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Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Hydrogeologic Conceptualization
  5. Methods
  6. Results and Discussion
  7. Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

Littlefield Springs discharge about 1.6 m3/s along a 10-km reach of the Virgin River in northwestern Arizona. Understanding their source is important for salinity control in the Colorado River Basin. Environmental tracers suggest that Littlefield Springs are a mixture of older groundwater from the regional Great Basin carbonate aquifer and modern (post-1950s) seepage from the Virgin River. While corrected 14C apparent ages range from 1 to 9 ka, large amounts of nucleogenic 4He and low 3He/4He ratios suggest that the carbonate aquifer component is likely even older Pleistocene recharge. Modeled infiltration of precipitation, hydrogeologic cross sections, and hydraulic gradients all indicate recharge to the carbonate aquifer likely occurs in the Clover and Bull Valley Mountains along the northern part of the watershed, rather than in the nearby Virgin Mountains. This high-altitude recharge is supported by relatively cool noble-gas recharge temperatures and isotopically depleted δ2H and δ18O. Excess (crustal) SF6 and 4He precluded dating of the modern component of water from Littlefield Springs using SF6 and 3H/3He methods. Assuming a lumped-parameter model with a binary mixture of two piston-flow components, Cl/Br, Cl/F, δ2H, and CFCs indicate the mixture is about 60% Virgin River water and 40% groundwater from the carbonate aquifer, with an approximately 30-year groundwater travel time for Virgin River seepage to re-emerge at Littlefield Springs. This suggests that removal of high-salinity sources upstream of the Virgin River Gorge would reduce the salinity of water discharging from Littlefield Springs into the Virgin River within a few decades.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Hydrogeologic Conceptualization
  5. Methods
  6. Results and Discussion
  7. Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

The Virgin River Gorge subarea includes part of the Virgin River Valley hydrographic area (HA 222; Rush 1968) and is defined herein as the region draining directly into the river between its confluence with the Santa Clara River, Utah, and Littlefield, Arizona (Figure 1). Littlefield Springs are the primary source of groundwater within this subarea, discharging along a 10-km reach of the Virgin River from the “Narrows” to Littlefield, Arizona (Trudeau et al. 1983). Previous studies have reported a seepage loss along the 25-km reach of the Virgin River upstream of Littlefield Springs (Trudeau et al. 1983; Cole and Katzer 2000). Pah Tempe Hot Springs, located about 60 km farther upstream in southwestern Utah, annually contribute about 90,000,000 kg of salt to the Virgin River, causing a substantial increase in its overall salinity (Gerner 2008). The Colorado River Basin Salinity Control Forum (“the Forum”), in trying to reduce salinity in the Colorado River, needs information on the ultimate fate of seepage and salinity load loss in the Virgin River Gorge to assess artificial removal of salt from Pah Tempe Hot Springs before it enters the Virgin River and ultimately the Colorado River at Lake Mead. If Virgin River seepage loss re-emerges at Littlefield Springs, then decreasing the salinity of the losing reach of the river would cause a larger reduction in salinity load of the Virgin River as it enters the Colorado River. Additionally, the average travel time of this groundwater flow path is needed to evaluate whether the benefits of this salinity reduction could be realized within the timeline of the salt mitigation project being considered by the Forum.

image

Figure 1. The Virgin River Gorge subarea and surrounding areas in Arizona, Nevada, and Utah, with previously published groundwater potentiometric contours, location of sampling locations, and 1940 to 2006 distribution of Basin Characterization Model (BCM) in-place recharge.

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The purpose of this study was to utilize geochemical and environmental tracers to test the previously proposed hypothesis that seepage loss from the Virgin River above the Narrows re-emerges as discharge to Littlefield Springs and, if so, to evaluate the percentage of springflow coming from stream seepage and its subsurface travel time. A related objective was an assessment of other potential groundwater sources and travel times to the springs. This work builds upon several previous studies of Littlefield Springs (Appendix S1, supporting information).

Hydrogeologic Conceptualization

  1. Top of page
  2. Abstract
  3. Introduction
  4. Hydrogeologic Conceptualization
  5. Methods
  6. Results and Discussion
  7. Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

The Virgin River Gorge subarea encompasses a region of about 2600 km2, including the Beaver Dam Wash drainage and the Beaver Dam, Bull Valley, Clover, and Virgin Mountains (Figure 1). Altitudes within the subarea range from about 550 m in the Virgin River Valley to more than 2300 m in the mountains. The Virgin River is the primary hydrologic feature in both the Virgin River Gorge subarea and the lower Virgin River Valley. Previous studies have shown that the Virgin River above the study area derives much of its flow from discharge of older groundwater from the Navajo Sandstone aquifer in Zion National Park and the Central Virgin River Basin (Heilweil and Freethey 1992; Herbert 1995; Heilweil et al. 2000). Littlefield Springs are comprised of over 200 individual springs (Trudeau et al. 1983) located between the USGS Virgin River gage above the Narrows (VR-14) and the USGS Littlefield gage (VR-15; Figure 1). Estimates of total discharge from these springs range from about 0.3 to 2.6 m3/s (Appendix S1). The springs range in altitude from about 550 to 600 m (Table S1) and emanate from a combination of Quaternary travertine deposits, the Tertiary Muddy Creek Formation, Mississippian Redwall Limestone, and the Upper Cambrian Nopah Formation (Billingsley and Workman 2000). Detailed geology of the area is presented in Trudeau (1979) and Trudeau et al. (1983).

Large regional springs such as Littlefield are aquifer discharge points where groundwater flow paths of different travel times typically converge, consistent with binary or exponential mixing of multiple recharge sources (Cook and Böhlke 2000). The two likely sources for Littlefield Springs are seepage loss from the Virgin River along the 25-km reach upstream of the Narrows and infiltration of precipitation to the regional Great Basin carbonate aquifer in and upgradient of the Virgin River Gorge subarea. The current seepage loss estimate, based on comparison of gage measurements for water years 1998–2010, is 0.93 ± 0.17 m3/s (see Appendix S1 for more detail and previously reported seepage-loss estimates). Darcy calculations indicate that the majority of this seepage loss travels through carbonate bedrock beneath alluvial deposits of the river channel (Appendix S1). The Basin Characterization Model (BCM) was used to estimate infiltration of precipitation to the carbonate aquifer in the Virgin River Gorge subarea (Flint et al. 2011). Results of the BCM indicate that the majority of infiltration occurs through bedrock in the Clover and Bull Valley Mountains, about 50 to 80 km north of the Virgin River Narrows (Figure 1; Appendix S1). The large amount of recharge in this northern region is due to higher average annual precipitation of up to 700 mm/year (Figure S1), exposed permeable Tertiary volcanic and sedimentary rocks in this recharge area (Figure S2), and carbonate formations that provide permeable north-south pathways to Littlefield Springs (Figure S3).

Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Hydrogeologic Conceptualization
  5. Methods
  6. Results and Discussion
  7. Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

Field parameters (Table S1) and geochemical samples were collected at 14 groundwater and 4 surface-water sites (Figure 1). Geochemical constituents included major and trace ions, stable and radioactive isotopes (δ18O, δ2H, 3H, 14C, δ13C, 87Sr/86Sr), noble gases (3He, 4He, 20Ne, 40Ar, 84Kr, 129Xe), and dissolved gases (CFCs, SF6). CFCs, SF6, and 3H were used to evaluate modern (post-1950s) recharge. Because of their use as industrial gases, the atmospheric concentrations of CFCs and SF6 began to rapidly increase in the 1950s and 1960s (Figure 2); these dating techniques are summarized in Plummer and Busenberg (2000) and Busenberg and Plummer (2000). Unfortunately, biodegradation of CFC-11 and crustal production of SF6 precluded their use for dating of Littlefield Springs (Appendix S1). Tritium (3H) was used to evaluate the presence or absence of modern (post-1950s) water. 3H concentrations in precipitation at Littlefield, Arizona from 1950 through 2009 (Figure 2) were estimated to range from <10 to more than 2200 TU (0.4 to 160 TU decay-corrected to 2010; Figure S4). For this study, water containing greater than 0.4 TU is assumed to have at least some modern (post-1950s) fraction. Helium derived from tritium decay (3He*) was also evaluated for 3H/3He* dating (Solomon and Cook 2000), but this was not successful at Littlefield Springs because of large nucleogenic 4He concentrations (Appendix S1).

image

Figure 2. Estimated atmospheric concentrations of chlorofluorocarbons (CFCs), sulfur hexafluoride (SF6), and tritium, 1950 through 2010. 3H concentrations for the study area are based on data from Sand Hollow, Utah (Heilweil et al. 2006) and from Albuquerque, New Mexico; Flagstaff, Arizona; and Salt Lake City, Utah (International Atomic Energy Agency, 2011; Robert Michel, U.S. Geological Survey, personal communication, 2011). CFC-11, CFC-12, CFC-113, and SF6 concentrations are for the Northern Hemisphere (Plummer and Busenberg 2008).

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20Ne, 40Ar, 84Kr, and 129Xe were used in the closed system equilibration (CE) model for determining noble-gas recharge temperatures (Aeschbach-Hertig et al. 2000; Kipfer et al. 2002). This model also yields A (amount of initially trapped air beneath the water table) and F (fractionation factor for partial dissolution of trapped air bubbles). Recharge altitude (the proxy for barometric pressure) is an unknown parameter, a typical situation in locations with a high topographic gradient. Because recharge temperature and recharge altitude are correlated, however, a range of noble-gas recharge temperatures (Tr) was estimated for each sample, as described by Manning and Solomon (2003). This method uses the minimum altitude (Hmin − typically the sample site) to calculate a maximum noble-gas recharge temperature (Trmax), and the maximum possible water table altitude (Hmax) to calculate a minimum noble-gas recharge temperature (Trmin). Hmax was estimated to be 1900 m based on published spring altitudes in the Virgin River Gorge subarea (Thomas et al. 1991). The recharge parameters (Tr, A, F) were evaluated using this range of recharge altitudes with a χ2 minimization code (an Excel spreadsheet program with the built-in “Solver” inversion routine) like that described by Aeschbach-Hertig et al. (1999) and Manning and Solomon (2003). A χ2 probability threshold (P < 5%) of 3.8 (based on four measured gases and three recharge parameters) was used to define an acceptable fit for Tr, Ae, and F.

14C was used to evaluate the age of the older (>1 ka) groundwater in the Virgin River Gorge subarea. Kalin (2000) provides a comprehensive review of the 14C groundwater dating method. The Ingerson and Pearson (1964) and Fontes and Garnier (1979) analytical methods were used in this study to correct for exchange with older (“dead”) 14C in the subsurface. 14C activities of the soil gas and aquifer minerals were assumed to be 100 and 0 pmC, respectively. Estimated soil-gas δ13C was −15.8 ‰, the mean of four vadose-zone measurements at Sand Hollow (15 km east of the study area in southwestern Utah with similar climatic conditions; Robert Streigl, U.S. Geological Survey, personal communication, 2004). The δ13C value for aquifer materials comprised of carbonate minerals was assumed to be 1.5‰, based on a range of −3.0 to 2.6 ‰ for marine carbonate sediments (Keith and Weber 1964; Plummer and Sprinkle 2001).

Nucleogenic helium (4HeNUC) produced by radioactive decay of uranium and thorium was also used to qualitatively evaluate older groundwater. Based on the method described by Solomon (2000), 4HeNUC was calculated by subtracting 4HeSOL (solubility equilibrium), 4HeEA (excess air), and 4HeMAN (mantle sources) from 4HeTOT (measured concentration). For this study, 4HeSOL was determined from Henry's Law solubility, 4HeEA was determined from other noble gases using the CE model, and 4HeMAN was considered to be negligible because of the lack of recent volcanic activity. 3He has both atmospheric (3H decay) and nucleogenic (6L fission) sources. Older groundwater generally has smaller 3He:4He ratios because of the larger amount of 4He produced by crustal rocks; helium produced in the crust typically has an R/Ra value of less than 0.1 (Mamyrin and Tolstikhin 1984), where R and Ra are the sample and atmospherically equilibrated 3He:4He ratios. Modern groundwater R/Ra values are generally about 1.0 because of the predominantly atmospheric source of helium.

Results and Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Hydrogeologic Conceptualization
  5. Methods
  6. Results and Discussion
  7. Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

Evidence for binary piston-flow mixing

Field parameters (Table S1) and major-ion chemistry from both current (Table S2) and previously published studies (Metcalf 1995; Yelkin 1996; Schaefer et al. 2006; Hershey et al. 2010) suggest that water discharging from Littlefield Springs is a mixture of seepage from the Virgin River and groundwater from the regional Great Basin carbonate aquifer. The trilinear diagram (Figure 3) shows two potential end-member waters in the Virgin River Gorge subarea: (1) Virgin River samples collected at VR-8 and VR-11 upstream of Littlefield Springs, and (2) a synthetic composite representing the carbonate aquifer. This composite was based on samples from the McCullough Well (MCW) completed in the Permian Kaibab Formation about 30 km east of the study area (Wilkowske and Heilweil 1998), Blue Point Spring (BPS) and Rogers Spring (RGS) discharging from gypsiferous carbonates near Lake Mead about 60 km southwest of the study area (Hershey et al. 2010), and Lost Spring (LST) discharging from the Permian Kaibab Formation in the Virgin Mountains (this report; Figure 1). LST is considered a carbonate-aquifer end member for major and trace-ion geochemistry, but not for age or recharge conditions since it is located south of the springs rather than receiving water from the primary recharge area in the northern part of the study area. Hershey et al. (2010) interpreted BPS and RGS as located at the terminal end of the carbonate aquifer based on chemical evolution as groundwater moves through the system. The similarity in major- and trace-ion chemistry between these springs and Littlefield Springs implies interconnection and discharge from a shared regional carbonate aquifer system (Yelkin 1996). Littlefield Springs (LFS-1 through LFS-13) and the Virgin River at Desert Springs (VR-DS) lie at an intermediate location between these two end members, indicating a binary mixture. Similarly, Littlefield Springs water temperatures (18 to 27 °C; Table S1) lie between the reported 30 °C temperature for BPS and RGS (Hershey et al. 2010) and the 3-year mean Virgin River temperature of 17 °C measured at VR-8 (Martin Schijf, Utah Division of Wildlife Resources, personal communication, 2011). Unlike the other Virgin River samples, VR-DS was sampled when the Virgin River was dry above Littlefield Springs, resulting in a weighted average of all the spring discharge above this location. Major-ion chemistry indicates that water from the Cedar Pocket Well (CPW), located adjacent to the Virgin River and screened from 80 to 110 m, is similar to VR-15 and likely is hydraulically connected to the Virgin River.

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Figure 3. Trilinear diagram showing major-ion chemistry of groundwater and surface water in the Virgin River Gorge subarea, Arizona, Nevada, and Utah.

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Cl:Br mass ratios can help differentiate different source waters (Davis et al. 1998). Relatively high ratios (>1000) have been reported for surface water from the Virgin River upstream of the study area, compared with generally low ratios (<300) found in regional groundwater (Heilweil et al. 2006; Heilweil and Marston 2011). Cl:Br ratios of Littlefield Springs range from 590 to 720 (Table S2), with an average value of 650 (n = 13). It is assumed that a representative end member Cl:Br ratio for the carbonate aquifer is about 160, the value for LST (this report). [Correction added after online publication February 21, 2013: Reference to Schaefer et al. 2006 has been removed from the preceding sentence.] An average Virgin River end-member ratio of 890 is based on three samples either from or influenced by the Virgin River (VR-8, VR-11, CPW). Binary mixing calculations using these end-member Cl:Br ratios indicate a 68:32 mixture of seepage from the Virgin River and groundwater from the carbonate aquifer.

87Sr/86Sr and Cl:F ratios are also useful for evaluating groundwater flow paths. Measured 87Sr/86Sr ratios for Littlefield Springs (LFS-1 through LFS-5) fall within a narrow range (0.7091 to 0.7092; Table 1) and indicate flow through marine carbonate formations. Because carbonate rocks contain very little Rb, and because the maximum 87Sr/86Sr ratio in sea water is 0.709, unaltered marine carbonates typically have ratios of less than 0.710; in contrast, silicate aquifers such as sandstones typically have higher 87Sr/86Sr ratios (McNutt 2000). Similarly narrow ranges have been documented in groundwater from other marine limestone aquifers, such as 0.7072 to 0.7073 for the Lincolnshire Limestone of Great Britain (Bishop et al. 1994) and 0.7079 to 0.7080 for the distal ends of flow paths in the Floridan aquifer (McNutt 2000). Hem (1985) calculated a global Cl:F ratio of 2.7 for carbonate-rock aquifers. Hershey et al. (2010) reported Cl:F ratios of  > 5 for regional springs in the Great Basin carbonate aquifer and 300 to 330 for Littlefield Springs. These higher values than the global mean for carbonate aquifers indicate less dissolution of F-bearing minerals. Cl:F ratios of Littlefield Springs sampled during the current study ranged from 290 to 530 (Table S2), with a mean value of 400 (n = 15). A binary mixture of Virgin River water upstream of Littlefield Springs having a mean Cl:F ratio of 640 (n = 10) with carbonate aquifer water having a Cl:F ratio of 120 (LST) indicates Littlefield Springs contain a 56:44 mixture of seepage from the Virgin River and groundwater from the carbonate aquifer.

Table 1. Isotopic Concentrations and 14C Ages of Groundwater and Surface Water from the Virgin River Gorge Subarea, Arizona, Nevada, and Utah
SiteDateδ18O (‰)δ2H (‰)3H (TU)87Sr/86Srδ13C (‰)14C (pmc)14C Age (Years)a14C Age (Years)b
  1. Notes: For site location see Figure 1. δ18O and δ2H values are relative to VSMOW; δ13C values are relative to VPDB; −, not reported.

  2. a

    According to Ingerson and Pearson (1964) model.

  3. b

    According to Fontes and Garnier (1979) model.

LSF-1March 11, 2009−12.6−94.50.90 ± 0.10
LFS-1June 25, 2009−12.6−94.70.72 ± 0.100.7092−2.978.4 ± 0.193008400
LFS-2June 24, 2009−12.6−94.80.70 ± 0.100.7092−2.399.7 ± 0.170006000
LSF-3June 23, 2009−12.6−94.60.93 ± 0.100.7091−3.099.4 ± 0.186007700
LFS-4June 23, 2009−12.7−94.20.80 ± 0.100.7091−3.129.0 ± 0.190008100
LFS-5June 24, 2009−12.4−92.31.2 ± 0.100.7091−3.9620 ± 0.236003100
LFS-6June 19, 2012−12.4−92.31.1 ± 0.07−3.4818 ± 0.139003200
LFS-6 replicateJune 19, 2012−12.4−92.30.98 ± 0.08−3.4818 ± 0.139003200
LFS-7June 20, 2012−12.4−93.50.76 ± 0.13−3.6715 ± 0.157005000
LFS-8June 20, 2012−12.5−94.60.60 ± 0.06−0.6610 ± 0.118001000
LFS-9June 19, 2012−12.6−94.90.42 ± 0.07−2.8810 ± 0.177006700
LFS-10June 20, 2012−12.7−95.20.49 ± 0.04−2.557.6 ± 0.193008300
LFS-11June 20, 2012−12.6−95.10.65 ± 0.06−2.969.4 ± 0.183007500
LFS-12June 20, 2012−12.6−95.00.60 ± 0.09−3.2912 ± 0.169006100
LFS-13June 20, 2012−12.5−94.81.0 ± 0.07−3.4714 ± 0.159005300
LFS-PalmJune 2, 2010−12.6−94.4−1.7814 ± 0.124001600
LSTFebruary 3, 2010−12.8−92.80.01 ± 0.1−13.286 ± 0.2Modern1900
CPWFebruary 4, 2010−12.3−92.32.3 ± 0.10−6.4363 ± 0.2ModernModern
VR-8March 11, 2009−12.1−92.12.0 ± 0.100.7089
VR-11February 4, 20102.0 ± 0.10−4.7465 ± 0.2ModernModern
VR-11July 21, 2010−10.7−85.5−6.4083 ± 0.3ModernModern
VR-DSJune 20, 2010−12.4−94.00.83 ± 0.09 −1.0712 ± 0.11800640

δ2H values for Littlefield Springs and VR-DS all have similar values of −92.3 to −94.8 ‰ (Table 1), lying mid-way between and likely comprised of a mixture of Virgin River and high-altitude carbonate aquifer water (Figure 4). While all the waters samples within the study area plot between the global meteoric water line (GML; Dansgaard 1964) and the local Arizona meteoric water line (Kendall and Coplen 2001), both the Virgin River and Littlefield Springs samples show some evaporative enrichment. Hershey et al. (2010) noted that the Littlefield Springs isotopic shift off the GML was larger than for other regional springs in the Great Basin carbonate aquifer. We propose that this larger shift is caused by mixing with evaporatively enriched Virgin River water. Similar to previous Great Basin carbonate aquifer studies by Thomas et al. (2001) and Lundmark et al. (2007), a deuterium mass-balance method was utilized to evaluate these mixing ratios. Assuming a mean δ2H value of −91 ‰ (n = 9) for samples collected along the losing reach of the Virgin River in the Virgin River Gorge (Table 1; Metcalf 1995) and a mean δ2H value of −100 ‰ for four springs at elevations between 2000 and 2200 m located north of the study area (Appendix S1), the mean δ2H value of about −94 ‰ (n = 14) for Littlefield Springs indicates a 67:33 mixture of seepage from the Virgin River and groundwater from the carbonate aquifer.

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Figure 4. Relation between δ18O and δ2H values in groundwater and surface water in the Virgin River Gorge subarea, Arizona, Nevada, and Utah.

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Like stable isotopes, noble-gas concentrations also provide insight into the recharge conditions of water emanating from Littlefield Springs. Both low total dissolved-gas pressures (about 0.9 to 1.0 atm; Table S1) and Δ20Ne values of −3 to −22 % for most of the Littlefield Springs samples suggest some degassing subsequent to recharge (Aeshbach-Hertig et al. 2008). Acceptable χ2 fits of <3.8 between measured 40Ar, 84Kr, 20Ne, and 129Xe concentrations and CE model calculations for all of the Littlefield Springs sites, however, indicate minimal gas stripping (Table 2). The average noble-gas recharge temperature (Trave) for Littlefield Springs is about 16 °C, which is slightly cooler than the 3-year mean Virgin River temperature of 17 °C measured at VR-8 and consistent with a mixture of waters from the Virgin River and high-altitude recharge in the carbonate aquifer. This mean value excludes LFS-2 because dissolved-gas re-equilibration likely occurred at this spring (Appendix S1).

Table 2. Noble Gases in Groundwater in the Virgin River Gorge Subarea, Arizona, Nevada, and Utah
SiteDateNoble GasesCE Model Results
4He (cm3 STP/g)R/Ra20Ne (cm3 STP/g)40Ar (cm3 STP/g)84Kr (cm3 STP/g)129Xe (cm3 STP/g)Δ20Ne (%)χ2FATrmax Using Site Altitudea (°C)Trmin Using Maximum Recharge Altitudeb (°C)Trave Using Median Altitudec (°C)Calculated 4HeNUC (cm3 STP/g)
  1. Notes: All samples collected in diffusion samplers except CPW, which was collected in a copper tube. R/Ra: 3He:4He ratio compared to atmosphere. χ2 is the chi-squared statistical fit. Δ20 Ne is neon excess. χ2, F, and A were calculated using site altitude (Trmax).

  2. a

    Site altitude is given in Table S1.

  3. b

    Maximum estimated water-table altitude is 1900 m.

  4. c

    Estimated mean altitude is median of site altitude and maximum recharge altitude.

  5. d

    Without 84Kr.

LFS-1June 25, 20091.6E−060.281.42E−072.73E−043.55E−082.29E−09−9.70.011.130.04019.515.417.51.5E−06
LFS-2June 24, 20091.0E−070.461.44E−072.50E−043.09E−082.05E−09−4.81.131.060.11725.020.323.26.1E−08
LFS-3June 23, 20092.3E−050.091.23E−072.67E−043.43E−082.40E−09−22.01.021.520.01118.314.016.32.3E−05
LFS-4June 23, 20092.5E−050.091.40E−072.98E−043.09E−082.64E−09−13.80.001.310.01015.3d11.0d13.3d2.5E−05
LFS-5June 24, 20095.9E−060.081.48E−072.77E−043.82E−082.55E−09−7.52.391.100.11616.713.415.65.9E−06
LFS-6June 19, 20129.8E−060.091.41E−072.99E−044.03E−082.42E−09−11.51.800.000.00017.012.014.49.7E−06
LFS-6 repl.June 19, 20121.0E−050.091.50E−073.16E−044.12E−082.48E−09−6.92.590.000.00016.010.013.41.0E−05
LFS-7June 20, 20122.3E−050.091.47E−073.14E−044.03E−082.64E−09−9.00.760.000.00015.711.313.12.3E−05
LFS-8June 20, 20123.6E−070.181.52E−072.88E−043.59E−082.36E−09−3.50.291.040.50019.115.617.73.2E−07
LFS-9June 19, 20122.4E−060.101.44E−072.92E−043.74E−082.44E−09−8.90.080.000.00018.513.815.72.3E−06
LFS-10June 20, 20126.7E−070.141.58E−072.66E−043.19E−081.94E−097.50.010.860.07827.823.625.36.3E−07
LFS-11June 20, 20124.4E−060.091.43E−072.89E−043.79E−082.31E−09−9.50.880.000.00019.114.416.34.4E−06
LFS-12June 20, 20121.3E−050.091.44E−073.03E−043.77E−082.61E−09−10.31.302.000.00016.811.914.21.3E−05
LFS-13June 20, 20122.7E−060.261.54E−073.04E−044.07E−082.38E−09−3.62.500.000.00017.414.715.72.1E−06
LSTFebruary 3, 20104.6E−081.011.70E−073.18E−044.11E−082.69E−0911.80.000.620.00013.311.712.51.5E−09
CPWFebruary 4, 20101.8E−060.121.73E−073.20E−043.88E−082.73E−099.01.630.000.00016.111.313.71.8E−06

14C activities of 7.6 to 86 pmC (Table 1) indicate a large range in groundwater age within the Virgin River Gorge subarea. The lowest activities (7.6 to 20 pmC) were measured in Littlefield Springs and VR-DS, consistent with values of 7.8 and 8.9 pmC reported by Hershey et al. (2010). In contrast, samples from CPW, LST, and VR-11 ranged from 63 to 86 pmC. Using measured δ13Caq, an estimated soil-gas δ13C value of −15.8 ‰, and an assumed δ13C value for aquifer materials of 1.5 ‰, the Ingerson and Pearson and the Fontes and Garnier models both yielded plausible ages of 0 and 1900 years, respectively, for LST. This is consistent with other data from the site indicating pre-1950s but late Holocene recharge: no detectable 3H (0.0 TU), low 4HeNUC (1.5 × 10−9 cm3 STP/g), and a high R/Ra value (1.01). Using these same models, the corrected 14C ages of Littlefield Springs are 1 to 9 ka. Assuming these are mixed waters, the carbonate aquifer component is likely much older.

Large 4HeNUC concentrations and low 3He:4He ratios similarly indicate that Littlefield Springs contain old groundwater. 4HeNUC of up to 2.5 × 10−5 cm3 STP/g (Table 2) are higher than previously published values for the Great Basin regional carbonate aquifer. This is supported by R/Ra values for 3He:4He as low as 0.09. A well in the carbonate aquifer of Rush Valley (western Utah) with 4HeNUC of 1 × 10−6 cm3 STP/g and 14C activity of 2.2 pmC yielded an apparent age of 20 to 30 ka (Gardner and Kirby 2011). Wells in the carbonate aquifer in the Snake Valley area of Utah and Nevada have 4HeNUC of 10−6 to 10−7 cm3 STP/g, 14C activities < 4 pmC, and corrected 14C ages >10 ka (Phillip Gardner, U.S. Geological Survey, personal communication, 2011). Thomas et al. (2003) reported 4HeTOT values for carbonate-aquifer springs in southern Nevada up to 1.8 × 10−6 cm3 STP/g and 14C activities of 2.2 to 3.0 pmC. The combination of high 4HeNUC, low 14C activities, and likely dilution with young river water suggests that the carbonate aquifer component of Littlefield Springs may be older than 10 ka.

CFC concentrations show that Littlefield Springs also contain some modern groundwater. Measured CFC and SF6 concentrations in groundwater and Virgin River samples were converted to equivalent atmospheric concentrations (Table 3) based on Henry's Law solubility using the interpreted noble-gas data, including recharge temperature and pressure, recharge salinity, and excess air. Recharge temperatures for the groundwater sites used in the conversion to equivalent atmospheric concentrations ranged from 13 to 25 °C (based on the Trave values given in Table 2). Estimated recharge pressure was determined from altitude/barometric relations using median estimated recharge altitude (average of Hmeas and Hmax) for each site. For the groundwater sites, these altitudes ranged from 1220 to 1580 m above sea level; the land-surface altitude of 650 m was used for VR-11. Estimated recharge salinities ranged from 2000 to 3000 mg/L and were based on binary mixtures of two recharge components: Virgin River seepage loss and carbonate aquifer groundwater (represented by LST). Groundwater excess-air values of 0.0 were based on the unfractionated model (UF) noble-gas concentrations. Figure 5 shows the relation between equivalent atmospheric concentrations of CFC-112 and CFC-13 for water samples collected within the study area. The solid black piston-flow line represents the hypothetical case where groundwater in equilibrium with atmospheric CFCs moves along one flowpath from the recharge area to the point of discharge with minimal longitudinal dispersion and mixing (Zuber 1986; Cook and Böhlke 2000). The colored lines represent binary mixtures of waters from two distinct recharge areas; their points of intersection with the piston-flow line indicates expected CFC concentrations of water that equilibrated with air during particular years (e.g., 1980, 1983, 1985, etc.). Based on their CFC-12 and CFC-113 concentrations, Littlefield Springs may be described with a lumped parameter binary piston-flow model (Eberts et al. 2012) in which young (1980s) recharge is mixed with pre-modern CFC-free groundwater (pre-1950s recharge). The component of modern water in this mix ranges from 31 to 89% (Table 3, excluding LFS-2), with a mean value of 57 ± 16% (1σ). This was determined by dividing the equivalent atmospheric CFC concentration by the measured atmospheric CFC concentration for the estimated recharge year (binary mixing lines in Figure 5). Assuming the modern component is seepage from the Virgin River, groundwater travel times range from 24 to 29 years (26 ± 1.6 years), indicating mid-1980s recharge. CFC concentrations from VR-11 indicate 100% recharge in 2010, as would be expected with water in equilibrium with the atmosphere.

Table 3. Chlorofluorocarbon and Sulfur Hexafluoride Concentrations and Age Estimates for Samples Collected from the Virgin River Gorge Subarea, Arizona, Nevada, and Utah
SiteDateCFC-11 Water Conc. (pmol/kg)CFC-12 Water Conc. (pmol/kg)CFC-113 Water Conc. (pmol/kg)CFC-11 Atm Conc. (pptv)CFC-12 Atm Conc. (pptv)CFC-113 Atm Conc. (pptv)CFC-12/ CFC-113 Binary Mix: Modern Component Recharge YearCFC-12/ CFC-113 Binary Mix: Modern Component Age (Years)CFC-12 Percent of Modern ComponentCFC-113 Percent of Modern ComponentSF6 Water Conc. (fmol/kg)SF6 Atm Conc. (pptv)SF6 Binary Mix: Modern Component Recharge Year
  1. Note: “Excess’ indicates that excess SF6 precluded SF6 dating.

  2. a

    Likely too large because of atmospheric re-equilibration prior to sample collection.

  3. b

    Using estimated recharge temperature of 15.0 °C.

  4. c

    CFC-113 degradation likely occurred.

LFS-1June 25, 20090.7020.5870.04658.518112.919802959561.686.75Excess
LFS-2June 24, 20091.381.190.15613441952.319872298a97a1.577.58Excess
LFS-3June 23, 20090.5820.4890.05443.113614.419852436341.887.21Excess
LFS-4June 23, 20090.5210.4720.05233.511410.919842631311.796.05Excess
LFS-5June 24, 20090.7731.090.09959.328022.819812889882.238.21Excess
LFS-6January 12, 20121.381.100.13096.929030.31985277875
LFS-7January 12, 20121.010.810.10166.520121.91986275654
LFS-8January 12, 20121.401.000.144102b271b34.8b1988256161
LFS-9January 13, 20120.9290.9640.13467.426232.31987256160
LFS-10January 13, 20121.080.9760.13378.1b265b32.0b1988245953
LFS-11January 13, 20120.9390.8200.10968.122326.31986265555
LFS-12June 20, 20120.7340.5690.07256.516318.41985274343
LFS-13June 20, 20121.210.8930.11493.225629.21986266162
CPWFebruary 4, 20101.241.560.21587.341249.919872395920.9032.871995
LSTFebruary 3, 20100.8411.960.23255.648749.919902010068c2.306.762010
VR-11February 4, 20102.932.710.44514953673.42010010097
image

Figure 5. Comparison of CFC-12 and CFC-113 equivalent atmospheric concentrations for groundwater and surface water in the Virgin River Gorge subarea, Arizona, Nevada, and Utah.

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Alternatives to a Binary Mixing Model

The simplest conceptualization of the mixed groundwater emanating from Littlefield Springs is a lumped parameter binary piston-flow model (BPP). Alternatively, seepage loss along the losing reach of the Virgin River above Littlefield Springs may be described as exponential mixing (E), defined as an infinite number of coalescing groundwater flow paths having an exponential age distribution and typified by uniform recharge to an unconfined aquifer (Cook and Böhlke 2000). The location of the Littlefield Springs samples between the exponential and piston-flow curves on the CFC-12/CFC-113 plot (Figure 5) indicates that in addition to the BPP, these samples could be consistent with BEP (binary model with piston-flow and exponential components), or DM (dispersion models). Such lumped parameter models have been previously invoked when interpreting large multiple-tracer data sets (Eberts et al. 2012). Exponential mixing and then piston flow (EP) would best represent recharge from Virgin River seepage loss if this recharge were relatively evenly distributed. EP has an initial exponential age distribution similar to an exponential mixing model, but is then shifted in time as this assemblage moves toward the discharge location under piston-flow conditions without receiving additional recharge (Cook and Böhlke 2000; Böhlke 2006). Recent seepage studies indicate, however, that almost half of this seepage loss is focused along a short (6-km) reach in the Virgin River Gorge containing known fault zones, including the Grand Wash Fault and Sullivans Canyon Fault (Figure S2). Also, an exponential age distribution would result in a trend of decreasing 3H concentrations (longer and older flow paths) with distance downstream along the 10-km reach of the spring complex, assuming aquifer homogeneity and uniform seepage loss from the Virgin River. In contrast, the similarity in measured 3H concentrations in water from LFS-1 through LFS-13 (0.8 ± 0.2 TU) suggests piston flow from a focused point source rather than an exponential mixture of a large range of flow paths and travel times from the river. An EP model may also describe the older carbonate aquifer component. Exponential mixing could be occurring with in the recharge zone in the northern part of the study area (Figure 1), followed by piston flow as this water moves towards Littlefield Springs. Because this groundwater is likely much older than 60 years and would be CFC-free, an EP mixing model for the carbonate aquifer component could not be evaluated with environmental tracers collected during this study.

Evaluating Virgin River Tritium Content

Virgin River samples VR-8 and VR-11 had 3H concentrations of about 2.0 TU (Table 1). These are lower than current concentrations in precipitation of about 6 TU (Figure S4). The 3H concentrations of Littlefield Springs (0.8 ± 0.2 TU; n = 14) indicate some modern recharge, but older than the Virgin River. The other Virgin River sample, VR-DS, had a lower 3H concentration than VR-8 or VR-11 because it was collected when there was no flow in the river above Littlefield Springs. The estimated 60:40 binary mixture of Virgin River and carbonate aquifer water derived from other environmental tracers (Table 4) and a mean travel time of 26 years based on CFCs (Table 3) can be used for evaluating the initial tritium concentration of seepage from the Virgin River. It is likely that the carbonate-aquifer component of flow discharging from Littlefield Springs is tritium-free based on the long groundwater flow path from the recharge area in the northern part of the study area to Littlefield Springs (Figure 1). Using the mean 3H concentration of Littlefield Springs (0.8 TU), the 60:40 mixture implies that the decay-corrected 3H concentration of Virgin River seepage prior to mixing with the older groundwater was about 1.4 TU. Assuming seepage occurred around 1985, followed by 26 years of radioactive decay (estimated travel time of Virgin River seepage reaching Littlefield Springs), the initial 3H concentration in the losing reach of the Virgin River would have been about 6 TU. This is about one-third of the estimated atmospheric tritium content of 20 TU for 1985 (Figure 2). Virgin River 3H concentrations reported by Trudeau et al. (1983) between 1976 and 1977 have a mean value of 17 TU (excluding those with uncertainty exceeding 30%). Comparing this mean value with the estimated atmospheric tritium content of 52 TU for 1976–1977 yields the same ratio of one-third. This indicates a predominantly tritium-free groundwater source to the Virgin River upstream of the study area, consistent with the discharge of large quantities of old groundwater (14C ages of 1 to 4 ka) from springs along the Virgin River in Zion National Park (Heilweil and Freethey 1992; Kimball and Christensen 1996). This predominantly old Virgin River water is further supported by a comparison of 3H and CFC-12 equivalent atmospheric concentrations (Figure 6), showing that water discharging from Littlefield Springs contains less 3H than expected based on CFC-12 concentrations. Assuming a binary mix of pre-modern carbonate aquifer water with Virgin River seepage loss having a mid-1980s apparent age, the samples would plot between the 1980 and 1990 mixing lines if the Virgin River contained only modern precipitation. VR-11 and CPW (containing seepage loss from the adjacent Virgin River) also have lower 3H concentrations than expected based on CFC-12 concentrations. In contrast to dissolved gases such as CFCs, 3H is a conservative tracer when in contact with the atmosphere. While the low 3H concentrations show that the river water is primarily derived from upstream discharge of old groundwater, higher CFC concentrations are caused by atmospheric re-equilibration of dissolved gases in the river.

Table 4. Estimated Percent of Virgin River Seepage in Littlefield Springs Discharge
AnalysisPercent from the Virgin River Seepage
CFC-12:CFC-11357 ± 16 (average of 11 values)
Cl:F56
Cl:Br68
δ2H67
Average60 ± 7
image

Figure 6. Comparison of 3H and CFC-12 equivalent atmospheric concentrations for groundwater and surface water in the Virgin River Gorge subarea, Arizona, Nevada, and Utah. 3H values for the piston-flow line are atmospheric tritium concentrations decay-corrected to 2010.

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Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Hydrogeologic Conceptualization
  5. Methods
  6. Results and Discussion
  7. Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

Geochemical and environmental tracer data from groundwater and surface-water sites in the Virgin River Gorge subarea suggest that discharge from Littlefield Springs is a mixture of modern (post-1950s) seepage from the Virgin River upstream of the springs with older groundwater in the regional Great Basin carbonate aquifer. Using a lumped-parameter model, this is interpreted to be a binary mixture of two piston-flow components (BPP), although exponential mixing within both recharge areas is possible. Assuming the BPP model, the mixture is estimated to be about 60% seepage from the Virgin River and 40% from the regional carbonate aquifer based on (a) Cl/F and Cl/Br mass ratios from Littlefield Springs indicating mixtures containing 56 and 68% river water, respectively, (b) a deuterium mass balance indicating 67% river water, and (c) CFC-12 and CFC-113 concentrations indicating a mixture of 57 ± 16% river water (Table 4). This suggests that about 0.9 m3/s of the estimated 1.6 m3/s discharge from Littlefield Springs is derived from the Virgin River, similar to the 0.93 ± 0.17 m3/s current study estimate for upstream Virgin River seepage loss. CFC concentrations indicate that the subsurface travel time of this seepage loss is about 26 ± 1.6 years before re-emerging at Littlefield Springs. δ2H and δ18O values for Littlefield Springs are consistent with a mixture of isotopically depleted high-altitude carbonate aquifer recharge with enriched Virgin River water. Littlefield Springs 14C, 3H, and 87/86Sr values are generally higher than previously reported values for carbonate aquifer groundwater and lower than for Virgin River water, supporting this mixing interpretation. Calculated noble-gas recharge temperatures for Littlefield Springs are generally cooler than the mean annual temperature of water in the Virgin River and are consistent with the relatively depleted stable-isotope concentrations, indicating a high-altitude recharge source area for the carbonate aquifer. This is supported by the majority of BCM recharge occurring in the northern part of the study area, along with hydrogeologic cross sections and potentiometric contours that indicate both permeable path ways and hydraulic gradients for north-to-south groundwater flow. Our conceptual model advances the knowledge of this groundwater system by integrating new environmental tracer data, additional water-budget calculations, hydrogeologic interpretations, and regional hydraulic gradients. These conclusions suggest that removal of high-salinity sources to the Virgin River upstream of the Gorge would cause a reduction of salinity in water discharging from Littlefield Springs into the Virgin River (eventually entering the Colorado River at Lake Mead) within a timeframe of several decades.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Introduction
  4. Hydrogeologic Conceptualization
  5. Methods
  6. Results and Discussion
  7. Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

This work was funded in part by the Colorado River Basin Salinity Control Forum. The authors thank Alan Flint, U.S. Geological Survey, for providing Basin Characterization Model results for the Virgin River Gorge subarea; J.K. Böhlke for his interpretive assistance with environmental tracer mixing models; Niel Plummer, Ed Busenberg, Jerry Casile, and Julian Wayland, U.S. Geological Survey, for their laboratory analysis and interpretive assistance of CFC and SF6 samples; D. Kip Solomon and Alan Rigby, University of Utah Dissolved-Gas Service Center, for their laboratory analysis and interpretive assistance of noble-gas and tritium samples; and R. Michel, U.S. Geological Survey, for providing recent atmospheric tritium data from Albuquerque, New Mexico.

References

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  2. Abstract
  3. Introduction
  4. Hydrogeologic Conceptualization
  5. Methods
  6. Results and Discussion
  7. Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information
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Authors' Note: The authors do not have any conflicts of interest or financial disclosures to report.

Supporting Information

  1. Top of page
  2. Abstract
  3. Introduction
  4. Hydrogeologic Conceptualization
  5. Methods
  6. Results and Discussion
  7. Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information
FilenameFormatSizeDescription
gwat12030-sup-0001-TableS1.pdfPDF document62KTable S1. Field measurements in groundwater and surface water from the Virgin River Gorge subarea, Arizona, Nevada, and Utah.
gwat12030-sup-0002-TableS2.pdfPDF document8KTable S2. Selected chemical constituents in groundwater and surface water from the Virgin River Gorge subarea, Arizona, Nevada, and Utah.
gwat12030-sup-0003-FigureS1.pdfPDF document354KFigure S1. Spatial distribution of average (1971–2000) precipitation in the Virgin River Gorge subarea, Arizona, Nevada, and Utah, based on Parameter-Elevation Regressions on Independent Slopes Model data (PRISM Group 2006).
gwat12030-sup-0004-FigureS2.pdfPDF document1962KFigure S2. Geology of the Virgin River Gorge subarea, Arizona, Nevada, and Utah.
gwat12030-sup-0005-FigureS3.pdfPDF document87KFigure S3. Generalized hydrogeologic cross sections within the Virgin River Gorge subarea, Arizona, Nevada, and Utah.
gwat12030-sup-0006-FigureS4.pdfPDF document32KFigure S4. Estimated (decay-corrected) atmospheric concentrations of tritium in precipitation for the Virgin River Gorge subarea, Arizona, Nevada, and Utah, 1950 through 2010.
gwat12030-sup-0007-FigureS5.pdfPDF document38KFigure S5. Comparison of CFC-11 and CFC-12 equivalent atmospheric concentrations for groundwater and surface water in the Virgin River Gorge subarea, Arizona, Nevada, and Utah.
gwat12030-sup-0008-FigureS6.pdfPDF document33KFigure S6. Comparison of SF6 and CFC-12 equivalent atmospheric concentrations for groundwater and surface water in the Virgin River Gorge subarea, Arizona, Nevada, and Utah.
gwat12030-sup-0009-AppendixS1.docxapplication/word40KAppendix S1. (a) Previous studies; (b) Hydrogeology, groundwater budget, Darcy calculations; (c) Collection and analysis methods; (d) Deuterium mass balance; (e) Evidence of dissolved gas re-equilibration, degradation, or crustal excess; (f) 14C for mixing calculations; (g) Comparison of findings to previous reports.

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