Natural uranium and strontium isotope tracers of water sources and surface water–groundwater interactions in arid wetlands – Pahranagat Valley, Nevada, USA
Graphical abstract
Introduction
An understanding of water budgets is critical for proper management of wetland resources. This can be difficult if contributions from different water sources are impossible to monitor due to diffuse discharge from phreatic water tables or complex patterns of surface flow. Geochemical methods employing the use of chemical concentrations and light stable isotopes can be used to help identify distinct mixing end members. However, identification of unique components contributing to wetlands in arid or semi-arid environments may be further complicated by physically and biologically mitigated processes that modify compositions of surface flow and shallow groundwater. Because both water chemistry and light stable isotopes can be strongly altered in these environments, more robust natural tracers that retain their unique signatures regardless of the intensity of near-surface processes are needed to identify various water sources and trace their interactions in these complex hydrologic settings.
This study focuses on radiogenic isotopes of Sr (87Sr/86Sr) and U (234U/238U). Both isotope systems have been used as natural tracers of hydrologic processes in a variety of settings (Négrel and Lachassagne, 2000, Négrel et al., 2001, Négrel et al., 2003, Négrel et al., 2004, Paces et al., 2002, Barbieri and Morotti, 2003, Frost and Toner, 2004, Barbieri et al., 2005, Suksi et al., 2006, Wang et al., 2006, Hogan et al., 2007, Martin and Moore, 2008, Chkir et al., 2009, Jin et al., 2009, Ryu et al., 2009, Gao et al., 2010, Li et al., 2010, Rango et al., 2010, Tichomirowa et al., 2010, Gourcy and Brenot, 2011, Vinson et al., 2011), but they have less commonly been considered in combination (Barbieri and Voltaggio, 1998, Roback et al., 2001, Durand et al., 2005). Use of these heavy radiogenic isotopes as natural hydrologic tracers has several distinct advantages over more conventional geochemical approaches: (1) both are abundant trace elements in most natural waters, (2) both have isotopic compositions that can vary widely in different hydrologic reservoirs, (3) isotope ratios of Sr and U are easy to measure at high levels of precision and accuracy, and (4) mass differences between isotopes are small compared to the overall mass of each element allowing ratios to remain unfractionated despite high rates of evaporation and transpiration present at desert oases. Therefore, the combination of Sr and U isotopic compositions in surface waters has the potential to provide a particularly powerful tool for evaluating source contributions in wetlands from arid settings where processes such as pan evaporation, plant transpiration, mineral precipitation, and solute gains and losses via sorption–desorption can have dramatic effects on compositions of near-surface water. Furthermore, these data can be used to establish mixing proportions for water present at different locations within the wetland even though multiple sources may be present, and track changes in those proportions in space and time.
To demonstrate the utility of this isotopic tool in an arid wetland environment, this study investigated samples of surface flow and shallow groundwater from a small wildlife refuge in southeastern Nevada using a combination of chemical and isotopic data. Hydrologic budgets for both agricultural activities and wetland habitats in the Pahranagat Valley (Fig. 1) depend on flow from several high-volume springs discharging from a regionally sourced carbonate aquifer as well as a number of small-volume springs and discharge areas in the valley. The relative contributions of water from those sources to the Pahranagat National Wildlife Refuge at the southern end of the valley (hereafter called the Refuge) are not well determined. The Refuge contains approximately 21.8 km2 of wet-meadows, marsh, open water, grasslands, and upland deserts and provides habitat for both migratory and resident bird species. Although relatively small, the Refuge is important given the paucity of wetland habitat in the region.
Increased demands for water resources by a growing population in the Las Vegas urban corridor have resulted in proposals to withdraw substantial amounts of groundwater from basins upgradient of Pahranagat Valley (Deacon et al., 2007, Southern Nevada Water Authority, 2011). Increased withdrawals from the regionally sourced carbonate aquifer could affect interbasin flow, resulting in lower discharge volumes and decreased surface flow in the valley. Because the Refuge is located at the downstream end of the valley’s surface and groundwater flow systems, the effects of reduced discharge from the regionally sourced aquifer is likely to affect water availability on the Refuge before other areas in the valley. To evaluate the possible effects of water-resource development on the high-quality wetland habitat at the Refuge, the U.S. Fish and Wildlife Service initiated an extensive hydrologic characterization of the Refuge between 2007 and 2009 that included an evaluation of the distribution and historic trends of wetland habitats, a description of surface-water and groundwater hydrology, and a history of water management on the refuge (Wurster, 2010). Geochemical data in this report were collected to support that effort.
The objective of the current study is to demonstrate that the combined use of Sr and U isotopes provides a robust method for evaluating the hydrologic contributions from both regional (interbasin) and local (intrabasin) groundwater sources supplying wetland habitats in an arid environment. Chemical and stable-isotope data for sites sampled in this report show a substantial degree of complexity caused by near-surface physical and chemical processes, making interpretation of water sources difficult. In contrast, patterns observed for combined Sr and U isotope ratios are much simpler to interpret and highlight their benefits as hydrologic tracers. We also show how simple binary mixing equations can be used to quantify relative contributions from compositionally distinct water sources. Finally, data presented here provide baseline chemical information for future changes in the hydrology of Pahranagat Valley caused by anthropogenic activities or shifting climate conditions.
Section snippets
Regional hydrogeology
Pahranagat Valley (Fig. 1) is a 70 km long by 11 km wide linear basin oriented roughly north–south within the Basin and Range Geological Province (Jayko, 2007, Sweetkind et al., 2007). Paleozoic marine carbonate rocks form an extensive, regionally sourced aquifer covering much of the southeastern part of Nevada (Mifflin, 1968, Winograd and Thordarson, 1975, Thomas et al., 1996, Knochenmus et al., 2007, Welch et al., 2007). These rocks are exposed in mountain ranges reaching altitudes of 3000–4000
Hydrochemical results
Water from 14 sites including springs, irrigation ditches, shallow monitor wells, and surface-water impoundments were used to characterize sources and evaluate mixing relations in the Valley and on the Refuge (Table 1). Wells were installed in 3- to 4-m-deep, hand-augered boreholes using 3.2-cm diameter polyvinyl chloride (PVC) pipe with 0.25-mm slotting in the lower half. Their primary purpose was to monitor fluctuations of the potentiometric surface at different times of the year. Wetland
Evaporation and transpiration
Evaporation is responsible for large shifts to heavy δ2H and δ18O values observed for surface water samples (Fig. 2A); however, samples with the heaviest H and O isotopes do not have the highest solute contents (Fig. 2C). Surface water from UPL outlet, Main Ditch, and Whin Dike define a steeply sloping Cl-versus-δ18O trend in Fig. 2C. Shallow groundwater samples from well NT11 located on the east margin of the Middle Marsh (Fig. S4 in supplement) also plot close to this trend. In contrast,
Conclusions
The combined use of Sr and U isotopic compositions of water in Pahranagat Valley represents a powerful tool for quantifying contributions of water from several different sources. Although the chemical and stable-isotope compositions of groundwater constituting these sources remain constant at discharge sites, evaporation during surface flow as well as near-surface physical and chemical processes in shallow groundwater within wetland areas (transpiration, evaporation, mineral precipitation, ion
Acknowledgements
This study was supported by funding from the Southern Nevada Public Lands Management Act Round 6. The authors wish to thank Kiyoto Futa, Loretta Kwak, Thomas Oliver, Cayce Gulbransen, Leonid Neymark, Craig Johnson, and Craig Stricker for assistance in collecting chemical and isotope data, and James King, Chris Morris, and Jim Docktor for collecting hydrologic data. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
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