mexico), a highly stressed arid-region river in which chronic water shortages threaten food production and limit economic development (Johnson et al., 2001). The pattern of water use and salinization in the Rio Grande is typical of irrigated rivers in arid climates. Currently, 89% of the available water supply is used to support 370,000 ha of irrigated agriculture (Ellis et al., 1993). Total dissolved solids increase from ~40 mg L–1 at the headwaters in central Colorado to 500–1500 mg L–1 at El Paso–Ciudad Juárez on the U.S.-Mexico border (Moore and Anderholm, 2002), leaving down-river users with water of marginal utility (Ellis et al., 1993; Moore and Anderholm, 2002). Although the cause of Rio Grande salinization has been investigated for 75 yr, no conclusive answer has been reached. Salinization has been attributed to (1) simple evapotranspirative concentration as the water is reused for irrigation (Lippincott, 1939; Haney and Bendixen, 1953); (2) displacement of shallow salty groundwater by irrigation (Wilcox, 1957); (3) regional saline groundwater discharge (Moore and Anderholm, 2002; Phillips et al., 2003); (4) riparian evapotranspiration (Moore and Anderholm, 2002); and (5) unspecified irrigation effects (Moore and Anderholm, 2002). As ~75% of river flow is lost to open-water evaporation, agricultural transpiration, and riparian transpiration, these processes clearly play a role in salinization. However, the results of a simple Cl mass-balance model indicate that they are insufficient to explain the greater than tenfold increase in salinity (Phillips et al., 2003). To test these competing salinization hypotheses we employ an approach that consists of (1) collecting water samples at a high spatial resolution along the full river length, and (2) using a suite of geochemical tracers to fingerprint and quantify salinity sources. PATTERNS OF SALINIZATION We performed synoptic sampling of the Rio Grande from the headwaters to ~150 km south of El Paso (Fig. 1) during a period of irrigation (August 2001) and non-irrigation (January 2002). In general, Cl concentrations along the river increased by about two orders of magnitude (Fig. 2). Concentrations were higher during the winter (particularly below Elephant Butte Reservoir at 800 km), a fact likely attributed to lower flows through- © 2007 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected] GEOLOGY, December 2007 Geology, December 2007; v. 35; no. 12; p. 1063–1066; doi: 10.1130/G23976A.1; 4 figures; Data Repository item 2007265. 1063 1000 106°W 108° 1 2 B 9 A 50 500 25 3 0 0 250 2 8 Middle Cl (mg L-1) Cl (mg L-1) 750 COLORADO NEW MEXIC O ARIZON A Upper Upper 1 Alluvial Basins 36° Lower 75 Irrigated Agriculture 38° 200 400 600 6 Distance (km) 4 5 7 August 2001 January 2002 0 Middle Albuquerque Alluvial Basins 1. San Luis 2. Española 3. Albuquerque 3C 3A. Santo Domingo 3B. Calabacillas 3C. Belen 4 34° 5 6 Lower Elephant Butte Reservoir 7 9 32°N 0 3A 3B 4. Socorro 5. San Marcial 6. Engle 7. Palomas 8. Mesilla 9. Tularosa - Hueco 8 TEXAS MEXICO North 108° El Paso / Juárez 400 600 800 Distance (km) 1000 1200 Figure 2. Chloride concentration versus river distance from the headwaters. Numbers correspond to locations where Rio Grande exits southern end of the alluvial basins shown in Figure 1. Locations marked with letters refer to additional salinity increases not associated with alluvial basin termini. water treatment plant, a hypothesis supported by an associated nitrate increase (Moore and Anderholm, 2002). However, this location (as well as a point at ~515 km, where there is a small increase in Cl) also corresponds to a structural high marking a subbasin boundary (Fig. 1). In the lower part of the river (800–1200 km) there are two small Cl increases above El Paso–Ciudad Juárez (most clearly seen during winter low flow) and a very large increase ~50 km south of El Paso. The first increase at the northern end of the Mesilla Basin is in an area of known geothermal activity (Reiter, 1999), the second is at the southern end of the Mesilla Basin, and the third is located within the Tularosa-Hueco Basin (B in Fig. 2), in a zone where high-salinity groundwater is believed to discharge to the Rio Grande (Hibbs and Boghici, 1999). 100 km 106° 104° Figure 1. Map of the Rio Grande study area outlining drainage area, river course, irrigated agriculture, and alluvial basins (Wilkins 1998) with Albuquerque subbasins (Grauch et al., 2001). out much of the river system, as water is stored in reservoirs until the irrigation season, and thus a greater proportion of groundwater discharge. The series of discrete Cl− concentration increases observed during both irrigation and non-irrigation periods is particularly interesting. Comparison with geologic structure of the Rio Grande rift (Fig. 2) shows that these increases are typically located at the southern, low-elevation end of alluvial sedimentary basins, raising the possibility that salinity increases result from the discharge of sedimentary brines. Note, however, that regions of irrigated agriculture are typically located within sedimentary basins (Fig. 1) and thus high-Cl agricultural return flows may also enter there. The salinity pattern of the upper Rio Grande can be roughly divided into three equal portions (Fig. 1). The upper portion (0–400 km) is typified by very low Cl concentration that increases in a roughly linear fashion with flow distance (Fig. 2). There is only one noticeable stepwise increase at ~215 km at the southern end of the San Luis Basin. In the middle portion (400–800 km), Cl concentration exhibits three stepwise increases that correspond with three alluvial basins (Albuquerque, Socorro, and San Marcial–Engle). A fourth increase (point A in Fig. 2) is located within the Albuquerque Basin and is likely associated with the Albuquerque waste- 1064 200 ENVIRONMENTAL TRACERS OF SALINIZATION To distinguish between saline groundwater discharge and effects associated with irrigated agriculture, we have employed 87Sr/86Sr and 36 Cl/Cl.·1015 ratios and δ234U isotopic values, along with the associated solute ratios Cl/Br and Ca/Sr, as environmental tracers. We selected these tracers for three reasons. First, these isotopic and solute ratios are minimally affected by evapotranspiration, and thus are tracers of solute inputs only. Second, in all three systems sedimentary brines are expected to have distinct compositions (GSA Data Repository Table DR11). Third, the high concentration of sedimentary brines results in mixing relationships with large changes in isotopic values or solute ratios with addition of small saline groundwater fluxes. The generally conservative nature of Cl and Br, combined with the distinct Cl/Br ratios (mass/mass) for atmospheric deposition (~100) and sedimentary brine (≥1000) (Whittemore, 1995; Davis et al., 1998), make it an extremely useful tracer. In general, Cl/Br ratios increase from ~100 to >1000 in a stepwise fashion (Fig. DR1; see footnote 1) mirroring the increasing Cl concentration. Comparing Cl/Br and Cl concentration changes (Fig. 3) provides a clear illustration of the combined effect of evapotranspirative concentration and solute addition in explaining down1 GSA Data Repository item 2007265, Figure DR1 (downriver variation in Cl), Figure DR2 (downriver variation in Sr), Figure DR3 (downriver variation in U), Figure DR4 (U isotope mixing plot), and Table DR1 (geochemical end members), is available online at www.geosociety.org/pubs/ft2007.htm, or on request from [email protected] or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA. GEOLOGY, December 2007 1500 1400 Mixing Line August 2001 January 2002 1149 km 1000 g Cl/Br (wt/wt) Mi Cl/Br (wt/wt) xin 1000 500 20% 800 1014 km 10% 600 801 km 415 km 5% 400 0 0 0 200 400 600 800 Cl (mg L-1) 243 km 1% 200 Evaporation / Transpiration 0 A 1200 1000 500 1000 36 1500 2000 15 Cl/Cl · 10 2500 1200 B 0.7102 20% stream salinization. If evapotranspirative concentration were the only factor, the Cl/Br ratio would not change as Cl concentration increased. Conversely, if only saline groundwater discharge was important, increases would follow the calculated mixing curve. Data for the Rio Grande are intermediate between these two cases, illustrating their combined effect. The 36Cl ratios, measured along the length of the study area, exhibit a decreasing trend from ~2500 in the headwaters to ~100 at the southern end (Fig. DR1). Headwater values are consistent with an atmospheric source; however, a 36Cl ratio of ~2500 is significantly higher than present-day precipitation (Phillips, 2000), indicating a component from nuclear weapons testing fallout in these waters. In contrast, the very low 36Cl and high Cl/Br ratios in the lower portion are indicative of a sedimentary brine source. When 36Cl ratios are plotted against Cl/Br ratios (Fig. 4A), a clear two-end-member mixing relationship is observed, with all river samples plotting close to the mixing curve. This mixing curve is calculated using Rio Grande headwater values and an estimated higher salinity end member based on groundwaters that are likely a mix of dilute shallow groundwater with a deeper sedimentary brine source (Table DR1). The volumes of saline groundwater contributions are small, <1% of river flow, in the upper part of the study area. In the middle portion of the study area, the cumulative saline groundwater contribution increases from 1% to ~5% of river flow as it passes through three large and deep sedimentary basins. In the lower portion a small volume of discharge occurs at the southern end of the Mesilla Basin. South of this increase there is a dramatic rise in the percentage of saline groundwater, reflecting both saline groundwater discharge from the Hueco Bolson and the diversion of almost all remaining water for irrigated agriculture. Overall, saline groundwater fluxes appear to be greatest from large and deep alluvial basins (Albuquerque and Tularosa-Hueco; Wilkins, 1998) or smaller basins with significant geothermal activity (Socorro, Engle, Mesilla; Reiter, 1999). Note that 36Cl and Cl/Br ratios indicate that this geothermal activity is not the source of the salinity (Table DR1), but rather it may mobilize the sedimentary brine. Closed—i.e., internally drained—basins (San Luis; Wilkins, 1998) and small and/or shallow basins (Española and Palomas; Wilkins, 1998) appear to make smaller contributions. In contrast to Cl/Br ratios, Ca/Sr ratios exhibit a significant change only in the middle portion (Fig. DR2), hence 87Sr/86Sr ratios were measured only for this stretch of the river (400–800 km). In the headwater regions, Sr is likely derived from two sources: atmospheric deposition and mineral weathering (basalt is the dominant lithology through which the upstream Rio Grande flows), both of which have ratios distinct from sedimentary GEOLOGY, December 2007 5% 0.7100 86 Sr/ Sr 10% 87 Figure 3. Cl/Br ratios versus Cl concentration mixing plot using illustrating combined effect of evapotranspirative concentration and solute addition in explaining Rio Grande salinization. 3000 1% 0.7098 August 2001 January 2002 0.7096 30 40 50 60 Ca/Sr (wt/wt) 70 80 Figure 4. Isotopic mixing diagrams quantifying saline groundwater discharge, with distance from headwaters noted for some samples. A: 36Cl vs. Cl/Br ratios. B: 87Sr/ 86Sr versus Ca/Sr ratios. Mixing lines were calculated using end members discussed in text. brines (Table DR1). As the Rio Grande enters the middle portion, 87Sr/86Sr ratios are ~0.709 and Ca/Sr ratios are ~75, consistent with a mixture of headwater sources with a small contribution of saline groundwater. In the middle section of the river, 87Sr/86Sr ratios increase while Ca/Sr ratios decrease (Fig DR2). As for the Cl isotopic system, Ca/Sr and Sr isotopes for river samples indicate a clear two-end-member mixing relationship (Fig. 4B) between river water entering the middle portion of the study area and sedimentary brine. Mixing calculations indicate that a cumulative saline groundwater contribution between 5% and 10% of river flow can explain the observed salinity increases in the middle portion of the study area. Uranium isotopes also help in salinity source interpretation. The reactive nature of uranium, as illustrated by both increasing and decreasing concentrations (Fig. DR3), limits use of this isotopic system to qualitative interpretation. Consistent with the 36Cl and 87Sr/86Sr, δ234U data are best explained by two-component mixing (Fig. DR4) where one end member is modern surface water and the other is sedimentary brine. The low δ234U value (~500) and elevated U concentration of the saline end member is consistent with very old brine. Typically groundwaters have high 234U excesses (Dickson and Davidson, 1985); however, with long residence times, on a scale of millions of years given the 234U half-life of ~245 k.y. (Cheng et al., 2000), this is reduced. CONCLUSIONS Our data show an increase of about two orders of magnitude in the Cl content of the Rio Grande across the study area. In contrast, estimates of evapotranspiration can only explain a factor of four increase in Cl content. The results of all three isotopic systems exhibit mixing relationships between only two end members: Rio Grande headwaters composed 1065 of atmospheric deposition plus mineral weathering and higher salinity groundwater of sedimentary brine origin. Thus, our calculations and observations lead us to conclude that natural discharge of saline groundwaters is more important than evapotranspiration associated with irrigation in increasing the salinity of the Rio Grande. The calculated percentages of saline groundwater needed to explain the observed salinity increases using different elemental and isotopic systems are relatively consistent. For example, in the middle portion of the river, which includes three distinct inputs, a 5%–10% saline groundwater contribution is capable of explaining the observed changes in both the Cl and Sr isotopic systems. During the summer of 2001, Rio Grande flow was 24 m3s–1, whereas in the winter of 2002 flow was 20 m3s–1. Using these flow rates and our calculated percentage implies a saline groundwater flux of 1–2 m3s–1 over this stretch of the river. These values, although uncertain due to a likely range in saline groundwater composition, are broadly consistent with recent discharge estimates from a groundwater model of the Albuquerque Basin (Sanford et al., 2004). Given that the saline groundwater end member is based on relatively shallow samples that are probably a mixture of shallow groundwater and deep brine, which may be an order of magnitude more concentrated (Table DR1), the actual flux of deep brine could be as low as 0.1 m3s–1. The above observations are significant for two reasons. First, they indicate that brines of geological origin may play a much larger role in the salinization of arid-zone rivers than previously suspected. Second, within the Rio Grande they limit the possible role of agricultural return flows as the major factor affecting river salinity. Recognition of saline groundwaters associated with alluvial sedimentary basin brines as a significant factor in river salinization has important management implications. The reduction of agricultural return flows by improving water use efficiency, lining of canals, and even reusing waters are commonly proposed to mitigate salinity increases (Chhabra, 1996). The sedimentary brine source identified for the Rio Grande in this study suggests that alternative approaches to salinity management, such as interception wells to capture sedimentary brines (e.g., McElwee, 1985; Gillespie and Hargadine, 1986), may offer more practical and effective long-term solutions for rivers with similar salinity sources. ACKNOWLEDGMENTS This material is based upon work supported by SAHRA (Sustainability of Semi-Arid Hydrology and Riparian Areas) under the Science and Technology Centers Program of the National Science Foundation, Agreement EAR-9876800. We thank Don Whittemore, Niel Plummer, and Jordan Clark for thoughtful reviews. REFERENCES CITED Allison, G.B., Cook, P.G., Barnett, S.R., Walk, G.R., Jolly, I.D., and Hughes, M.W., 1990, Land clearance and river salinisation in the western Murray Basin, Australia: Journal of Hydrology, v. 119, p. 1–20, doi: 10.1016/0022– 1694(90)90030–2. Cheng, H., Edwards, R.L., Hoff, J., Gallup, C., Richards, D.A., and Asmerom, Y., 2000, The half-lives of uranium-234 and thorium-230: Chemical Geology, v. 169, p. 17–33, doi: 10.1016/S0009–2541(99)00157–6. Chhabra, R., 1996, Soil salinity and water quality: Rotterdam, A.A. Balkema, 300 p. Davis, S.N., Whittemore, D.O., and Fabryka, M.J., 1998, Uses of chloride/ bromide ratios in studies of potable water: Ground Water, v. 36, p. 338–350, doi: 10.1111/j.1745–6584.1998.tb01099.x. Dickson, B.L., and Davidson, M.R., 1985, Interpretation of 234U/ 238U activity ratios in groundwaters: Chemical Geology, v. 58, p. 83–88, doi: 10.1016/ 0009–2541(85)90180–9. Ellis, S.R., Levings, G.W., Carter, L.F., Richey, S.F., and Radell, M.J., 1993, Rio Grande Valley, Colorado, New Mexico, and Texas: Water Resources Bulletin, v. 29, p. 617–646. 1066 Ghassemi, F., Jakeman, A.J., and Nix, H.A., 1995, Salinisation of land and water resources: Human causes, extent, management and case studies: Wallingford, UK, CAB International, 544 p. Gillespie, J.B., and Hargadine, G.D., 1986, Geohydrology of the Wellingtonalluvial aquifer system and evaluation of possible locations of relief wells to decrease saline ground-water discharge to the Smoky Hill and Solomon Rovers, central Kansas: U.S. Geological Survey Water-Resources Investigations Report 86–4110, 31 p. Grauch, V.J.S., Sawyer, D.A., Keller, G.R., and Gillespie, C.L., 2001, Contributions of gravity and aeromagnetic studies to improving the understanding of subsurface hydrogeology, Middle Rio Grande Basin, New Mexico, in Cole, J.C., ed., U.S. Geological Survey Middle Rio Grande Basin Study— Proceedings of the fourth annual workshop: U.S. Geological Survey OpenFile Report 00–488, p. 3–4. Haney, P.D., and Bendixen, T.W., 1953, Effect of irrigation runoff on surface water supplies: American Water Works Association Journal, v. 45, p. 1160– 1171. Herczeg, A.L., Simpson, H.J., and Mazor, E., 1993, Transport of soluble salts in a large semiarid basin: River Murray, Australia: Journal of Hydrology, v. 144, p. 59–84, doi: 10.1016/0022–1694(93)90165–6. Hibbs, B.J., and Boghici, R., 1999, On the Rio Grande Aquifer; flow relationships, salinization, and environmental problems from El Paso to Ford Quitman, Texas: Environmental and Engineering Geoscience, v. 5, p. 51–59. Johnson, N., Revenga, C., and Echeverria, J., 2001, Managing water for people and nature: Science, v. 292, p. 1071–1072, doi: 10.1126/science.1058821. Lippincott, J.B., 1939, Southwestern border water problems: American Water Works Association Journal, v. 31, p. 1–29. McElwee, C.D., 1985, A model study of salt-water intrusion to a river using the sharp interface approximation: Ground Water, v. 23, p. 465–475. Moore, S.J., and Anderholm, S.K., 2002, Spatial and temporal variations in streamflow, dissolved solids, nutrients, and suspended sediment in the Rio Grande Valley study unit, Colorado, New Mexico, and Texas, 1993–1995: U.S. Geological Survey Water-Resources Investigations Report 02–4224, 58 p. Phillips, F.M., 2000, Chlorine-36, in Cook, P., and Herczeg, A.L., eds., Environmental tracers in subsurface hydrology: Boston, Kluwer Academic Publishers, p. 299–348. Phillips, F.M., Hogan, J., Mills, S., and Hendricks, J.M.H., 2003, Environmental tracers applied to quantifying causes of salinity in arid-region rivers: Preliminary results from the Rio Grande, southwestern USA, in Alsharhan, A.S., and Wood, W.W., eds., Water resources perspectives: Evaluation, management, and policy: Developments in Water Science, v. 50: Amsterdam, Elsevier Science, p. 327–334. Postel, S., 1999, Pillar of sand: Can the irrigation miracle last?: New York, Norton, 328 p. Reiter, M., 1999, Hydrogeothermal studies in New Mexico and implications for ground-water resources: Environmental and Engineering Geoscience, v. 5, p. 103–116. Sanford, W.E., Plummer, L.N., McAda, D.P., Bexfield, L.M., and Anderholm, S.K., 2004, Use of environmental tracers to estimate parameters for a predevelopment ground-water-flow model of the Middle Rio Grande Basin, New Mexico: U.S. Geological Survey Water-Resources Investigations Report 03–4286, 102 p. Whittemore, D.O., 1995, Geochemical differentiation of oil and gas brine from other saltwater sources contaminating water resources; case studies from Kansas and Oklahoma: Environmental Geosciences, v. 2, p. 15–31. Wilcox, L.V., 1957, Analysis of salt balance and salt-burden data on the Rio Grande, in Duisberg, P.C., ed., Problems of the Upper Rio Grande: An arid zone river: Socorro, New Mexico, U.S. Commission for Arid Resource Improvement and Development, Publication no. 1, p. 39–44. Wilkins, D.W., 1998, Summary of the southwest alluvial basins regional aquifersystem analysis in parts of Colorado, New Mexico, and Texas: U.S. Geological Survey Professional Paper 1407A, 49 p. Manuscript received 9 April 2007 Revised manuscript received 10 July 2007 Manuscript accepted 15 July 2007 Printed in USA GEOLOGY, December 2007 Hogan et al. pg. 1 Data Repository Item Hogan J. F., Phillips F.M., Mills S.K., Hendrickx J., Ruiz J., Chesley J.T., Asmerom Y. Geologic origins of salinization in a semiarid river: the role of sedimentary brines, Geology. Article ID: G23976 Figure DR1. Chlorine isotope system: (A) Cl/Br ratios and (B) 36Cl versus distance down river. Open circles (○) are samples collected in August 2001, whereas solid squares (■) were collected in January 2002. Numbers correspond to locations where the Rio Grande exits the southern end of the alluvial basins shown in Figure 1. Locations marked with letters refer to additional salinity increases not associated with alluvial basin termini. (A) is the Albuquerque Wastewater Treatment Plant and (B) is a location within the Tularosa- Hueco Basin, see text for discussion. In the winter, marked variability is observed in the Cl/Br ratio of the lower Rio Grande as a result of very low flows due to highly restricted reservoir releases. The first increase occurs south of Elephant Butte Reservoir; ratios then decrease at Caballo Reservoir. South of Caballo, Cl/Br ratios increase until a second sharp decrease is observed at the point where El Paso wastewater enters the river. Hogan et al. pg. 2 Figure DR2. Strontium isotope system: (A) Ca/Sr ratios and (B) 87Sr/86Sr ratios versus distance down river. Open circles (○) are samples collected in August 2001 whereas solid squares (■) were collected in January 2002. Numbers correspond to locations where the Rio Grande exits the southern end of the alluvial basins shown in Figure 1. Locations marked with letters refer to additional salinity increases not associated with alluvial basin termini. (A) is the Albuquerque Wastewater Treatment Plant and (B) is a location within the Tularosa- Hueco Basin, see text for discussion. Hogan et al. pg. 3 Figure DR3. Uranium isotope system: (A) U and (B) δ234U values versus distance down river. Only samples collected during August 2001 (○) were analyzed. Numbers correspond to locations where the Rio Grande exits the southern end of the alluvial basins shown in Figure 1. Locations marked with letters refer to additional salinity increases not associated with alluvial basin termini: (A) is the Albuquerque Wastewater Treatment Plant and (B) is a location within the Tularosa-Hueco Basin, see text for discussion. In general the headwaters have high δ234U values and low U concentrations indicative of recent water-rock interaction. In contrast, waters downstream have low δ234U and high U concentration values. Note that there are two marked decreases in concentration that reflect the non-conservative behavior of this solute, likely as a result of removal during application onto irrigated lands (first decrease) or by processes within Elephant Butte Reservoir (second decrease). Hogan et al. pg. 4 Figure DR4. Isotopic mixing diagram of δ234U values versus U concentration. Mixing lines were calculated using the end-members in Table DR1. Percentages are not included on the U-isotope mixing plot because of its non-conservative behavior. The 234U/238U ratio of waters, expressed as δ234U per mil deviation from the secular equilibrium ratio, reflects both the extent of water-rock interaction and the residence time. 234U is preferentially leached from rocks because its crystal site is damaged when 238 U decays to 234U, thus, groundwater with short residence times and/or strong waterrock interaction typically has high δ234U values (Dickson and Davidson, 1985). If the residence time is significant with respect to the half-life of 234U, the δ234U value is lowered by radioactive decay of 234U (Dickson and Davidson, 1985). Hogan et al. pg. 5 TABLE DR1. ISOTOPIC AND SOLUTE RATIOS FOR POTENTIAL SOLUTE SOURCES AND SELECTED END-MEMBER COMPOSITIONS. End-member Cl (mg/l) Cl/Br (wt/wt) Atmospheric deposition 0.2 to 2* 40 to 200* 36 Cl/Cl*1015 Ca (mg/l) 700† Basalt weathering Rio Grande headwaters** Sr/86Sr 430 to 603§ 0.709 to 0.713§ 100 to 240# 0.704 to 0.705# 75 0.7096 2500 Magmatic geothermal sources 250 to 350†† 1 to 6†† Sedimentary brines 1000 to 10,000 * 5 to 35§§ 10 to 50## 0.708 to 0.716*** Rio Grande saline groundwaters 400 to 1300††† 35§§§ 20 to 44††† 0.7093 to 0.7118### 1300 35 33 0.71015 1000 15 87 100 Saline groundwater end-member**** 10 Ca/Sr (wt/wt) 350 U (μg/l) δ234U 2 800 500 §§§ 5 500 Note: Concentrations are reported only for end-members used in the mixing calculations. Basalt weathering is not considered to be a major source of chloride, thus associated values are not included. Likewise we report no values for Sr in geothermal waters as values reflect the local rock type (Goff and Gardener, 1994) not a magmatic source. For the emerging U-isotope system, limited data on potential solute sources explains the lack of reported values. *Precipitation values from Davis et al. (1998) for Nevada and Arizona; †Values from Phillips (2000); §Values from Graustein and Armstrong (1983); #Values from Dugan et al. (1986); **End-member values based on measured samples from the headwater of the Rio Grande; ††Values from Fehn et al. (1992); §§Value based on secular equilibrium (Phillips, 2000); ## Values from Banner (1995) and Musgrove and Banner (1993); *** Values from Musgrove and Banner (1993) and McNutt (2000); note individual basins have significantly narrower ranges; †††Data compiled from Brandvold (2001) and Plummer et al. (2004) using only samples over 500 ppm Cl; §§§This study, n=1; ### Saline groundwater samples collected by Plummer et al. (2004), measured for this study, n=5; ****For saline groundwater end-member we used the upper end of concentration values compiled from Brandvold (2001) and Plummer et al. (2004) which represent shallow saline groundwaters; saline waters at depth are likely more concentrated. The isotope and solute ratios were selected to be within the observed values for saline groundwaters in the Rio Grande as well as to best fit the observed mixing trend. Hogan et al. pg. 6 Additional References Brandvold, L., 2001, Arsenic in ground water in the Socorro Basin, New Mexico: New Mexico Geology, v. 23, p. 2-8. Banner, J.L., 1995, Application of the trace element and isotope geochemistry of strontium to studies of carbonate diagenesis: Sedimentology, v. 42, p. 805-824. Dungan, M.A., Lindstrom, M.M., McMillan, N.J., Moorbath, S., Hoefs, J., and Haskin, L.A., 1986, Open system magmatic evolution of the Taos Plateau volcanic field northern New Mexico: Journal of Geophysical Research v. 91, p. 5999-6028. Fehn, U., Peters, E.K., Tullai-Fitzpatrick, S., Kubik, P.W., Sharma, P., Tang, R.T.D., Grove, H.E., and Elmore, D., 1992, 129I and 36Cl concentrations in waters of the eastern Clear Lake area, California: Residence time and source ages of hydrothermal fluids: Geochimica et Cosmochimica Acta, v. 56, p. 2069-2079. Graustein, W.C., and Armstrong, R.L., 1983, The use of strontium-87/strontium-86 ratios to measure atmospheric transport into forested watersheds: Science, v. 219, p. 289-292. McNutt, R.H., 2000, Strontium Isotopes, in Cook, P., and Herczeg, A.L., eds., Environmental Tracers in Subsurface Hydrology, Kulwer Academic Publishers, p. 233-260. Musgrove, M., and Banner, J.L., 1993, Regional ground-water mixing and the origin of saline fluids: mid-continent, United States: Science, v. 259, p. 1877-1882. Plummer, L.N., Bexfield, L.M., Anderholm, S.K., Sanford, W.E., and Busenberg, E., 2004, Geochemical characterization of ground-water flow in the Santa Fe Group aquifer system, Middle Rio Grande Basin, New Mexico, U.S. Geological Survey, p. 395.
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