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2 Geothermal Resources Council TRANSACTIONS, VOL 9 - PART I, August 1985 KINITORING THE HYD- SYSTEM IN LQNG VALLEY CALDERA, CALIFORNIA 2 Christopher D. Farrar' and Michael L. Sorey 1 *U.S. U.S. Geological Survey, Santa ~osa, California Geological Survey, Men10 Park, California ABSTRACT An ongoing program to monitor the hydrothermal system in Long Valley for changes caused by volcanic or tectonic processes has produced considerable data on the water chemistry and discharge of springs and fluid temperatures and pressures in wells. Chemical and isotopic data collected under this program have greatly expanded the knowledge of chemical variability both in space and time. Although no chemical or isotopic changes in hot spring waters can be attributed directly to volcanic or tectonic processes, changes in hot spring chemistry that have been recorded probably relate to interactions between and variations in the quantity of liquid and gas discharged. Stable carbon isotope data are consistent with a carbon source either from the mantle or from metamorphosed carbonate rocks. Continuous and periodic measurements of hot spring discharge at several sites show significant coseismic and aseismic changes since INTRODUCTION The Long Valley area is located in southwestern Mono County in east-central California, about 20 miles south of Mono Lake and 30 miles northwest of Bishop, California. The study area includes the Long Valley caldera and parts of the sur rounding mountains (fig. 1). The topographic expression of the caldera is largely the result of structural collapse following an eruption of 600 km3 of rhyolite ash about 0.7 m.y. ago (Bailey and others, 1976). As the magma chamber emptied, subsidence of the overburden occurred along ring faults. Later volcanic eruptions produced lavas and pyroclastic rocks of rhyolitic to basaltic compositions. The postsubsidence eruptions built a resurgent dome in the west-central part of the caldera and several smaller cones near or along the ring faults. Intermittent volcanic activity has continued until as recently as years ago (Miller, 1985). Geothermal energy development in Long Valley began in 1959 when about 20 exploratory wells were drilled in the Casa 'Diablo and Fish Hatchery areas., Because of the engineering capabilities and economic considerations prevailing at the time, geothermal energy development was temporarily abandoned in the early 1960's. During the mid-l970's, interest in the geothermal resources of Long Valley led energy companies to renew exploration programs. By 1983 two sites for intermediate-temperature geothermal production had been picked and powerplant designs completed. The site at Casa Diablo is now operational; plant testing began in late The second site, near the Hot Creek Fish Hatchery, is currently being drilled. In addition, exploration is continuing for hightemperature 'geothermal reservoirs in other parts of the caldera. Earthquakes of magnitude near 6 occurred in the Long Valley caldera in May 1980 and January This activity, along with detection of associated ground deformation and increased fumarolic discharge, increased concern over the possibility of a volcanic eruption in the near future (Miller and others, 1982). Since January 1983, lower levels of earthquake activity and reduced rates of ground deformation have lessened the likelihood of imminent volcanic activity. Nevertheless, the Long Valley area continues to exhibit significantly higher rates of microearthquake activity and ground deformation than other areas in California and is still recognized as having the potential for volcanic activity. 423

3 ~arrar and Sorey The Long Valley area contains an active hydrothermal system that should be affected by seismicity and ground deformation, Changes in the discharge characteristics of hot springs in the Long Valley caldera have been noted following, and possibly preceeding earthquake shocks of magnitude 5 or greater (Sherburne, 1980, p. 130; Sorey and Clark, 1981). Such observations suggest that hydrologic monitoring in a volcanic area could be used as an aid to predict the next occurrence of a volcanic eruption or to detect the subterranean movement of magma. This study began with the purpose of detecting hydrologic changes caused by volcanic processes. However, as development of geothermal resources in the area expands, the baseline-data collected to date and data from continued monitoring will provide a basis for assessment of the impact of geothermal energy production on the hydrothermal system in Long Valley. THE MONITORING PROGRAM The types of data collected during this study include: (1) both periodic and continuous recording of hydraulic head in the ground-water system, (2) measurements of spring-flow by direct and indirect means, (3) chemical analyses of water samples from springs and wells, (4) stable-isotope analyses, (5) subsurface temperature data, (6) atmospheric pressure, (7) precipitation, and (8) lithologic and stratigraphic information from exploratory drilling. Additional monitoring currently being conducted by other agencies and institutions involves the chemistry and isotopic content of gas from springs and fumaroles and helium, radon, and mercury contents in soil gas. This paper reports only selected data from the eight-point monitoring program noted above. Discussion here is limited to hot spring discharge rates and recently collected chemical and stable isotope data. All the data collected through 1984 will be presented in a Geological Survey report currently in pr epa r ati on. During the course of the monitoring program a literature search was made to tabulate historic chemical and isotopic analyses for comparison with more recent analyses. Variations in water chemistry are evident in these data for sites that have been sampled periodically. Some of the variability may be due in part to differences in analytical or collection procedures. Although increases in spriny flow have been recorded following earthquakes in the Long Valley region and some chemical variations accompany changes in flow rate, to date no observed chemical or isotopic changes are attributable directly to tectonic or volcanic processes. The discussion presented here will be limited to selected recent analyses (table 1) that characterize different water types and include at least one analysis from each main hot spring area. HOT SPRING DISCHARGE The main thermal spring areas in Long Valley include: Casa Diablo, Hot Creek Fish Hatchery, Hot Creek Gorge, and Little Hot Creek (figure 1). Total hot spring flow from each of these areas has been computed either by direct measurements or indirectly using the measurement of chemical loads in streams. Liquid discharge from hot springs at Casa Diablo has been variable, although not well documented, over the past several decades. Accurate flow measurements at this site did not begin until mid A flume was in place during the period August-December, 1984 at a site where approximately half of the total flow of the springs passes. Discharge fluctuated considerably over the short term but averaged about 9 L/s, until November. In early November a sudden but as yet unexplained doubling of the discharge occurred and above normal rates have persisted until March 1985, when continuous fluid production from nearby geothermal wells was initiated. The total spring flow from Casa Diablo was measured during December 1984 and January 1985 as 43 L/s and 34 L/s, respectively. At the Fish Hatchery numerous springs discharge mixed waters (average temperature = 14oC) that include an estimated 2 percent thermal component (Sorey, 1975). This equates to an equivalent total thermal-water discharge of about 20 L/s. A measurement of 1020 L/s for the total flow of springs at the Fish Hatchery made in July 1984 is within 5 percent of a similar measurement made in The total 'flow from hot springs discharging within Hot Creek gorge has been estimated from measurements of the increase in chloride and boron flux in Hot Creek between sites upstream and downstream of the area of spring discharge. Such estimates, based on eight sets of observations made between October 1972 and December 1980, average 271 L/s. Although a temporary increase in total hot-spring discharge may have accompanied the earthquakes of May 1980, 424

4 chemical flux measurements made in July 1980 yielded an estimated spring discharge of 267 L/s (Sorey and Clark, 1981). Measurements of flow and specific conductance at a site on Hot Creek downstream of all hot spring inflow (in operation since August, 1983) allow a means of obtaining a continuous record of hot spring discharge. The method relies on establishing a relation between specific conductance and chloride and boron concentrations. Preliminary analysis of these data indicate that an increase of approximately 50 L/s in hot spring discharge accompanied a M5.8 earthquake in late November Hot springs at Little Hot Creek include five main vents that discharge a combined flow of about 11 L/s. Sorey and Clark (1981) note that following earthquakes of M6 in May 1980, total discharge increased by as much as 45 L/s, but spring flow returned to normal several hours after each 1 ar ge-magnitude earthquake. WATER CHEMISTRY Ground water in Long Valley can be classified as non-thermal, thermal, or mixed. Non-thermal springs and wells tapping shallow aquifers discharge water low in dissolved solids and at temperatures less than about 120C. Hot springs and wells tapping the hydrothermal system discharge water containing about mg/l dissolved solids at temperatures near boiling for the ambient pressure. As thermal water rises from depth, it may mix with the shallow non-thermal wat.er in varying proportions to produce water of intermediate temperature and ionic composi tion. The chemical analyses of water samples from Laurel spring and Bald Mountain spring (table 1) typify the chemistry of non-thermal waters discharging after a short travel path from the recharge area. The water is alkaline, with less than 100 mg/l dissolved solids and a temperature near the mean annual air temperature. Conce ntr at i ons of el em ent s char act e r i st i c of hot springs such as fluoride, boron, and arsenic are all low. A trilinear diagram (fig. 2) shows water from Laurel spring plots in a distinctive position relative to the hot-spring analyses. Calcium is the dominant cation and bicarbonate the dominant anion. The thermal waters are generally near neutral to slightly alkaline (ph 6.5 to 8.5), sodium-chloride rich, and contain between mg/l dissolved solids. A few hot springs with low-ph waters (ph<5) have been observed in the Casa Diablo area (Sulfate Springs 2 and 3). The relative proportions of major ions can be seen in figure 2. For the near neutral to alkaline springs, sodium is the dominant cation ranging from 210 mg/l in Meadow Spring near Casa Diablo to 410 mg/l in springs at Hot Creek gorge and Little Hot Creek. In terms of milliequivalent s, sodium and potassi um ions account for about 98 percent of the cations; calcium and magnesium account for only about 2 percent. Chloride and bicarbonate are the dominant anions, but sulfate contributes up to 30 per cent of the total anions. Silica is present in concentrations generally between 120 and 230 mg/l except at Little Hot Creek, where the concentration is 82 mg/l. The high concentrations of sodium, potassium, and silica result from water-rock reactions with the highly siliceous acid volcanic and intrusive rocks within the hydrothermal system. Among the minor elements characteristic of hot springs are arsenic, boron, fluoride and lithium. In unmixed thermal waters, arsenic concentrations generally range from mg/l, boron 9-13 mg/l, fluoride 8-12 mg/l, and lithium mg/l. The variability of water chemistry between various spring vents in the Casa Diablo area is greater than differences between vents in other areas. Casa Diablo is the only location where some hot springs discharge acidic waters. The variable chemistry may be the result of mixing a low-ph, more dilute water with the more prevalent alkaline water higher in dissolved solids. The low-ph waters are characterized by dissolved solids concentrations of about 850 mg/l. When compared to the alkaline waters the lowph waters contain higher proportions of calcium, magnesium, and sulfate; with lower proportions of sodium, potassium, chloride, and alkalinity. The acidic waters contain lower concentrations of the minor elements arsenic, boron, fluoride, and lithium. The low ph allows greater concentrations of iron and manganese to be carried in solution. In terms of the quantity of fluid discharged, the acid springs contribute only a very minor portion of the total hot spring discharge from Casa Diablo. The discharge of acidic waters from individual vents is probably short lived. Observations of Sulfate Spring 2 (table 1) demonstrate that distinctive changes in chemistry can occur over a period of a 425

5 few days. As the ph of this spring rose changes in He-3/He-4 ratios (Rison et from 4.5 to 6.8, the concentrations of al., 1983), could provide evidence of the major elements calcium, magnesium, future intrusions of magma beneath the and potassium decreased and alkalinity, cal der a. chloride, and sulfate increased. The large variations observed in sulfate and iron concentrations suggest that redox reactions are involved. REFERENCES CITED The acid springs probably develop as steam and gas, mainly C02 and H2S, dissolve in the shallow ground-water system as they move toward the surface from an upflowing mixture of hot water and steam. The rapid changes in chemistry probably result from changes in the quantity and flaw paths of gas and steam, followed by rapid re-equilibration of water with rocks and soil. ISOTOPIC COMPOSITION The results of selected stable isotopic analyses are given in table 2. Hydrogen and oxygen ratios in thermal waters plot to the right of the meteoric water line (fig. 3). This relation has long been recognized as resulting from watet/rock reactions at elevated temperatures pref errentially exchanging rock 0-18 for water 0-16 but with little change in hydrogen isotope ratios because of the paucity of hydrogen in rocks, The isotopic data for the thermal waters are consistent with the hypothesis that deep circulation of meteoric water recharges the hydrothermal system and juvenile water does not contribute significantly to the flow of hot water. The hydrogen and oxygen data demonstrate ground-water recharge from more than one source area, Fractionation causes isotopically heavier precipitation to fall on the Sierra (Mineret sp. d D = - 111, = -14.9) than on the mountains north and northeast of the caldera (Waterson troughs sp. (5D = -131 and = -17.4). As discussed by Sorey, Lewis, and Olmsted (1978), recharge to the hydrothermal system appears to originate around the west rim of the caldera. The ratio of carbon-13 to carbon-12 is useful for interpretation of the origin of carbon in the system. The 6C-13 ratios for thermal springs given in table 2 range from -3.4 to -6.1 and are consistent with values obtained for mantle derived carbon (Faure, 1977) However, the observed C-13 ratios for thermal springs are also included in the range of ratios (0 to -11.9) determined for carbonate rock samples from Sierran roof pendants (Harold Wollenberg, Lawrence Be r kel ey Labor a tory, w r it ten communi cation, 1985). Continued monitoring of C- 13 ratios, along with measurements of Bailey, R.H., Dalrymple, G.B., Lanphere, M.A., 1976, Volcanism, structure, and geochronology of Long Valley caldera, Mono County, California : Journal of Geophysical Research, 81, no. 5, p Faure, G., 1977: Principles of isotope geology: John Wiley and Sons, New York, 464p. Miller, C.D., 1985, Holocene eruptions at the Inyo volcanic chain, California: implications for possible eruptions in Long Valley caldera: Geology, v. 13, p Miller, D.C., Mullineaux, D.R., Crandell, D.R., and Bailey, R.H., 1982, Potential hazards from future volcanic eruptions in the Long Valley - Mono Lake Area, eastcentral California and southwest Nevada - A preliminary assessment: U. S. Geological Survey Circular 877, 10 p. Rison, W., Welham, J.A., Poreda, R., and Craig, H., 1983, Long Valley: Increase in the He-3/He-4 ratio from 1978 to 1983: EOS transactions, American Geophysical Union, v. 64, no. 45, p. 891., ed., 1980, Mammoth Lakes, Sherburne, R.W. California, earthquakes of May 1980 : California Division of Mines and Geology special report 150, 141p. Sorey, M.L., 1975, Potential effects of geothermal development on springs at the Hot Creek Fish Hatchery in Long Valley, Mono County, California: U. S. Geological Survey open-file report , 10p. Sorey, M.L., Lewis, ROE,, and Olmsted, F.H., 1978, The hydrothermal system of Long Valley Caldera, California: U. S. Geological Survey Professional Paper 1044-A, 60p. Sorey, M.L. and Clark, M.D., 1981, Changes in the discharge characteristics of thermal springs and fumaroles in the Long Valley caldera, California, resulting from earthquakes on May 25-27, 1980: U. S. Geological Survey open-f ile report , 22p. 426

6 Figure 1. Index niap showing locations of sample sites (circles with tails are springs, circle without tail is a well). Dashed line is outline of caldera floor, dashed-dot line is outline of resurgent dome. Colton Sp. Meadow Sp. Milky Pool 1 Milky Pool 2 North Sp. South Sp. Geyser Sp. Sulfate Sp. 2 Sulfate Sp co <lo < Morning Glory Pool Spring above bridge a Geysers 'Well CH-IOA Hot Creek Gorge Area Flume A AB supply H-IX,IXI supply Little Hot Creek Area HIXED WATERS Laurel Sp Bald Htn Sp COLD SPRIffiS

7 Figure 2. Major ion compositions of water from selected springs and wells. Points are identified by numbers corresponding to ID numbers in tables. Axes are in percent milliequivalents per liter. Table 2. Stable isotope date MAP COLLECT1 ON 6 D 6l80 6% ID SPRIX NAME DATE 0100 otoo Colton li4 t / 20 '7 2 Ueadow North Geyser Morning Glory Above bridge Geysers Flume Laurel Uineret Waterson Troughs OXYGEN-18. */e* Figure 3. Stable isotope plot showing differences in hydrogen and oxygen isotopic ratios relative to SMOW. ID numbers correspond to those used in tables and on location map. 428