Executive Summary
Agua Caliente Spring, in downtown Palm Springs,
California, has been used for recreation and medicinal therapy
for hundreds of years and currently (2008) is the source of
hot water for the Spa Resort owned by the Agua Caliente
Band of the Cahuilla Indians. The Agua Caliente Spring is
located about 1,500 feet east of the eastern front of the San
Jacinto Mountains on the southeast-sloping alluvial plain of
the Coachella Valley. The objectives of this study were to
(1) define the geologic structure associated with the Agua
Caliente Spring; (2) define the source(s), and possibly the
age(s), of water discharged by the spring; (3) ascertain the
seasonal and longer-term variability of the natural discharge,
water temperature, and chemical characteristics of the spring
water; (4) evaluate whether water-level declines in the
regional aquifer will influence the temperature of the spring
discharge; and, (5) estimate the quantity of spring water that
leaks out of the water-collector tank at the spring orifice.
A gravity survey was conducted to define the thickness
of the valley-fill deposits or depth to the basement complex
beneath the Agua Caliente Spring area and to delineate
geologic structures associated with the spring. The gravity
data indicated that the Agua Caliente Spring is located within
the inferred trace of the Palm Canyon fault, where the density
boundaries suggest that the fault steps laterally to the west.
The thickness of the valley-fill deposits is irregular along
the western margin of the Coachella Valley, with a shallow
buried ridge that strikes east-northeast as much as 10,000 feet
away from the mountain front that appears to be a subsurface
continuation of the steep ridge to the north of Tahquitz
Canyon. The Agua Caliente Spring is located on the southeast
flank of this buried basement ridge, where the valley-fill
deposits are estimated to be 830 feet thick.
Shallow-depth seismic refraction and reflection surveys
were conducted along three lines near the Agua Caliente
Spring to help delineate and image geologic structures
associated with the spring. Consistent with observations from
nearby wells, analysis of the seismic velocity images suggests
that a perched groundwater table occurs in the upper 30 feet of
sediments near the spring. The seismic reflection data indicate
that the basement complex is about 830 feet below land
surface directly beneath the Agua Caliente Spring and that
the basement complex rises from south to north, indicating
the presence of a buried basement ridge to the north of the
Agua Caliente Spring; this interpretation is consistent with the
gravity data. The migrated seismic reflection images indicate
the presence of a density contrast above the seismic interpreted
depth to basement complex, which is interpreted as the
contact between overlying unconsolidated valley-fill deposits
and underlying indurated valley-fill deposits. The seismic
interpreted contact between the unconsolidated valley‑fill
deposits and the indurated valley-fill deposits is about 500 feet
below land surface directly beneath Agua Caliente Spring and
rises to about 200 feet below land surface less than 500 feet
east and north of the spring. These seismic reflection images
also show disruptions in the layering and changes in the
character of reflectors in the strata beneath the Agua Caliente
Spring, which probably are related to the north‑south trending
Palm Canyon fault. Faulting of the basement complex (along
the buried ridge) and indurated valley-fill deposits could
provide a pathway for deep thermal water to rise from an
underlying geothermal reservoir, and is the probable source of
the Agua Caliente Spring.
Interferometric Synthetic Aperture Radar was used in
this study to help identify ground-surface deformation and
locate structures such as faults that may affect groundwater
movement. Analysis of 18 interferograms representing time
periods ranging from 35 to 595 days between October 2003
and September 2005 indicates that little deformation (less
than 0.6 inches) occurred in the study area for the time periods
represented by the interferograms. With so little deformation,
none of the interferograms had sufficient contrast to provide
information on the location of possible buried faults near the
Agua Caliente Spring.
Historical records indicate that the Agua Caliente Spring
discharge has varied from 5 to 60 gallons per minute over the
past century. For this study, discharge at Agua Caliente Spring
was measured by using two methods to obtain a reliable
continuous record of discharge during the 2-year study period.
Data collected for this study indicate that the discharge varied
from a high of about 24 gallons per minute in the summer of
2005, following 2 years that had above‑normal precipitation,
to a low of about 9 gallons per minute in the summer of 2006,
a year with below-normal precipitation. These observations
suggest that the discharge of Aqua Caliente Spring is
influenced by recent precipitation, although discharge data
need to be collected over a period spanning multiple wet
and dry cycles to establish the relation with a high degree of
confidence.
Available records indicate that the temperature of the
Agua Caliente Spring has been relatively constant over the
past century, ranging from a low of 37.8 degrees Celsius in
1917 to a high of 42.2 degrees Celsius in 1953. Measured
water temperatures at Agua Caliente Spring during this study
were nearly constant, ranging from 40.7 to 41.8 degrees
Celsius between April 2005 and September 2006. The
temperature of the spring does not appear to be influenced by
recent precipitation.
Seasonal water-quality data collected during this study
and available historical data were used to define the source(s)
and age(s) of water discharged by the Agua Caliente Spring,
and to ascertain the seasonal and longer-term variability
of chemical characteristics of the spring discharge. A large
contrast in sodium fraction and pH values indicates little or no
contribution from groundwater in the regional aquifer to the
thermal Agua Caliente Spring. Chemical composition changed
minimally in the Agua Caliente Spring during 2005–06, either
seasonally or annually, indicating an absence of response
to changing discharge or precipitation. Comparison with
historical data indicates water quality at the spring has not
changed appreciably in the last 100 years. Together, this
indicates an absence of contribution to the spring from the
regional aquifer and suggests an old age for the source water.
Comparison of chemical concentrations between the
Agua Caliente, Fenced, and Chino Warm Springs indicates
differences are much greater for major ions than for several
trace elements; hence, a single common source for the
geothermal water at the three sites is unlikely. Also, there are
large differences between the trace element concentrations in
the Agua Caliente Spring and the surrounding groundwater,
which supports the inference based on major-ion
concentrations that no mixing occurs between the thermal
water and regional aquifer.
Temperature estimates for the geothermal reservoirs
(geothermal source water) of Agua Caliente, Fenced, and
Chino Warm Springs made by using an empirical relationship
between sodium, potassium, and calcium concentrations and
by using calculations based on aqueous equilibration with
chalcedony (silica) range from 61 to 71 degrees Celsius and
from 50 to 80 degrees Celsius, respectively. Both methods
confirm a moderate temperature, far below the boiling point
of water, for the geothermal source water for all three warm
springs.
Use of dissolved-gas-concentration data yield calculated
recharge temperatures of about 14 degrees Celsius for Agua
Caliente Spring, 16 degrees Celsius for Fenced Spring, and
19 degrees Celsius for Chino Warm Spring. Partial loss of
gas, either during sampling or by re-equilibration with soil
gas as groundwater nears the surface, will cause temperature
estimates based on gas concentrations to be high. The
calculated recharge temperature for Chino Warm Spring
probably is several degrees higher than the actual recharge
temperature because excess-air data collected from the sample
indicate that gasses were “stripped” from the sample during
the sampling process.
Delta deuterium values range from about –70 per mil in
Fenced Spring to almost –80 per mil in Agua Caliente and
Chino Warm Springs. The lighter (more negative) deuterium
ratios in Chino Warm and Agua Caliente Springs are consistent
with an older and(or) higher-altitude source of recharge for
these springs. The altitude of recharge was estimated by using
deuterium data from the spring discharge and the isotopic
composition of precipitation from a monitoring station on Mt.
San Jacinto. The altitude of recharge was estimated to be about
7,740 feet for Chino Canyon Creek, 7,260 feet for Chino Cold
Spring, 7,750 feet for Chino Warm Spring, and 7,290 feet
for Agua Caliente Spring. The calculation yields a recharge
altitude of about 6,100 feet for Fenced Spring; however,
recharge probably was a few hundred feet higher because it is
likely that evaporation has caused the isotope ratios to become
less negative at this site.
Tritium is present at low concentrations in Chino Cold
Spring and in a sample from the regional aquifer, indicating
at least some contribution from water that is younger than
1950 (post-bomb). The complete absence of tritium at Agua
Caliente Spring is consistent with the lack of mixing with
groundwater in the regional aquifer.
Carbon-14 activities for samples from the Agua
Caliente, Chino Warm and Fenced Springs range from 16
to 30 percent modern carbon. Calculated 14C ages range
from about 15,000 years before present at Agua Caliente
Spring to 7,000 years before present at Chino Warm
Spring. Carbon-13/12 ratios indicate some exchange with
radiocarbon‑dead carbonate in the soil, suggesting actual time
since recharge is about 3,000 years less than these calculated
ages.
Numerical models of fluid and temperature flow were
developed for the Agua Caliente Spring to (1) test the validity
of the conceptual model that the Agua Caliente Spring enters
the valley-fill deposits from fractures in the underlying basement complex and rises through more than 800 feet of
valley-fill deposits by way of a washed-sand conduit and
surrounding low-permeability deposits (spring chimney) of
its own making, (2) evaluate whether water-level declines
in the regional aquifer will influence the temperature of
discharging water, and (3) determine the source of thermal
water in the perched aquifer. A radial-flow model was used
to test the conceptual model and the effect of water-level
declines. The observed spring discharge and temperature
could be simulated if the vertical hydraulic conductivity of the
spring orifice was about 200 feet per day and the horizontal
hydraulic conductivity of the orifice (spring chimney) was
about 0.00002 feet per day. The simulated vertical hydraulic
conductivity is within the range of values reported for sand;
however, the low value simulated for the horizontal hydraulic
conductivity suggests that the spring chimney is cemented
with increasing depth. Chemical data collected for this study
indicate that the water at Agua Caliente Spring is at saturation
with respect to both calcite and chalcedony, which provides
a possible mechanism for cementation of the spring chimney.
A simulated decline of about 100 feet in the regional aquifer
had no effect on the simulated discharge of Agua Caliente
Spring and resulted in a slight increase in the temperature
of the spring discharge. Results from the radial-flow- and
three-dimensional models of the Agua Caliente Spring area
demonstrate that the distribution and temperature of thermal
water in the perched water table can be explained by flow
from a secondary shallow-subsurface spring orifice of the
Agua Caliente Spring not contained by the steel collector tank,
not by leakage from the collector tank.