Thermal and Trace-Element Anomalies in the Eastern Snake River Plain Aquifer: Toward a Conceptual Model of the EGS Resource

Transcription

1 GRC Transactions, Vol. 39, 2015 Thermal and Trace-Element Anomalies in the Eastern Snake River Plain Aquifer: Toward a Conceptual Model of the EGS Resource J. A. Welhan Idaho Geological Survey, Idaho State University, Pocatello, Idaho Keywords EGS, ESRP, rhyolite, thermal water, chemical tracers, conceptual model Abstract Data on temperature and chemistry of thermally-influenced ground water in the eastern Snake River Plain aquifer were examined to determine how such information could be used to inform on heat transport from the hot rhyolitic rocks that underlie the aquifer. The U.S. Geological Survey s NWIS database reveals the existence of several thermally-influenced areas located near the margins of the aquifer, where ground water temperatures are associated with distinctive water chemistry. The associated enrichments in F, Li and B in these waters are diagnostic of water-rock chemical reactions with felsic rocks under thermal conditions. The chemistry of thermal waters sampled in a deep borehole drilled into rhyodacite and welded tuff beneath the Idaho National Laboratory (INL) to a depth of 3.2 km bears the geochemical fingerprint of such interaction with hot felsic rock. Based on an analysis of temperature data in a thermal anomaly located in the central ESRP aquifer south of the INL, an estimate of the thermal water flux through the base of the aquifer, derived via a two-component mixing model, is incompatible with core-scale data on hydraulic conductivity (K) of the rhyolite. This suggests that advective heat transport from the rhyolite is not spatially uniform but focused in localized, high-k preferential flow paths within the rhyolite and mineralized basalts that overlie the rhyolite. This interpretation is consistent with data on the fracture-flow characteristics of rhyolitic basement and sheds new light on the spatial distribution of thermally influenced ground water near the margins of the ESRP aquifer in the area of the INL. Introduction The tremendous geothermal potential of hot rhyolitic rock beneath the eastern Snake River Plain (ESRP) along the track of the Yellowstone hotspot has long been recognized (MIT, 2006), but information necessary for effective exploration and development of this resource, such as its hydraulic characteristics and advective vs. conductive heat transfer mechanisms, has been hard to come by. Except for a very few deep geotechnical and monitoring wells on and near the Idaho National Laboratory (INL) and a few such wells in other locations on the ESRP, too little is known of the rhyolites to effectively constrain its regional thermal structure or our conceptual models of EGS-mineable heat. In the WSRP, for example, a study by Arney et al. (1982) illustrated the relevance of a wide variety of geophysical and geochemical information to assess EGS potential. As part of the Snake River Geothermal Consortium the Idaho Geological Survey is working with INL colleagues on the FORGE initiative to develop an EGS test site on the ESRP, by helping to refine geologic and hydrogeologic conceptual models of this thermal resource. This report provides a preliminary analysis of thermal and geochemical data from the ESRP aquifer that have not previously been evaluated in a geothermal context. 363

2 Welhan Geohydrologic Background The ESRP comprises a complex sequence of volcanic materials that record the passage of the Yellowstone hotspot beneath the western North American plate beginning in early to middle Miocene time (Brott et al., 1981). A carapace of fractured and highly permeable basalt lava flows of Pliocene and younger age, intercalated with minor amounts of fine to coarse eolian, fluvial and playa sediments, hosts an active, fast-flowing aquifer (Smith, 2004). Total basalt thickness approaches 2 km in the central portion of the basin, but secondary mineralization in the lower basalt section has reduced porosity and permeability by orders of magnitude (e.g., Morse and McCurry, 2002), comprising an effective hydraulic base to the ESRP aquifer that precludes regional ground water flow below depths of ca. 100 to 500 meters in the vicinity of the INL (e.g., McLing et al., 2014). Underlying the mineralized base of the aquifer are more than 3 kilometers of ignimbrite, welded tuff, rhyolite and granitic basement (hereafter collectively referred to as rhyolite ) that reflect pervasive silicic volcanism and caldera collapse in the hotspot s wake. Total volume of erupted felsic material has been estimated to range from 750 km3 to >1800 km3 (Morgan and McIntosh, 2005). These felsic rocks retain considerable heat, possibly augmented by young rhyolite volcanism as expressed in late Quaternary rhyolite domes and crypto-domes on the ESRP. Although the flow of ground water in the basalt aquifer masks the province s high heat flow, three quarters of the area of the ESRP s hot rhyolite is thought to exceed 200 oc at 4 km depth, making it one of the highest-value EGS exploration targets in the U.S. (MIT, 2006; Podgorney et al., 2013; McLing et al., 2014). NWIS Database The USGS s National Water Information System (NWIS) database ( gov/mapper/index.html) contains a wealth of data on the temperature and chemistry of ground waters in the ESRP aquifer, including trace-element data (Figure 1). The data reported on here have not been filtered by well depth, sampling time or aquifer lithology of the producing zone and represent samples collected from individual wells that have been sampled at various times, in some cases spanning a period of decades, by pumping from fixed depths within the aquifer. This report represents a preliminary evaluation of the NWIS data for the purpose of determining whether it sheds useful light on conditions at the base of the ESRP aquifer and of formulating testable hypotheses with which to constrain hydrogeologic conceptual and numeric models of thermal inputs from the rhyolites. As such, this report represents an in-progress analysis at the time of the manuscript s peer review (May, 2015). Final results will be presented at the GRCReno annual meeting. Essentially all of the wells in the NWIS data set represent ground water drawn from the actively flowing, shallow basalt aquifer; only a very small subset of sampled wells are completed in rhyolite of the ESRP in the Ashton and Newdale Figure 1. All available temperature data from USGS National Water Information System database for ground waters sampled in southeast Idaho from 1911 to Figure 2. Locations of thermally affected areas in the eastern Snake River Plain aquifer, which may inform on the mechanisms of heat transport from underlying rhyolitic rocks (the EGS resource) into the shallow flow system as well as on the hydraulic characteristics of the resource. 364

3 areas. As shown in Figure 2, the NWIS data define several geographic areas that have ground water temperatures above ambient (ca o C). The tendency of anomalous temperatures to cluster near the margins of the aquifer indicates that the source of thermal water lies beneath these areas. Whether these manifestations reflect upwelling directly from the underlying hot rhyolitic rocks through the mineralized base of the aquifer or whether thermal water rises along structural heterogeneities at the basin margins and is distributed laterally into the basalt aquifer is not known, but has implications for the manner in which heat is stored in, and migrates out of, the rhyolitic basement. Trace-element signatures that accompany the NWIS thermal signals may inform on the water-rock reaction history of these fluids. Preliminary results of this analysis are the subject of this report. Trace-Element Abundances Ground waters in these thermal areas display certain trace-element enrichments that may reflect the water-rock history of these thermal waters. Enrichment of Cl, F, Li and B have been reported in thermal waters of the Idaho batholith as well as in ground waters along the margins of the western SRP (Waag and Wood, 1987; Krahmer, 1995; Druschel, 1998) and have been sampled within a deep borehole drilled into hot rhyolite beneath the ESRP aquifer (Mann, 1986). Work by Fisher et al. (2012) presented the latest assessment of some of these data in the immediate vicinity of the INL, demonstrating that shallow tributary recharge from the highlands in the vicinity of the aquifer s margins encroaches on and dilutes diagnostic tracers like Li and B in a systematic fashion. Enrichments of Li and B as well as F are characteristic of thermal waters that have reacted with granitic rocks (e.g., Ramirez-Guzman et al., 2004) and the presence of these natural tracers in the ESRP s thermally affected ground waters suggests that these waters reflect interaction with felsic rocks at elevated temperatures. Ramirez-Guzman s modeling results suggest that equilibrated Cl concentrations may be more reflective of the source water s Cl content (e.g., meteoric vs. Cl-rich deep flow system water) rather than from the felsic rock. Figure 3 summarizes concentrations of Cl, F and T measured in the Newdale and Ashton thermal areas, as defined in Figure 2. Unlike the great majority of NWIS sampling sites on the ESRP, the highest-temperature wells in these two thermal areas are completed in rhyolitic rocks that outcrop on the southern margin of the ESRP. The cross-plots of Cl-T, F-T and F-Cl display considerable scatter, possibly because different wells and sampling periods are represented. However, the correlations shown are statistically significant at 95% confidence in the Newdale area. The Ashton F-T and Cl-T correlations are much weaker, but the strong F-Cl correlation and the overall similarity to Newdale trends suggests that these correlations are real. The observed trends are interpreted to reflect dilution of upwelling thermal water with shallow ground water, possibly with involvement of a Clrich sedimentary source either before or after thermal equilibration with rhyolite. More complex Cl-F-T behavior is seen in the Rexburg and Idaho Falls thermal areas, as shown in Figure 4. Mixing patterns are strikingly similar in the two areas, but both display (1) much lower F concentrations (< 1 mg/l, a reflection of fluorite solubility Figure 3. Variations of Cl, F and measured temperatures in (A) Newdale-area wells and (B) Ashton-area wells. Data reflect historic sampling of wells on the ESRP through 2012 (NWIS, 2012). 365

4 control in response to mixing with calciumrich ground waters of the ESRP aquifer, e.g., Welhan, 2012), and (2) evidence of possible anthropogenic effects in which temperature is uncorrelated with Cl and F (e.g., due to infiltration from drain wells, septic fields, and pavement runoff). The most complex relationships are observed in wells of the INL thermal area, where site-wide patterns such as those shown in Figures 3 and 4 are swamped by the sheer range and type of variability that is operative in this area, not only reflecting potential local contamination but also the variety of boreholes that are open over a wide range of depths and wells sampled at multiple depths. As in the Rexburg-Idaho Falls thermal areas, F concentrations are limited to <1 mg/l, so mixing relationships cannot be deduced on the basis of F data. In a sitewide plot of 67 samples in which both elements were analyzed, Li correlates with B at >99% confidence (Figure 5A) and Li correlates with temperature in all but a small subset of samples (Figure 5B); this subset is not site specific and is geographically scattered. Far fewer B data are available sitewide (not shown), and do not show a correlation with temperature. A number of multi-level sampling systems have been installed in wells recently drilled on the INL, providing discrete temperature and water quality data vs. depth (e.g. Bartholomay and Twining, 2010). Preliminary analysis of these data sets and a comparison with the NWIS data presented in this paper suggests that the NWIS temperature data from such installations displays substantial secular variability in selected wells and the possible effects of long-lived thermal re-equilibration following drilling. Analysis of these data sets is underway to determine whether NWIS trace element concentrations also reflect significant temporal variability. Figure 6 summarizes the principal types of Cl - T behavior observed in individual wells of the INL thermal area. The majority display essentially isothermal behavior but with large Cl variations (Figure 6B, C). Possible reasons are that Cl is introduced locally from anthropogenic sources (storage ponds, disposal wells, storm water retention basins), and intra-borehole flow in wells that are open over large intervals may promote mixing of interconnected hydrostratigraphic units (e.g., Bartholomay and Twining, 2010). Figure 4. Correlated Cl, F and temperature measured in (A) Rexburg-area wells and (B) Idaho Falls-area wells. Note that F concentrations are much lower compared to Newdale and Ashton waters. Simple, two-component mixing trends are not apparent, although mixing trends in the two areas are very similar. Figure 5. (A) Sitewide NWIS data on Li and B in INL ground waters, showing statistically significant correlation (> 99.5% confidence level); far fewer B data (not shown) that are available do not correlate with temperature. (B) Aside from high sitewide Li values (grey symbols) greater than the mean of values in (A), the bulk of Li concentrations sitewide correlate with temperature at > 99.5% confidence level. 366

5 Welhan Discussion Table 1 summarizes data on basaltic and felsic rocks from southern Idaho, showing that rhyolitic rocks of the Snake River Plain / Yellowstone volcanic province have much higher Cl and F, elevated Li and slightly higher B compared to SRP basalts. It is also clear that the Cl and F content of rhyolites in this area are in the same range as the granite composition assumed by Ramirez-Guzman et al. (2004) in their modeling (Figure 7). Note that the two available B analyses of SRP basalt are very similar to mid-ocean ridge basalt (MORB), whereas the available analyses of the evolved basalts at Craters of the Moon are consistent Figure 6. Examples of most common types of Cl and temperature variability seen in INL-area wells. A minority display no evidence of a mixing relationship (A). The overwhelming majority of wells are characwith their evolved nature. terized by isothermal behavior with large variations in Cl (B, C). Relatively few wells display clear evidence The NWIS data of of two-component mixing, such as between ambient ground water with a thermal water having low Cl, Figures 3 through 5 suggest such as in (D), or elevated Cl, such as in (E, F). that coupling of F, Li and B and possibly Cl concentrations with temperature may be a characteristic feature of Snake River Plain ground water that has reacted with felsic igneous rocks at elevated temperatures (e.g., Waag and Wood, 1987; Welhan, 2012), prior to its mixing with Ca-rich shallow ground water. For example, Cl concentrations in the highest-temperature springs of the Idaho batholith (ca. 85 oc) are characteristically low, ranging from 7 to 15 mg/l (Krahmer, 1995; Druschel, 1998), which is similar to thermal waters from a deep INL borehole in rhyodacite and welded tuff (12 to 17 mg/l at T > 60 oc; Mann, 1986) and to the low-cl, Na-HCO3-type thermal water that is the product of reaction of meteoric water with granitic rock along a thermal gradient of oc, followed by conductive cooling during ascent (Figure 7; Ramirez-Guzman et al., 2004). However, as in the Newdale and Ashton areas (Figure 3), the thermal component appears to be enriched in Cl. Table 2 summarizes key major and traceelement characteristics of ground water from thermally influenced areas in southern Idaho that Figure 7. Origin of high-ph thermal waters in granitic rocks as a consequence of a reflect the geochemical fingerprint of thermal multi-step reaction path process. After Ramirez-Guzman et al. (2004), based on reacreaction with felsic rock. Relative to ambient, tion path simulation using the CHILLER reaction-path code of Reed and Spycher (1984). 367

6 Table 1. Comparative summary of available data on Cl, F, Li, and B contents of basalts and rhyolites of the SRP and vicinity. wt. % ppm Rock Type Sample Provenance SiO 2 CaO Na 2 O Cl F Li B Source Basalt, MORB Basalt, SRP Basalt, evolved Rhyolite Famous area, MAR; Tamayo FZ; Juan de Fuca Ridge; EPR; 48 analyses (3-17) 1.8 (.3-11) Gerritt basalt, Mesa Falls (~ 0.2 Ma) Fissure basalt, Spencer Kilgore (~ 0.5 Ma) Kimama flow (15 ka) Lava Creek flow, Craters of the Moon (13 ka) Average SRP Basalt << 20* << 100* Devils Orchard flow, Craters of the Moon (~ 5 ka) Highway flow, Craters of the Moon (2.4 ka) Serrate flow, Craters of the Moon (2.4 ka) Average COM Basalt Cougar Point tuff, Bruneau-Jarbridge eruptive center (11.5 Ma), three analyses Lava Creek Tuff Member B, east of Magic Reservoir (0.64 Ma) Big Southern Butte, obsidian (0.3 Ma) Big Southern Butte, obsidian (0.3 Ma) Moonstone Mountain, NW of Magic Reservoir < Wedge Butte flow, SE of Magic Reservoir (3.0 Ma) < Ammon quarry, ash-flow tuff (4.1 Ma) < Lost River Range near Howe, vitrophyre Nez Perce flow, Yellowstone plateau (0.15 Ma) Canyon flow, Yellowstone plateau (0.48 Ma) Crystal Springs flow, Yellowstone plateau (70 ka) Obsidian Cliffs flow, Yellowstone plateau (0.17 Ma) Average SRP Rhyolite < Topaz Rhyolite China Hat flow (57 ka), seven analyses Topaz Rhyolite ** , 6 ± 5.6 ± 0.9 ± 0.3 ± 247 ± 1386 ± 18 ± 7 * inferred from contents of MORB vesicles (Kagoshima et al., 2012) 3 - Kuntz et al. (1992) ** mean and 2σ range 4 - Leeman (1982) 1 - Ryan and Langmuir (1987, 1993) 5 - Lewis and Kauffman (2013) 2 - Savov et al. (2009) 6 - Ford (2005) 1 Table 2. Summary of dominant major ions and Cl, F, Li and B levels (mg/l) characteristic of thermal waters from felsic rock environments of the Snake River Plain and Idaho batholith. Area T, C ph Ca Mg Na K SiO 2 HCO 3 CO 3 SO 4 Cl F Li B Water Type Sources INL rhyodacite (INEL-1 deep borehole) Idaho batholith Boise Front Mountain Home area Banbury Hot Springs Mean Na-HCO 3 Mann, σ n=3 Min Max Mean Na-HCO 3 Druschel, σ Krahmer, 1995 n=17 Min White, 1967 Max Young, 1985 Youngs, 1981 Mean Na-HCO 3 Berkeley Group, n=8 2 σ Min Mayo & others Max Mean Na-HCO 3 Arney et al., 1982 n=11 Min Max σ Mean Na-HCO 3 -Cl Lewis and Young, 2 σ n=7 Min Max

7 non-thermal ground water characteristic of the ESRP aquifer, these thermal waters display a striking shift in major-ion composition to Na-HCO 3 -type water, with decreased Ca, Mg and increased SiO 2, consistent with thermodynamic predictions of Ramirez-Guzman et al. (2004). They are also greatly enriched in F (indicating they are not influenced by mixing with Ca-rich ground water), as well as Li and B. The fact that thermal waters from the Banbury area are much higher in Cl, while sharing all other characteristics, suggests that the Cl content of reacting water may be a determining factor, born out by Ramirez-Guzman s modeling results, which showed that Cl content following the reaction path shown in Figure 7 remained essentially the same as in the reacting meteoric water. As is clear from the data on the Rexburg, Idaho Falls and INL areas, anthropogenic influences as well as water-rock reactions other than with felsic volcanics would limit the utility of geochemical fingerprinting. However, where such influences can be ruled out, trace element and temperature information appear to be diagnostic indicators of thermal waters that have originated within the hot rhyolitic rocks of the ESRP. Figure 8 depicts the shift in major-ion and trace-element chemistry that occurs in deep borehole INEL-1. That well was drilled to a depth of 3160 m near the margin of the ESRP, where rhyodacite and welded tuff comprise the section between 750 m depth and TD (Mann, 1986). Consistent with reaction-path modeling, INEL-1 s thermal waters also display characteristic major and minor element relationships that reflect thermal reactions with felsic rock (Table 2), including the striking contrast with the shallow basalt aquifer (Ca-HCO 3 -type above ca. 750 m depth, with very low F, Li), and the underlying welded tuff and rhyodacite, which host a Na-HCO 3 -type water considerably enriched in F, Li and B. The step-like changes reflect the shift from low-temperature water-rock reactions characteristic of the ESRP aquifer s recharge areas and reactions with felsic rocks at elevated temperature. Implications for Thermal Inputs From Hot Rhyolite In areas where land use, flood-irrigation and other near-surface thermal and geochemical influences are minimal and where felsic-mineral assemblages control the geochemical evolution of thermal ground water, two-component mixing models may be appropriate to explain the dilution of thermal water that issues from rhyolitic basement (e.g., Figure 3). In areas such as Newdale and Ashton, Cl and F correlate in a statistically significant manner, suggesting that water of T > 40 o C with Li and B derived from hot rhyolite mixes with shallow ground water (10-12 o C) to produce the observed patterns. Too few Li and B analyses are available in the NWIS database to ascertain whether similar mixing relationships hold for these elements, but if water-rock reaction with rhyolite is responsible, then their behavior would be expected to be similar. The data from INEL-1 clearly define the geochemical nature of thermal water that has equilibrated with hot rhyolite in one area of the ESRP. The fact that INEL-1 s deep thermal water is > 35k years old (Mann, 1986) is consistent with the hypothesis that its Na-HCO 3 composition reflects reaction with felsic volcanic rocks at elevated temperatures. The existence of a substantial upward vertical hydraulic gradient (ca m/m; Mann, 1986) between rhyolite and basalt implies that the hydraulic conductivity of the rhyolite and/or the mineralized basalts and sedimentary interbeds at the base of the ESRP aquifer must be very low, in order to promote silicate-water mass exchange over such long times. This conjecture is corroborated by hydraulic conductivity measurements on these rocks which indicate that K < 0.01 m/day (e.g., Mann, 1986). Even with such a low K, however, Mann (1986) estimated that the flux of thermal water across the base of the ESRP aquifer due to the large vertical hydraulic gradient between the rhyolitic rocks and the shallow aquifer could be ca. 2 x 10 6 m 3 /year over the area of the INL. Figure 8. Comparison of thermal waters sampled in deep borehole INEL-1 drilled to 3.2 km in ESRP rhyodacite and welded tuff (Mann, 1986). (A) Compositional change in major-ion chemistry between the upper basalt aquifer (Ca-HCO 3 ) and the lower rhyolite (Na-HCO 3 ). (B) enrichments in fluorine and lithium that accompany the major-ion compositional shifts. If this proves to be typical of other thermally affected areas of the ESRP aquifer (Figure 2), then the magnitude of thermal recharge derived from the rhyolite could comprise a significant fraction of the ESRP aquifer s overall water budget. Determining whether this is the case would benefit from an evaluation of the NWIS thermal and chemical data. Doing so could provide information on the hydraulic connection between the rhyolite and the overlying aquifer as well as the manner in which heat is stored and transported within the rhyolite. Stated another way, if dilution ratios between ambient ground water and thermal water can be inferred from temperature or trace element data, such information may 369

8 provide independent estimates of the rhyolite s hydraulic conductivity and the manner in which thermal waters discharge from the rhyolite. Hypothesis Testing It would be difficult to evaluate such an hypothesis in areas like the INL, Idaho Falls or Rexburg thermal areas where anthropogenic and/or non-rhyolite water-rock end members appear to play a major role (Figures 4, 5). An area of the ESRP aquifer affected by thermal water inputs but not by anthropogenic disturbances or the influence of sedimentary water/rock geochemistry is required. Figure 9 shows all NWIS wells sampled between 2000 and 2012, together with a kriged interpolation of regional temperature variations in the ESRP aquifer. The data define a subdued thermal high that extends along the axis of the ESRP aquifer southwest of the INL, apparently for a distance of more than 80 km. This thermal anomaly is defined on the basis of wells sampled since about 1995 and therefore is a persistent feature of the aquifer s thermal structure, making it a good candidate to evaluate hypotheses concerning the thermal influence of hot rhyolite beneath the aquifer. Its geographic proximity to the INL means it is directly relevant to the FORGE test site and, being located in the middle of the basin, it should be minimally influenced by both anthropogenic inputs and water-rock influences from Phanerozoic rocks outside the ESRP basin. Figure 10 shows the locations of ten wells that define this axial thermal zone and that will be used to illustrate how NWIS temperature and trace element data can inform on the hydraulic connection between rhyolite thermal waters and the ESRP aquifer. Figure 11 summarizes temperature and major-ion mixing trends for selected wells indicated in Figure 10. A mixing zone exists along the aquifer s northern margin (Figure 10) within which Li and B concentrations are diluted by shallow tributary recharge originating in the highlands outside the aquifer (Fisher et al., 2012). Ground waters east of this zone, including the area of the axial thermal high, tend to have higher Na/Ca ratios, much higher Li and B concentrations, and higher Cl, all apparent indicators of felsic rock thermal influence. The elevated Cl may be sedimentary and/or anthropogenic in origin, possibly derived from regional flow system inputs to the rhyolite and/or from agricultural activities upgradient of the INL. The higher Na, Li, and B contents east of the mixing zone are consistent with the hypothesis that Na-HCO 3 thermal waters derived from the underlying rhyolite, as sampled by deep borehole INEL-1 (Figure 8), recharge the ESRP aquifer through its base. Ground water within the mixing zone is Li- and B-poor due to dilution by Ca-rich tributary recharge from the range front. At the time of writing, trace- and major-element data from the NWIS database were still being compiled for wells in the vicinity of the proposed FORGE test site, so only Cl and T data are discussed in what follows. For the purpose of this computation, it is assumed that the chemistry of ground water in the area of the axial thermal high is not affected by anthropogenic or sedimentary influences and only reflects mixing between ESRP regional ground water flow and thermal water that discharges from the rhyolite. The average water temperature of 54 measurements in the 10 wells within the axial thermal zone is 14.9 C (2 σ = 1.6). Figure 12 shows available temperature and Cl data from these wells. Note that the pattern of Cl-T variations is reminiscent of the pattern seen in the majority of INL monitoring wells (e.g., Figures 5B, 5C). To perform a meaningful twocomponent mixing analysis requires defining two end members as well as the resulting mixture. In this case, these components are: (1) the thermal water issuing from the rhyolite beneath the axial thermal high; (2) local shallow ground water immediately upgradient of the mixing zone; and (3) the composition of the mixture within the mixing zone. Figure 9. All NWIS temperature data from the period 2000 to 2012 overlain on regional temperature trends as interpolated by ordinary kriging. 370

9 Welhan Figure 10. Selected wells in the NWIS database are shown, which define the axial thermal high and major ion water chemistry of the regional ESRP aquifer in the vicinity of the INL. Well names correspond to NWIS site identifiers. The dashed line through the middle of the INL defines the eastern boundary of a ground water mixing zone along the margin of the ESRP (Fisher et al., 2012). The colored underlay, taken from Whitehead (1986), represents interpreted basalt thickness based on resistivity data, a proxy for rhyolite depth (contours are in feet). Figure 11. Representative major-ion data from selected INL wells shown in Figure 10. (A) Selected temperature and major-ion data from Busenberg at al. (2000) showing evidence for a thermal end member. (B) Major-ion data from Busenberg at al. (2000) and Fisher et al. (2012), showing mixing trends with high-cl and high-bicarbonate components. (1) Thermal end member: Ostensibly, the data from INEL1 would seem to constrain the characteristics of thermal water that feeds the axial thermal high. However, recognizing that such characteristics may be spatially variable, the best estimate of the Cl composition of thermal water that sustains the axial thermal high is assumed to be the minimum Cl concentrations shown in Figure 12: ca. 7.5 mg/l. Although low, this is still well within the range observed in thermal waters that have interacted with felsic rocks in other areas of the SRP (Table 1) and is consistent with INL wells that exhibit mixing behavior with a low-cl thermal component (e.g., Figure 6D). The thermal end member s temperature should be defined at the base of the aquifer where it mixes with ESRP ground water. The depth to the base of the aquifer beneath the axial thermal zone is not known, so INEL-1 s thermal gradient (Mann, 1986) and the depth to the base of the aquifer at that location (102 m; Podgorney et al., 2013) were used to constrain the temperature of the thermal end member where it enters the ESRP aquifer (25 oc). Unfortunately, information from the nearby Kimama deep well (e.g., Freeman, 2013) is irrelevant to Figure 12. Available temperature-cl data for wells that define the axial thermal high south of INL. Data from individual wells are plotted with different symbols; site identifiers and locations correspond to Figure

10 this discussion because at 1070 m depth, those waters are an unusual Na-Cl composition, and are non-representative of a rhyolite-influenced thermal endmember. (2) Upgradient (nonthermal) end member: The nearest NWIS wells from which upgradient ground water conditions can be estimated occur more than 20 km from the axial thermal zone; data from more than 70 such wells within a distance of 60 km define a mean T of 11.6 o C (2 σ = 3.0) and Cl = 17.6 mg/l (2 σ = 30.0). Considering the distances involved and the large relative uncertainties in these averages, a better estimate of local ground water flowing through the axial thermal high cane derived from the 119 NWIS analyses of the two highest-cl wells in Figure 12 whose mean T = 14.7 o C (2 σ = 1.4) and Cl = 21.2 mg/l (2 σ = 8.0). These values are similar to, but somewhat higher than, those defined by the 70 upgradient wells. They still carry considerable relative uncertainty, but for the purpose of this computation, they are considered the best available estimates. (3) Mixed water: The thermal water rising through the base of the aquifer mixes with ground water moving through the axial thermal zone, a mixture whose composition was estimated from the average of 27 measurements in the lowest-cl wells shown in Figure 12: T = 15.6 C (2 σ = 1.5) and Cl = 10.3 mg/l (2 σ = 6.0). Note that the 60% relative uncertainty in this Cl estimate renders any mixing calculation based on it highly uncertain. With the above estimates in hand, the mixing fractions based on a two-component mixture of the thermal and nonthermal end members can be determined using the following mixing equation: T mix = f * T thermal + (1-f) * T non-thermal (1) where T mix = 15.6 o C, T thermal = 25 C, T non-thermal = 14.7 C, and f is the flux ratio of thermal water in the mixture, defined as Q thermal /(Q thermal + Q non-thermal ), where Q thermal is the specific Darcy flux: Q thermal /m 2 = K rhyolite * I rhyolite. (2) Solving for f in equation (1) yields an estimate of the mixing fraction based on temperature data. In this case, f = 0.087, suggesting that Q thermal through the base of the aquifer is an order of magnitude smaller than Q non-thermal. In contrast, the flux ratio estimated from the Cl values is quite unreasonable, being almost an order of magnitude larger (f = 0.82). Given the large relative uncertainties in the estimates of Cl end member concentrations, this discrepancy is to be expected. (4) Inferred hydraulic conductivity of rhyolite: Based on linear tracer velocities and porosities (η = ) reported by Ackerman et al. (2006) for basalts beneath the INL in the vicinity of the axial thermal zone, the specific Darcy flux in the aquifer, Q non-thermal /m 2, is estimated at 1.5 to 13 m/day. Mann (1986) estimated a vertical hydraulic gradient of 0.07 m/m in the upper part of INEL-1. Assuming the vertical gradient beneath the axial thermal zone also is in the range of 0.05 to 0.1 m/m, then the flux ratio estimated from f in equation (1) and the specific Darcy flux in the basalt aquifer allows K rhyolite to be estimated using equation (2), yielding a range of 1.5 to 25 m/day. This estimate is orders of magnitude larger than the hydraulic conductivity of unfractured rhyolite below the base of the ESRP aquifer ( to 0.01 m/day; Mann, 1986). The only way to reconcile the estimates derived from a mixing calculation is to conclude that either (1) the flux ratio estimate derived on the basis of temperatures is grossly inaccurate or (2) the thermal flux into the aquifer is localized and focused along preferential flow paths of much higher hydraulic conductivity, such as large fractures and/or vertical heterogeneities along the feeder dikes of the nearby Big Southern Butte and other smaller rhyolite domes. Possibility (1) cannot be ruled out, because flux ratios < 0.01 can be derived for T thermal >> 25 C and/or T non-thermal values very close to T mix. Possibility (2) could apply to either or both the fractured rhyolite and/or mineralized basalt that overlies the rhyolite and defines the base of the ESRP aquifer. The colored underlay in Figure 10, taken from Whitehead (1986), represents interpreted basalt thickness based on resistivity data; since basalt outcrops at or near the surface in this part of the ESRP, its thickness may also be a proxy for depth to rhyolite. Note that the axial thermal high in Figure 10 originates near where rhyolite appears to shallow to within m of the surface before deepening rapidly to the northeast, south and southwest. This fact alone could facilitate the transport of thermal water from the rhyolite into the shallow aquifer at this location. The clustered nature of thermal occurrences on the ESRP (Figure 2) also is in keeping with the highly fractured nature of rhyolitic rocks that have been drilled in the Snake River Plain. For example, Moos and Barton s (1990) analysis of INEL-1 s high-resolution televiewer log confirmed that fracture density varies considerably in different depth zones, that large-aperture fractures exist in the rhyolite, and even though most fractures are sealed due to past hydrothermal deposition, open fractures do exist and are associated with elevated temperatures indicating that thermal water is capable of actively moving within the rhyolite. Together with the inferences made from the mixing calculation, these facts suggest that thermal water makes its way from the rhyolite into the ESRP aquifer only in certain areas, possibly where the rhyolite is shallowest and/or where local preferential flow paths channel thermal water into the shallow aquifer. 372

11 Implications for a Hydrogeologic Conceptual Model Figure 13, modified from Ackerman et al. (2006), summarizes the local-scale complexity of thermal signals in this part of the ESRP aquifer. The spatial variation of aquifer temperatures reflects several juxtaposed processes: (1) fast-flow paths that conduct non-thermal ground water from tributary recharge sources (Johnson et al., 2000); (2) different hydrostratigraphic zones in the basalt aquifer (e.g., Twining and Fisher, 2015) sampled by boreholes of different depths; (3) inputs of thermal water from structures along the basin margin ( A thermal zones in Figure 13); and/or (4) via upwelling through the base of the ESRP aquifer ( B thermal zones in Figure 13), as in the axial thermal high of Figures 9 and 10. The failure of the two-component mixing calculation to produce estimates of K rhyolite that are in the range of corescale hydraulic conductivities may simply reflect gross errors in the values of the end-member temperatures used in the mixing calculation, as well as the assumed vertical hydraulic gradient, or the magnitude of the lateral Darcy flux in the ESRP aquifer. On the other hand, it may suggest that the preferential flow paths which characterize the basalt aquifer are also important in the rhyolite. The association of thermal anomalies with open fractures in the INEL-1 s rhyolite (Moos and Barton, 1990), together with more recent data on fracture geometry in deep boreholes drilled into rhyolite in other parts of the Snake River Plain (Shervais et al., 2013; Moody et al., in prep.), indicates that open fractures and systematic changes in fracture density with depth will need to be considered in any conceptualization of the hydraulic conditions within the EGS target beneath the INL. Acknowledgements The author wishes to thank Rob Podgorney, Mitch Plummer and Tom Wood for their support of this work and for INL-LDRD project funding. Anh Lay graciously allowed an extension of time to submit the manuscript, and Pat Dobson provided a very thorough and helpful review and offered editorial suggestions that materially improved the manuscript. References Figure 13. Detail showing local-scale complexity of temperature distribution near the margin of the ESRP aquifer and evidence for two possible sources of thermal inputs: (A) upwelling along structural features or heterogeneities at the aquifer margin, within the mixing zone of Fisher et al. (2012); and/or (B) upwelling through the base of the ESRP aquifer, as modeled in the axial thermal high to the south. Figure modified from Ackerman et al. (2006). Ackerman, D.J., G.W. Rattray, J.P. Rousseau, L.C. Davis and B.R. Orr, 2006, A conceptual model of ground-water flow in the eastern Snake River Plain aquifer at the Idaho National Laboratory and vicinity with implications for contaminant transport; U.S. Geological Survey, Sci. Investigations Rept , 62 pp. Arney, B.H., F. Goff and Harding Lawson Associates, 1982, Evaluation of the hot dry rock geothermal potential of an area near Mountain Home, Idaho, Los Alamos National Laboratory Report LA-9365-HDR, 65 pp. Bartholomay, R.C. and B. V. Twining, 2010, Chemical Constituents in Groundwater from Multiple Zones in the Eastern Snake River Plain Aquifer at the Idaho National Laboratory, Idaho, ; U.S. Geological Survey, Sci. Investigations Rept , 82 pp. Berkeley Group, 1990, Boise geothermal aquifer study; Idaho Dept. of Water Resources, Final Report, DOE/ID/ , 119 pp. plus appendices. Brott, A., D.D. Blackwell, and J.P. Ziagos, 1981, Thermal and tectonic Implications of heat flow in the eastern Snake River Plain: Journal of Geophysical Research, v. 86, p. 11,709 11,

12 Busenberg, E., L.N. Plummer, M.W. Doughten, P.K. Widman and R.C. Bartholomay, 2000, Chemical and isotopic composition and gas concentrations of ground and surface water from selected sites at and near the Idaho National Engineering and Environmental Laboratory, Idaho, 1994 through 1997: U.S. Geological Survey Open-File Report (DOE/ID 22164), 51 p. Christiansen, E.H., M.F. Sheridan and D.M. Burt, 1986, The Geology and Geochemistry of Cenozoic Topaz Rhyolites from the Western United States: Geol. Soc. Am. Special Paper 205, 82 pp. Druschel, G.K., 1998, Geothermal systems in the Idaho batholith: Geology and geochemistry; Washington State University, M.S. thesis, 183 pp. Fisher, J.C., J.P. Rousseau, R.C. Bartholomay and G.W. Rattray, 2012, A comparison of U.S. Geological Survey three-dimensional model estimates of groundwater source areas and velocities to independently derived estimates, Idaho National Laboratory and vicinity, Idaho; U.S. Geological Survey, Sci. Investigations Rept , 130 pp. Ford M.T., 2005, The petrogenesis of Quaternary rhyolite domes in the bimodal Blackfoot Volcanic Field, southeastern Idaho: M.S. thesis, Idaho State University, 133 pp. plus one plate, Freeman, T.G., 2013, Evaluation of the geothermal potential of the Snake River Plain, Idaho, based on three exploration holes; Utah State University, M.S. thesis, 90 pp. Johnson, T. M., R.C. Roback, T.L. McLing, T.D. Bullen, D.J. DePaolo, C. Doughty, R.J. Hunt, R.W. Smith, L.D. Cecil and M.T. Murrell, 2000, Groundwater fast paths in the Snake River Plain aquifer: Radiogenic isotope ratios as natural groundwater tracers; Geology, v.28, p Kagoshima, T., N. Takahata, J. Jung, H. Amakawa, H. Kumagai and Y. Sano, 2012, Estmation of sulfur, fluorine, chlorine and bromine fluxes at Mid- Ocean Ridges using a new experimental crushing and extraction method; Geochem. Journal, v.46, p Krahmer, M.S., 1995, The geology and geochemistry of the geothermal system near Stanley, Idaho; Washington State University, M.S. thesis, 169 pp. Kuntz, M.A., H.R. Covington and L.J. Schorr, 1992, An overview of basaltic volcanism of the eastern Snake river Plain, Idaho; in P.K. Link, M.A. Kunz, and L.B. Platt (eds.), Regional Geology of Eastern Idaho and Western Wyoming, Geol. Soc. America, Memoir 179, p Leeman, W.H., 1982, Rhyolites of the Snake River Plain-Yellowstone Plateau Province, Idaho and Wyoming: A summary of petrogenetic models; in B. Bonnichsen and R. Breckenridge (eds.), Cenozoic Geology of Idaho, p Lewis, R.S. and J.D. Kauffman, 2013, Selected radiometric and analytical data for the China Hat Dome area, NURE Project, southeastern Idaho: Idaho Geological Survey Digital Analytical Data Series DAD-9, Radiometric_and_Analytical_Data_for_the_China_Hat_Dome_Area,_NURE_Project,_Southeastern_Idaho Lewis, R.E. and H.W. Young, 1982, Thermal springs in the Payette River basin, west-central Idaho; U.S. Geological Survey, Water Resources Investigations Rept , 23 pp. plus plates. Mann, L.J., 1986, Hydraulic properties of rock units and chemical quality of water for INEL-1: A 10,365-foot deep test hole at the Idaho National Engineering Laboratory; U.S. Geological Survey, Water Resources Investigations Rept , 23 pp. Mayo, A.L., A.B. Muller and J.C. Mitchell, 1984, Geochemical and Isotopic Investigations of Thermal Water Occurrences of the Boise Front Area, Ada County, Idaho; Idaho Dept. of Water Resources, Water Information Bull. No. 30, Part 14, Geothermal Investigations in Idaho, 55 pp. McLing, T., McCurry, M., Cannon, C., Neupane, G., Wood, T., Podgorney, R., Welhan, J., Mines, G., Mattson, E., Wood, R., Palmer, C. and Smith, R., 2014, David Blackwell s Forty Years in the Idaho Desert, The Foundation for 21st Century Geothermal Research; Geothermal Resources Council Transactions, v.38, p MIT, 2006, The Future of Geothermal Energy: Impact of Enhanced Geothermal Systems (EGS) on the United States in the 21 st Century; Massachusetts Inst. Technology, Report INL/EXT , 372 pp. Moody, A., J. Fairley and M. Plummer, in prep., Designing an engineered geothermal reservoir in the welded tuffs of the Snake River Plain; Geol. Soc. America Special Paper. Moos, D. and C.A. Barton, 1990, In-situ stress and natural fracturing at the INEL site, Idaho; a report to EG&G Idaho, Inc., EGG-NPR Morgan, L.A., and W.C. McIntosh, 2005, Timing and development of the Heise volcanic field, Snake River Plain, Idaho, western USA; Geological Society of America Bulletin, v. 117, p Morse, L.H. and M. McCurry, 2002, Genesis of alteration of Quaternary basalts within a portion of the eastern Snake River Plain Aquifer; in P.K. Link and L.L. Mink (eds.), Geology, hydrogeology, and environmental remediation: Idaho National Engineering and Environmental Laboratory, eastern Snake River Plain, Idaho, Geological Society of America Special Paper 353, p Podgorney, R., M. McCurry, T. Wood, T. McLing, A. Ghassemi, J. Welhan, G. Mines, M. Plummer, J. Moore, J. Fairley and R. Wood, 2013, Enhanced Geothermal System Potential for Sites on the Eastern Snake River Plain, Idaho; Geothermal Resources Council Transactions, v. 37, p Ramirez-Guzman, A., Y. Taran and M.A. Armienta, 2004, Geochemistry and origin of high-ph thermal springs in the Pacific coast of Guerrero, Mexico; Geofísica Internacional, v. 43, p Reed, M.H., and Spycher, N.F., 1984, Calculation of ph and mineral equilibria in hydrothermal water with application to geothermometry and studies of boiling and dilution; Geochim. Cosmochim. Acta, v.48, p Ryan, J.G. and C.H. Langmuir, 1987, The systematics of lithium abundances in young volcanic rocks; Geochim. Cosmochim. Acta, v.51, p Ryan, J.G. and C.H. Langmuir, 1993, The systematics of boron abundances in young volcanic rocks; Geochim. Cosmochim. Acta, v.57, p Savov, I.P., W.P. Leeman, C.A. Lee and S.B. Shirey, 2009, Boron isotopic variations in NW USA rhyolites: Yellowstone, Snake River Plain, eastern Oregon; J. Volcanology and Geothermal Research, v. 188, p Shervais, J.W., D.R. Schmitt, D.L. Nielson, J.P. Evans, E.H. Christiansen, L. Morgan, W.C.P. Shanks, T. Lachmar, L.M. Liberty, D.D. Blackwell, J.M. Glen, D. Champion, K.E. Potter and J.A. Kessler, 2013, First results from HOTSPOT: The Snake River Plain Scientific Drilling Project, Idaho, USA; Scientific Drilling, v. 15, p

13 Smith, R.P., 2004, Geologic setting of the Snake River Plain aquifer and vadose zone; Vadose Zone Journal, v. 3, p Twining, B.V. and J.C. Fisher, 2015, Multilevel Groundwater Monitoring of Hydraulic Head and Temperature in the Eastern Snake River Plain Aquifer, Idaho National Laboratory, Idaho, ; U.S. Geological Survey, Sci. Investigations Rept , 49 pp. Waag, C.J. and S.H. Wood, 1987, Evaluation of the Boise geothermal system; Idaho Dept. of Water Resources, Geothermal Investigations in Idaho, Final Report, 70 pp. Wedepohl, K.H. (ed.), 1974, Handbook of Geochemistry, Springer-Verlag Berlin. Welhan, J.A., 2012, Preliminary hydrogeologic analysis of the Mayfield area, Ada and Elmore Counties, Idaho; Idaho Geological Survey, Staff Report S-12-2, 41 pp. White, D.E., 1967, Mercury and base-metal deposits with associated thermal and mineral waters; in H.L. Barnes (ed.), Geochemistry of hydrothermal ore deposits; Holt, Rinehart and Winston, NY, p Whitehead, R.L., 1986, Geohydrologic framework of the Snake river Plain, Idaho and eastern Oregon; U.S. Geological Survey, Hydrologic Investigations Atlas HA-681; three plates. Young, H.W., 1985, Geochemistry and hydrology of thermal springs in the Idaho batholith and adjacent areas, central Idaho; U.S. Geological Survey, Water Resources Investigations Report , 44 pp. Youngs, S.W., 1981, Geology and geochemistry of the Running Springs geothermal area, southern Bitterroot lobe, Idaho batholith; Washington State University, M.S. thesis, 125 pp. 375

14 376