Mixing of thermal and nonthermal waters in the margin of the Rio

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Mixing of thermal and nonthermal waters in the margin of the Rio Grande
Rift, Jemez Mountains, New Mexico
Frank W. Trainer, 1975, pp. 213-218
in:
Las Cruces Country, Seager, W. R.; Clemons, R. E.; Callender, J. F.; [eds.], New Mexico Geological Society 26th
Annual Fall Field Conference Guidebook, 376 p.
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New Mexico Geol. Soc. Guidebook, 26th Field Conf., Las Cruces Country, 1975
MIXING OF THERMAL AND NONTHERMAL WATERS
IN THE MARGIN OF THE RIO GRANDE RIFT,
JEMEZ MOUNTAINS, NEW MEXICO*
by
FRANK W. TRAINER
U.S. Geological Survey
Albuquerque, New Mexico
INTRODUCTION
Chemical analyses of ground water from the marginal fault
zone of the Rio Grande rift, in the southwestern Jemez Mountains, show that thermal water from depth mixes with local
ground water. Mixing of the waters provides evidence of
ground-water flow paths in this part of the rift, and general
conclusions reached in study of the mixing may be useful in
investigations of thermal waters elsewhere.
The geology of the southwestern Jemez Mountains has been
described by Wood and Northrop (1946), Ross, Smith, and
Bailey (1961), and Smith, Bailey, and Ross (1970). Trainer
(1974) summarized the geohydrology and presented representative chemical analyses of ground water. The Jemez Mountains (Fig. 1) comprise a complex pile of late Tertiary and
Quaternary volcanic rocks that lies athwart the fault zone at
the west margin of the Rio Grande rift. San Diego Canyon has
been excavated, in part, in the marginal fault zone. The Jemez
River, flowing through the canyon, drains Valles Caldera.
Rocks exposed in San Diego Canyon include Precambrian
granitic rock, Paleozoic and Mesozoic sedimentary rocks, and
Tertiary and Quaternary volcanic rocks and alluvial fill. Water
discharged by thermal and cold mineral springs in San Diego
Canyon is believed to be derived in part from a hydrothermal
reservoir in Valles Caldera and to have flowed out along the
fault zone, largely in limestone that overlies the granitic rock.
Other hydrothermal features include areas of solfataras and
hydrothermal alteration, believed to result from the activity of
deep thermal water; and several thermal springs, on the flanks
of young volcanoes in the ring-fracture zone of the caldera,
that are believed to discharge water which has circulated to
only relatively shallow depths.
ACKNOWLEDGMENTS
The writer gratefully acknowledges the provision of water
samples by Los Alamos Scientific Laboratory from its deep
test well GT-2 in the Jemez Mountains; discussion of geohydrologic problems with W. D. Purtymun and F. G. West of
Los Alamos Scientific Laboratory; and discussion with and
assistance from Ivan Barnes, J. K. Kunkler, R. H. Mariner, C.
S. Smith, and D. E. White of the U.S. Geological Survey.
CHEMICAL CHARACTER OF THE
GROUND WATER
Analyses summarized by White, Hem, and Waring (1963)
characterize compositional groups typical of dilute ground
water in several types of rock. Figure 2B illustrates two of
these groups: (1) Points near the left vertex of the diagram are
*Publication approved by Director, U.S. Geological Survey.
213
214
TRAINER
representative of water in carbonate rock. l he water is of a
calcium magnesium bicarbonate composition; points distributed along the upper left side of the diagram represent
carbonate-rock water that also contains moderate proportions
of sulfate. (2) Points distributed largely near the left vertex
and along the lower left side of the diagram are representative
of water in igneous rocks. The water is of a calcium sodium
bicarbonate composition; calcium is dominant in some samples
and sodium in others, a fact that is not surprising when we
consider the range in chemical composition of the feldspars. A
third group of points, near the right vertex and lower right side
in Figure 28, represents waters from a variety of rocks but a
single environment; these are thermal waters in geyser areas of
volcanic regions. White (1957) has discussed the composition
of thermal waters of volcanic origin. White (1957, p. 1646)
explained the sodium chloride composition of much of the
water by the solution of alkali halides in high-density steam at
depth and their accumulation in the hydrothermal fluid because of their low volatility in low-density steam nearer the
land surface. With long-continued accumulation the fluid may
become a brine containing minor constituents typical of some
igneous minerals (for example, arsenic, boron, fluoride, and
lithium). (See also Mahon, 1970, p. 1312-1313.)
Figure 2C illustrates the composition of these three types of
ground water in samples from the southwestern Jemez Mountains and nearby areas. (1) Points near the left vertex of the
diagram represent water in carbonate rocks east of the east marginal fault zone of the Rio Grande rift near Albuquerque (Fig.
1). These carbonate rocks, which are in the Magdalena Group
of Pennsylvanian and Permian age, are similar to the Magdalena rocks in the Jemez Mountains. Water analyses from
these other areas are used in Figure 2C because samples collected from carbonate rocks in the Jemez Mountains appear to
be mixed waters.
(2) Points near the lower left side of the diagram represent
cold dilute water in near-surface volcanic rocks in the Jemez
Mountains and warm dilute water in volcanic rocks on or near
the young volcanoes in Valles Caldera. The composition of
these dilute waters corresponds to that of the dilute waters in
igneous rocks shown in Figure 28. The warm dilute waters are
believed to be examples of a type described by White, Hem,
and Waring (1963, p. F55) as thermal waters that are probably
entirely meteoric. In the Jemez Mountains this dilute thermal
water is more dilute than ground water normally found in the
Permian sandstone and shale that underlie the volcanic rocks.
Hence the dilute thermal water must have migrated to only
relatively shallow depths and gained its heat from young, nearsurface volcanic rocks, perhaps near the feeder dikes or pipes
where heat flow must be higher than it is in the somewhat
Mountains (crosses). (Data from southeast of Albuquerque
from R. E. Smith, 1957; numbered samples listed in Table 7;
other data from files of U.S. Geological Survey.)
D. Composition of mixed waters in southwestern Jemez
Mountains, in carbonate rock (open circles) and in alluvium
(solid circles). Shaded patterns show approximate distribution
fields in Figure 2C; the mineral water, right field, is itself a
mixed water. (Numbered samples listed in table 1; other data
from files of U.S. Geological Survey.)
E. Composition of selected thermal waters from the Rio
Grande rift outside the Jemez Mountains. (Sample numbers as
in Table 1.)
MIXING OF THERMAL AND NONTHERMAL WATERS
older ash-flow tuffs that rest on old sedimentary rocks outside
the caldera. These thermal waters have not evolved chemically
in the way thermal waters in geyser regions (Fig. 2B) appear to
have done. Such a chemical distinction between thermal
waters of different provenance and history should be useful in
geothermal exploration.
(3) Points near the right vertex of the diagram represent
mineral waters, largely thermal, from San Diego Canyon. The
points are farther from the vertex and lower right side of the
diagram than the examples in Figure 2B because the Jemez
Water has flowed through carbonate rock and contains somewhat greater proportions of calcium and bicarbonate, and
correspondingly lesser proportions of sodium and chloride,
than would be expected in water from deep-lying and hot
igneous rocks.
The diagrams in Figure 2 show the composition but not the
concentration of dissolved solids in the waters. Average concentrations of dissolved solids for the samples shown in Figure
2C are approximately as follows: water in carbonate rocks,
350 mg/I (milligrams per litre); dilute water in volcanic rocks,
200 mg/I; and mineral water, 6,500 mg/I.
MIXED WATERS
Water in alluvium along the Jemez River is derived principally from dilute ground water in volcanic rocks, but many
points representing samples of water from the alluvium (Fig.
2D) fall outside the field typical of dilute water in the volcanic
rocks (Fig. 2C). The reason for this atypical composition is
clear: this water occurs down-canyon from mineral springs in
the canyon floor, and the water is a mixture of local ground
water and of mineral water from the springs and probably
from faults beneath the alluvium. The points representing this
mixed water in Figure 2D lie between the field for dilute water
in volcanic rocks and the field for mineral water in San Diego
Canyon.
Ground water in carbonate rock in San Diego Canyon has a
high concentration of calcium and bicarbonate, but it also
contains unexpected proportions of sodium and chloride.
Points representing these samples (Fig. 2D) fall outside the
field typical of water in carbonate rocks and lie between that
field and the field for mineral water. This intermediate position suggests that these waters are also mixed.
Ground-water samples from San Diego Canyon include
many that appear, on the basis of major-ion composition, to
be mixtures of dilute and mineral waters. Mixing of the
thermal and dilute non-thermal ground waters is evidently
widespread in this part of the marginal fault zone of the rift.
Further examination of mixing of the waters is desirable.
Most of the springs in San Diego Canyon that yield mixed
waters have low rates of discharge, and use of a mixing model
described by Fournier and Truesdell (1974) is not promising.
However, enough data are available to permit investigation of
the mixing by calculating the volumes of original mineral
water and dilute water required to provide the resultant waters
collected at springs and wells in San Diego Canyon. Four constituents—lithium, chloride, bromide, and boron—were used
for these calculations, and "standard" concentrations in the
original mineral water were assumed on the basis of analyses of
water samples from a deep well drilled in the Jemez Mountains
by the Los Alamos Scientific Laboratory (LASL) of the University of California. LASL test hole GT-2, a short distance
outside Valles Caldera and west of the head of San Diego
215
Canyon (Fig. 1, Sample Site 2), penetrated water-bearing
zones in carbonate rock of the Magdalena Group and in the
upper part of the underlying Precambrian granitic rock. Water
samples collected during the drilling appear to have been
affected by reaction of native water with drilling fluid or
grout, but three samples from the granitic rock (Table 1) contain fairly consistent concentrations of the constituents used
in this study, and these concentrations are believed to be
representative of the native water. The following concentrations of these constituents, in milligrams per litre, were
assumed for the calculations: lithium, 20; chloride, 3,000;
bromide, 15; and boron, 25. The 3,000-mg/l chloride value
was selected because one thermal water outside the caldera
(water from "Warm Spring," near San Ysidro; Trainer, 1974,
table 1, sample 11), which may have come from Valles Caldera, contains about 3,000 mg/l chloride. The other concentrations were selected arbitrarily from study of data for
samples 2, 3 and 4 in Table 1.
Calculations were made by using a simple equation:
(volmin)(concmin) + (voldil)(concdil) = (volmin + voldil) (concmix), in which
vol is volume, conc is concentration of a selected constituent,
and subscripts are min for mineral water, di/ for dilute water,
and mix for mixed water. The dilute water probably enters the
ground by infiltration through the bed of Jemez River and San
Antonio Creek (Fig. 1). A check analysis of water in Jemez
River in San Diego Canyon above the major mineral springs,
during base runoff, showed 6 mg/I chloride and 0.1 mg/I
each, or less, lithium, bromide, and boron. These concentrations
are so small, relative to those in the mineral water, that they
can be neglected in an approximate calculation. If, in
addition, we assume a unit volume of mineral water, the
foregoing equation can be rewritten
co n c min co n c m i x + (v ol d i l )( con c m i x ) , or
voldil ~ concmin — concmix
concmix
The apparent dilution factors listed in Table 2 were calculated using this relationship and data in Table 1. The computed values are reasonably consistent for each sample site
tested. They suggest that sample 8 results from the mixing of
one part of original mineral water and about one part of dilute
water, and that samples 9 and 11 require about two parts of
dilute water. The computed values for sample 7 suggest mixing
with about 10 parts of dilute water, and values for sample 10
suggest mixing with a much larger proportion of dilute water,
perhaps as much as 60 parts.
The four values computed for each sample are not averaged
in the foregoing discussion, and the apparent dilution factor
for each sample is treated as an estimate, because of possible
differences in the effectiveness of the four constituents as
indicators of dilution. For example, errors in analysis for
chloride should be much smaller than those in analysis for
bromide. Lithium may substitute for sodium in some reactions. The four values determined may therefore not be of
comparable precision.
This wide range in mixing proportions is believed to represent considerable differences in the nature of the aquifer:
samples such as 8, 9, and 11 are evidently from principal fractures in the main fault zone, whereas the more dilute mixtures
MIXING OF THERMAL AND NONTHERMAL WATERS
that the silica was derived entirely from quartz and that it had
not been increased by the escape of steam, a concentration of
170 mg/l indicates that at equilibrium with quartz the water
°
was at a minimum temperature of about 169 C (Fournier and
Rowe, 1966, Fig. 5, p. 694). On the basis of temperaturegradient and other data from hole GT-2 (Pettit, 1975, p. 18
and Appendix A) the rock temperature at 3,557 ft is com°
puted to have been about 90 C. The well data show that the
water at and above 3,557 ft must be moving principally laterally; under this condition the only likely source of the heat
indicated by the higher water temperature is the caldera east
of the well (Fig. 1). Surface-geophysical studies (Jiracek,
1975) have revealed a belt of low resistivity in what is probably part of the ring-fracture zone, inside the caldera near well
GT-2. This segment of the ring-fracture zone, apparently not
occupied by younger igneous rock, is probably a conduit for
the flow of hot mineralized water.
Two conclusions are derived from the foregoing discussions
of the source and flow pattern of the thermal water. (1) Along
some flow paths a dissolved load of volcanic-derived constituents, including several (for example, arsenic, fluoride and
perhaps boron) that could affect downstream uses of the
water, is carried far from the center of the volcanic region
before it is dispersed in other ground water or released to
streams. The quantity of this dissolved material and its disposition beyond the Jemez Mountains region, at the surface and in
the subsurface, require further study. (2) Dilution of thermal
water from Valles Caldera by equal or greater amounts of
shallow ground water results in marked decrease in the temperature of the thermal water during its migration, in addition to
that due to heat loss by conduction through wall rock, and
this consideration suggests that only under very favorable
conditions is water hot enough for geothermal-power development through processes now in general use likely to occur
outside the caldera.
THERMAL WATERS ELSEWHERE
IN THE RIO GRANDE RIFT
A brief study was made of the chemical composition of
selected thermal waters from the Rio Grande rift in New
Mexico outside the Jemez Mountains. The major-ion composition of these waters (Fig. 2E) is of interest in light of the
foregoing discussion because it suggests that most of them are
not in equilibrium with the near-surface rocks. Many investigators have shown that geothermal waters for which adequate
isotopic data are available are almost entirely of meteoric
origin. If this is true of the thermal waters in the Rio Grande
rift, these waters have undergone substantial change in chemical composition since entering the ground. In an area where
chemical data are available for the shallow ground water upgradient from thermal springs, in likely recharge areas for the
thermal aquifer, it may be possible to show to what extent the
composition of the thermal water differs from that of the
shallow ground water, and hence to what degree the thermal
water may have evolved under high temperature and pressure.
Figure 2E justifies the initial working hypothesis that most of
these thermal waters have so evolved, through direct heating or
by the addition of steam carrying igneous-derived material.
Several factors may contribute to the dispersion of points in
Figure 2E. It would be surprising if some of the waters, all but
one of which are discharged from springs at relatively low
217
altitudes in their respective regions, were not mixtures of
thermal water and shallow ground water. Samples 14 and 15
appear, from their proportions of the principal ions, to be
most likely to have resulted from such mixing. A second explanation could lie in differences in the degree to which different waters have evolved toward an ideal sodium chloride
composition. Third, some waters may simply be ordinary
ground water that has been warmed through contact with hot
rock in the subsurface. This may be a reasonable explanation
for sample 15, which appears to have a dissolved-solids content
of a few hundred milligrams per litre (figure not given in available analyses) and relatively low antecedent temperature (estimates are about 77°C by the SiO2 geothermometer and 55°C
by the Ca-Na-K geothermometer). It seems much less likely for
sample 14, for which the corresponding values are about 1,250
°
°
mg/l, 113 C, and 156 C, respectively. Another possible explanation for sample 14 is the injection of high-density steam
carrying minor constituents typ ical of volcanic water into
dilute ground water. This explanation is consistent with the
sodium bicarbonate composition of the water, which is typical
of dilute water in igneous rock, and with its intermediate
chloride concentration (270 mg/l). Further work could include
study of the flow patterns and chemical character of shallow
ground water near these thermal springs as a possible aid in
resolution of these uncertainties.
Data on selected minor constituents in these thermal waters
were compared with corresponding data from waters in the
Jemez Mountains. Figure 3 summarizes this comparison using
Li/Na, B/Cl, and Br/Cl, three of the ratios of constituents
discussed by White, Hem, and Waring (1963). Omitting consideration of the dilute waters, for which White, Hem, and
Waring (1963, Table 29, p. F59) found new data, we see that
the magnitudes of the ratios shown in Figure 3 are consistent
with median ratios for thermal waters. Of the ratios in Figure
3, B/Cl and Br/Cl appear likely to be most useful in distinguishing mineral and dilute waters in the Jemez Mountains.
218
TRAINER
Among the thermal waters elsewhere in the rift the range in
value for each ratio overlaps the ranges for both dilute and
mineral waters in the Jemez Mountains. This feature of the
data can be reasonably explained as reflecting a wide range in
the nature of mixing among thermal waters in the Rio Grande
rift, but it may also be due to a greater diversity of host rocks
in the rift generally than in the Jemez Mountains. Integration
of minor-constituent studies of water with the hydrologic and
hydrochemical investigations mentioned in the preceding paragraph, and with study of the geologic environment in each
thermal-spring area, will be needed to evaluate the likely significance of mixing and dilution of the waters. As in the Jemez
Mountains, the degree to which mixing of thermal and cold
waters has occurred will have an important effect on the value
of the thermal-water resource in other parts of the Rio Grande
rift.
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