Data Series 280
U.S. GEOLOGICAL SURVEY
Data Series 280
The Nevada Test Site (NTS), occupying approximately 3,500 km2 in southern Nevada (fig. 1), has been the primary site for underground testing of nuclear weapons by the United States since the late 1950’s. Radioactive byproducts have been emplaced above and below the regional water table as a result of this testing (Laczniak and others, 1996, p. 2). In an effort to understand the movement of these radioactive contaminants (radionuclides) away from areas of underground testing, the U.S. Department of Energy (DOE) has established an Environmental Restoration Program to investigate the hydrologic pathways and travel times associated with regional ground-water flow beneath the NTS and vicinity (U.S. Department of Energy, 1991, p. 2). Fractures in Middle Cambrian through Devonian limestones and dolomites constituting the regional lower carbonate aquifer (LCA) are the most likely pathways for the transport of radioactive nuclides (Winograd and Thordarson, 1975, p. 119; Laczniak and others, 1996, p. 15).
Geochemical and isotopic data from ground-water samples are being used to evaluate possible ground-water flow paths in the DOE Environmental Restoration Program (Thomas and others, 2002; Hershey and others, 2005; Rose and others, 2006; Farnham and others, 2006). Conservative tracers (isotopes or elements that undergo negligible modification by water-rock interaction) typically are used to delineate ground-water flow paths and evaluate mixing of water masses from upgradient sources. These tracers are combined with other dissolved constituents in advection-reaction geochemical modeling (NETPATH, PHREEQC) to evaluate chemical evolution along postulated ground-water flow paths. However, to understand the potential contributions of rock and mineral constituents, chemical compositions of aquifer components are required as inputs in geochemical models. Isotope tracers of geochemical processes can help discriminate between alternative flow models whose solutions may remain non-unique based on chemical concentrations alone. Like other chemical constituents, an understanding of the range of compositions in both water and rock is required for constraining flow paths and for evaluating possible aquifer-water interactions.
The isotopic composition of strontium (87Sr/86Sr) can be used as a natural tracer of ground-water flow because the strontium ion (Sr+2) is soluble in aqueous solutions at levels allowing high-precision isotope measurement, because variations in rubidium-to-strontium (Rb/Sr) ratios in rocks of different type and petrogenetic origin result in distinctive 87Sr/86Sr ground-water signatures, and because fractionation of heavy radiogenic isotopes (87Sr from 86Sr) is negligible for geologic processes (Peterman and Stuckless, 1992; McNutt, 2000). However, unlike conservative tracers, 87Sr/86Sr values can be modified along ground-water flow paths by reaction with the aquifer rock. Studies have shown that Sr dissolved in ground water may not be in isotopic equilibrium with the aquifer through which it flows (Peterman and Stuckless, 1992, and references therein). This disequilibrium commonly provides important insight into the ground-water flow system for which conceptual models may otherwise be oversimplified. For example, in several carbonate aquifer studies, 87Sr/86Sr ratios in ground-water samples are larger than those of the aquifer rock (Chaudhuri and others, 1987; Banner and others, 1989, p. 396; McKenna and others, 1992; Peterman and others, 1992, p. 827; Frost and Toner, 2004). This disequilibrium requires ground-water contributions from associated Precambrian rocks having higher 87Sr/86Sr signatures, but which typically were considered to be impermeable.
The behavior of Sr and 87Sr/86Sr in ground-water flow systems depends on numerous factors including length of flow path, water-to-rock mass ratio, chemical reaction rates, Sr concentration and 87Sr/86Sr in solid and fluid phases, flow velocity, advective fluid mixing, dispersion, and ion exchange (Johnson and DePaolo, 1994, 1997). In aquifers dominated by fracture flow, the Sr isotopic composition of water is less likely to be modified by changes in rock 87Sr/86Sr values along a flow path than in aquifers dominated by matrix flow because of faster flow velocities, larger water-to-rock mass ratios, and slower reaction rates (Johnson and others, 2000). Therefore, in aquifers dominated by fracture flow or high permeability matrix flow, such as alluvium, Sr may be useful as a tracer of water from upgradient sources (Johnson and DePaolo, 1994, p. 1576), at least at path lengths commensurate with the scale of the NTS.
Chemical processes resulting in nonconservative behavior of Sr in ground water affect Sr concentrations to a greater extent than 87Sr/86Sr values. Losses of Sr from solution due to sorption and mineral precipitation will not change the 87Sr/86Sr ratio in the remaining fluid. Furthermore, desorption or cation exchange from reactive mineral surfaces along flow paths may not significantly affect 87Sr/86Sr values if the dynamics of flow and sources of Sr have remained unchanged over long periods (steady-state flow conditions). In contrast, rock dissolution can add Sr containing very different isotopic compositions to solution, particularly if ground water that is unsaturated, with respect to calcite or dolomite, flows into the LCA.
Use of 87Sr/86Sr as a tracer in advection-reaction geochemical models of ground-water flow in the LCA requires knowledge of rock Sr concentrations and 87Sr/86Sr compositions. The structures of minerals constituting marine carbonate deposits result in rocks with relatively high Sr concentrations and low Rb concentration. Therefore, the 87Sr/86Sr values inherited from seawater will not change appreciably with time due to the decay of 87Rb. However, previous studies demonstrated that at least some Paleozoic carbonate rocks in the NTS vicinity have nonmarine 87Sr/86Sr compositions indicating that radiogenic 87Sr from crustal sources was introduced into the carbonate rocks at some time in the past (Peterman and others, 1994; J. Kenneally, Lawrence Livermore National Laboratory, written commun., 1995). These studies indicate that Paleozoic carbonate rocks at the NTS can have a much wider range of 87Sr/86Sr values that are more radiogenic than values derived from a seawater origin.
Ground water flowing through the LCA on the east side of the NTS commonly has 87Sr/86Sr values that are greater than values expected from seawater sources (Hershey and others, 2005; Farnham and others, 2006). Advection-reaction geochemical models of ground-water evolution between sample sites commonly require small amounts of mineral dissolution that would likely affect the 87Sr/86Sr of dissolved Sr in the water. However, because 87Sr/86Sr compositions of most carbonate rocks on the NTS generally are not available, the influence of water-rock reaction during chemical evolution along flow paths cannot be evaluated effectively. Therefore, it is necessary to better constrain rock 87Sr/86Sr values present along potential flow paths to more reliably utilize 87Sr/86Sr as a ground-water tracer.
Because ground-water flow through the LCA represents the most likely scenario for transporting radionuclides beyond the eastern and southern boundaries of the NTS, characterization of 87Sr/86Sr in Paleozoic carbonate rocks constitutes the focus of this study. A small number of older carbonate rocks as well as a few samples of Paleozoic silicate rocks are also included. Most rocks constituting the LCA were deposited as chemical precipitates from Paleozoic seawater. Because this reservoir is well mixed on a global scale by vertical and lateral circulation patterns, its Sr isotope evolution has been well documented by examining samples from many other areas. Both the stratigraphy of pre-Tertiary rocks and the evolution of seawater 87Sr/86Sr are described in the following sections.
Pre-Tertiary rocks of southeastern Nevada consist of a thick sequence of older (Neoproterozoic to Devonian in age) shallow-water marine sediments deposited in a miogeoclinal (passive continental shelf) environment and younger (Devonian to Mississippian in age) marine sediments deposited in a foreland basin along the Cordilleran continental margin. A generalized stratigraphic succession of the approximately 11,500-m-thick sequence of pre-Tertiary rocks present on the NTS is shown in figure 2. The summary presented below is compiled from more detailed descriptions presented by Burchfiel (1964), Barnes and Christiansen (1967), Stewart and Poole (1974), Winograd and Thordarson (1975), Monsen and others (1992), Laczniak and others (1996), Trexler and others (1996; 2003), Slate and others (1999), and Page and others (2005). Age estimates for stratigraphic units from the NTS and surrounding areas are listed in table 1.
The oldest sedimentary rocks on the NTS consist of a 2,800-m-thick series of Neoproterozoic and lower Cambrian deposits dominated by marine clastic sediments (quartzite, arkose, siltstone, and shale/argillite). Silty carbonate units, such as the Noonday Dolomite and dolomitic units in the Wood Canyon and Johnnie Formations, are present locally in this sequence. The well-cemented to moderately metamorphosed rocks of this sequence generally are considered to be poor transmitters of ground-water flow and constitute the lower clastic confining unit of Winograd and Thordarson (1975). The Lower-to-Middle Cambrian Carrara Formation contains siltstone and shale with thin, but persistent, limestone beds that become more common up-section. These time-transgressive lithologic changes reflect the regional transition to a depositional environment dominated by marine carbonate sedimentation.
The 5,200-m-thick section of Middle Cambrian through Devonian rocks is composed of marine limestones and dolomites with minor interbeds of sandy or silty clastic units (Barnes and Christiansen, 1967; Stewart and Poole, 1974; Trexler and others, 1996, 2003; Slate and others, 1999). Aquifer tests indicate very high degrees of water transmissivity through these units due to extensive secondary permeability along a well-connected network of solution-enhanced fractures, faults, and breccias (Winograd and Thordarson, 1975, p. 117; Laczniak and others, 1996). Collectively, these rocks constitute the LCA and provide the main pathways for ground-water flow between Tertiary basins. Because the few clastic units within this sequence (Dunderberg Shale Member of the Nopah Formation, the Ninemile Formation of the Pogonip Group, and the Eureka Quartzite) are thin and structurally disrupted, they do not constitute regionally effective confining units, though they may impede flow locally (Laczniak and others, 1996, p. 15).
Mississippian strata overlying Devonian and older carbonates of the LCA are mostly clastic rocks (conglomeritic sandstone, siltstone, shale, bioclastic limestone, and rare quartzite) to the west of Yucca Flat and carbonate to the east of the NTS (Winograd and Thordarson, 1975, p. 9; Trexler and others, 1996, 2003). These rocks reflect a change from sedimentation along a passive continental margin to clastic-dominated depositional environments in foreland basins associated with convergent tectonics of the Antler orogeny (Poole, 1974; Cole and others, 1997; Cole and Cashman, 1999; Trexler and others, 2003). Up to 3,000 m of predominantly synorogenic siliciclastic rocks constitute the Late Devonian to Middle Mississippian Eleana Formation and laterally equivalent Gap Wash Formation as well as the overlying Late Mississippian Chainman Shale/Captain Jack Formations (Cole and Cashman, 1999; Trexler and others, 2003). Although silty carbonate beds present in the Late Mississippian Captain Jack Formation (and, to a lesser extent, Chainman Shale) reflect a return to shallow-water marine environments, the thick siliciclastic sediments have low permeability and form the upper clastic confining unit in western Yucca Flat and Jackass Flat that isolates most regional ground-water flow in the LCA (Winograd and Thordarson, 1975, p. 43). Approximately 1,100 m of Pennsylvanian to early Permian Tippipah Limestone are preserved in two localities in the central NTS that unconformably overlie muddy shelf deposits of the Chainman Shale (Cole and Cashman, 1999). Winograd and Thordarson (1975, p. 30) included these rocks in the upper-carbonate-rock aquifer, but recognized their restricted distribution within the saturated zone and the limited role of the upper carbonate aquifer in regional ground-water movement. East of the NTS, the upper clastic confining unit is not present and Paleozoic carbonate rocks, regardless of age, form a single regional carbonate-rock aquifer (Laczniak and others, 1996, p. 15).
The isotopic composition of seawater has evolved through time due to the balance of fluxes from continental (fluvial inputs), mantle (volcanic and hydrothermal activity at mid-ocean spreading centers), and continental shelf (diagenesis and dolomitization of marine carbonate) sources (Peterman and others, 1970; Burke and others, 1982; Faure, 1986; Peterman and Stuckless, 1992; Veizer and others, 1999, Faure and Mensing, 2005, p. 440). Determination of the seawater 87Sr/86Sr evolution curve is possible because Sr concentrations in biogenic carbonate are high (Sr readily substitutes for Ca, especially in the aragonite crystal structure) whereas radioactive 87Rb [the parent isotope for 87Sr with a half-life of 48.8 million years (m.y.)] is excluded. Therefore, seawater Sr is incorporated into the crystal structure of marine calcite or aragonite and remains unchanged with time in rocks and carbonate fossils that have remained unmodified by secondary processes. Values of 87Sr/86Sr for Triassic through Cambrian brachiopods, belemnites, and conodonts and Neoproterozoic carbonate rocks from many locations around the world have been compiled to define the Paleozoic seawater 87Sr/86Sr evolution curve, shown in figure 3 along with bounding curves that represent probable uncertainties for mean seawater compositions at any one time period. Seawater 87Sr/86Sr values fluctuated from a high of around 0.7092 during the late Cambrian Period to a low of around 0.7070 in the late Permian Period with numerous fluctuations in between.
Post-depositional modification of 87Sr/86Sr can occur by several mechanisms including (1) accumulation of 87Sr from the radioactive decay of small amounts of 87Rb incorporated in the rock, especially in finely dispersed clay particles, (2) exchange of Sr with fluids during diagenesis or dolomitization, or (3) introduction of exotic strontium by hydrothermal fluids carrying radiogenic 87Sr derived from basement sources (Peterman and others, 1994). These processes typically drive rock 87Sr/86Sr compositions toward higher (more radiogenic) values. Dolomites with low Sr concentrations may be particularly susceptible to subsequent introduction of exotic Sr.
The purpose and scope of this report are to present chemical and 87Sr/86Sr data from cuttings and core samples of Paleozoic carbonate rocks from NTS boreholes (table 2) where ground-water data are available. Paleozoic rock samples from 20 boreholes in the Yucca Flat, Rainier Mesa, Frenchman Flat, and Mercury Valley were collected in 2005 and 2006 from zones within the LCA that produced water. In addition to these recently acquired data, chemical and isotopic data acquired from the same laboratory also are reported for rock samples obtained in the early and middle 1990s. These samples are from outcrops located outside the NTS and from two boreholes in the southwestern part of the NTS. These data are discussed in Peterman and others (1994), but were not published previously. Data in this report are presented in geospatial, tabular, and graphic forms, and are discussed in terms of the degree to which the compositions may have been modified by post-depositional chemical processes. However, no attempt is made to interpret these data with respect to water-rock interactions, reactive-transport mechanisms, or ground-water flow-path identification.
U.S. Department of the Interior | U.S. Geological Survey
URL: https://pubs.usgs.gov/ds/2007/280/
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