Geologic origins of salinization in a semi-arid river

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
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.
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.
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.
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]
v. 35; no. 12; p. 1063–1066; doi: 10.1130/G23976A.1; 4 figures; Data Repository item 2007265.
B 9
Cl (mg L-1)
Cl (mg L-1)
Alluvial Basins
Irrigated Agriculture
600 6
Distance (km)
4 5
August 2001
January 2002
Alluvial Basins
1. San Luis
2. Española
3. Albuquerque
3A. Santo Domingo
3B. Calabacillas
3C. Belen
Elephant Butte
4. Socorro
5. San Marcial
6. Engle
7. Palomas
8. Mesilla
9. Tularosa - Hueco
El Paso /
Distance (km)
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
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-
To distinguish between saline groundwater discharge and effects
associated with irrigated agriculture, we have employed 87Sr/86Sr and
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, or on
request from [email protected] or Documents Secretary, GSA, P.O. Box
9140, Boulder, CO 80301, USA.
GEOLOGY, December 2007
Mixing Line
August 2001
January 2002
1149 km
Cl/Br (wt/wt)
Cl/Br (wt/wt)
1014 km
801 km
415 km
Cl (mg L-1)
243 km
Evaporation / Transpiration
Cl/Cl · 10
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
Sr/ Sr
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.
August 2001
January 2002
Ca/Sr (wt/wt)
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.
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
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.
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.
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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
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
0.2 to
40 to
Basalt weathering
Rio Grande
430 to
0.709 to
100 to
0.704 to
geothermal sources
250 to
1 to 6††
Sedimentary brines
1000 to
10,000 *
5 to 35§§
10 to
0.708 to
Rio Grande saline
400 to
20 to
0.7093 to
Saline groundwater
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
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Banner, J.L., 1995, Application of the trace element and isotope geochemistry of
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