Environmental Tracers Applied to
Quantifying Causes of Salinity in AridRegion Rivers: Preliminary Results
from the Rio Grande, Southwestern
USA
Fred M. Phillips, James Hogan,
Suzanne Mills, and Jan M.H.
Hendrickx
New Mexico Tech & SAHRA, USA
Center for Sustainability of semi-Arid
Hydrology and Riparian Areas
(SAHRA)
This research was funded by SAHRA
under the Science & Technology Center
Program of the U.S. National Science
Foundation
TDS of the Rio Grande
Winter 01
Summer 00
Winter 00
1500
Albuquerque
TDS (mg/L)
2000
1000
500
El Paso
2500
Elephant Butte
3000
0
0
200
400
600
Distance (km)
800
1000
1200
Origins of Salts
„Cyclic
salts
„ Atmospheric
deposition
„ Weathering of minerals
„Subsurface
„ Solution of
salts
evaporites
„ Diagenesis
„ Connate
waters
„ Geothermal waters
Origins of Salts
„ Anthropogenic
„ Dietary
salt
„ Industrial salts
„ Road salt
„ Livestock salts
„ Fertilizers
Evapotranspirative Concentration
„ Salts
are subsequently concentrated
by
„ Evaporation from open water
„ Transpiration
by riparian vegetation
„ Evapotranspiration from irrigated fields
„ Consumptive municipal and industrial use
How to Quantify Sources and Causes of
Salinization ?
„ Traditional
approach: Measure
discharge and salt concentrations at gaging
stations and compute salt burden
„ Alternative Approach:
Measure
environmental tracers at high spatial
resolution and employ dynamic simulation
to interpret results
Potential Tracers
„ Cl
„ Cl/Br
„ 36 Cl
„ δ37Cl
„ δ18O
and δ2H
„ 87 Sr/86 Sr
„ 234U/238U
Cl/Br and 36Cl/Cl to fingerprint salt
sources
1500
Meteoric
1000
Sedimentary Brines
Geothermal
500
0
0
500
1000
36
Cl/10
15
Cl
1500
Halide tracers for mixing versus evaporation
1500
Mixing
1000
Sedimentary
Brine
500
Geothermal
Meteoric
Evaporation
0
0
2000
4000
Cl (mg L
6000
-1
)
8000
Study Area
= sedimentary
basin
Near Creede , CO
Escondida, NM
TDS and Cl (1994- 1995 average)
1000
400
350
800
300
TDS
250
600
200
-
Cl
400
150
100
200
50
0
0
0
200
400
600
Distance from Headwaters (km)
800
1000
Hypotheses to Explain Salinization
„ Evaporation of river
water
„ Flushing by irrigation or natural flooding of salts
accumulated from prehistoric evaporation of river
water
„ Displacement of natural saline groundwater by
irrigation
„ Natural sedimentary basin hydrodynamics
δ18O vs Flow Distance (Summer ‘01)
-6
-8
-10
-12
Rio Grande
Drains
Tributaries
-14
-16
0
400
800
Flow Distance (km)
1200
δ18O vs δ 2H (Summer ‘01)
-40
MWL
-60
-80
Rio Grande
Tributaries
Drains
Meteoric Water Line
-100
-120
-15
-12
-9
δ18O (per mil)
-6
Significance of Stable Isotopes
„ Strong
enrichment shows much
evaporation
„ Simple Rayleigh distillation model
indicates ~35% of inflow is
evaporated
„ River gaging indicates ~75% of inflow
lost to atmosphere
„ Shows losses are about half
evaporation and half transpiration
TDS in Summer and Winter 2001
2500
Winter '02
Summer '01
2000
1500
1000
500
0
0
300
600
Flow Distance (km)
900
1200
Halides in Rio Grande (Summer ‘01)
300
A.
200
Elephant Butte Res
100
El Paso
Lobatos
Albuquerque
0
1500
B.
1000
Elephant Butte Res
500
El Paso
Lobatos
Albuquerque
0
0
200
400
600
distance (km)
800
1000
1200
Cl/Br versus Cl - Summer ‘01
1600
1200
800
El Paso
Mesilla Basin
Palomas Basin
Socorro Basin
Albuquerque Basin
San Luis Basin
400
0
0
300
600
-
Cl (mg/L)
900
Influences on Rio Grande Cl/Br
1600
Mixing
1200
800
400
Evaporation
0
0
300
600
-
Cl (mg/L)
900
Implication: Neither simple
evaporation nor leaching of
prehistoric Rio Grande salts
is adequate to explain
salinization of Rio Grande
Influence of Drains
600
El Paso
400
Mesilla Basin
Palomas Basin
Socorro Basin
Albuquerque Basin
San Luis Basin
200
San Luis Drains
Albuquerque Drains
Socorro Drains
Palomas Drains
Mesilla Drains
0
0
50
100
150
-
Cl (mg/L)
200
250
Implication: Although drains
are flushing some old saline
water in the Palomas and
Mesilla Basins, in general,
drains are not the source of
the salinization
We can quantify evaporation
and salt inputs using simple
mass balance equations on
Cl and Br
Salt Balance Equations
mCl = e[mCl,subsurface • f + mCl,cyclic (1− f )]
mBr = e[mBr,subsurface • f + mBr,cyclic (1− f )]
e = ( fraction _ evaporated)−1
f = mixing _ fraction
Evaporation as a Function of Flow
Distance
1.4
1.0
El Paso
Albuquerque
1.2
0.8
0.6
0.4
0.2
0.0
0
400
800
Distance (km)
1200
Fraction Cl Added versus Distance
1.0
0.8
0.6
0.4
0.2
0.0
0
300
600
Flow Distance (km)
900
1200
Study Area
= basin
terminus
Fraction Cl Added versus Distance
0.3
EB
0.2
SLB
AB
0.1
(AB)
0.0
0
200
400
Flow Distance (km)
600
800
Locations of Chloride Addition
1.0
SB
0.8
0.6
0.4
PB
MB
0.2
0.0
0
300
600
Flow Distance (km)
900
1200
From Heath, R. C. (1984) Ground-Water Regions of the United States. U.
S. G. S. Water-Supply Paper 2242, 78 pp.
Diagenesis
Connate
Implications of Cl/Br Data
„ Most
salt added to the Rio Grande
originates from deep groundwater
sources
„ Agricultural drains flush only a small
amount of this salt
„ Most of the salt is apparently added by
sedimentary basin discharge at the
downstream ends of the basins
Summary of Findings
„ Salinity of
Rio Grande increases from
~15 mg/L to ~800 mg/L in 1200 km
flow
„ About 400 mg of salt are added to
each original liter of water
„ Remainder of concentration in crease
is due to evapotranspiration of water
Summary of Findings ( cont.)
„ Irrigation probably plays
only a minor
role in salinization
„ Most added salts originate from points
of sedimentary basin discharge
Practical Implications
„ Changes
in agricultural practices will
probably do little to alleviate
salinization
„ Areas of sedimentary basin discharge
appear to be localized
„ Interception may
be practicable
Future Work
„ Greater
emphasis on groundwater
sampling to identify salinity sources
„ Measurement of additional tracers
(36Cl, δ37Cl, 87Sr/86 Sr, 234U/238U) to
better fingerprint sources
„ Use of dynamic systems modeling
(PowerSim) to quantify salt inputs