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.

First posted December 1, 2011