Scientific Investigations Report 2008–5044
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2008–5044
The general approach used to conceptualize ground-water flow through the study area was to delineate the extent of the rocks forming the three primary aquifer types—volcanic, upper carbonate, and lower carbonate (fig. 3). Discrete aquifers identified within each of these aquifer types are classified as either continuous or isolated aquifers (fig. 3). Continuous aquifers are hydraulically connected to adjacent aquifers and together form part of a larger flow system, whereas isolated aquifers are hydraulically restricted and generally drain only to adjacent confining units. One or more continuous aquifers form tributary flow systems (fig. 3), a term used in this report to imply a small or intermediate flow system that feeds water to a more extensive regional flow system. Water levels in each of the continuous aquifers delineated in the study area were contoured to determine general flow directions and interactions with other continuous aquifers and adjacent confining units.
The first step in the flow conceptualization process was to identify and delineate the continuous and isolated aquifers in the RMSM area. These aquifers were identified and mapped using a composite hydrostratigraphic framework developed by merging previously constructed three-dimensional hydrostratigraphic framework models (HFMs) for the RMSM, Yucca Flat, and Pahute Mesa areas (Bechtel Nevada, 2002, 2006; National Security Technologies, LLC, 2007). During each of these framework-development efforts, a base HFM was constructed that represented the geologist’s favored interpretation of the distribution of hydrostratigraphic units across the modeled area. Additionally, several alternative HFMs were developed within each area to represent the different unit distributions that are geologically possible and potentially would alter ground-water flow paths away from areas of underground nuclear testing. In the RMSM HFM report (National Security Technologies, LLC, 2007), one base and four alternative frameworks were developed.
Each HFM is composed of hydrostratigraphic units (HSUs) that consist of one or more stratigraphic units with similar geologic and hydraulic properties. The 45 HSUs identified in the RMSM base and alternative HFMs (National Security Technologies, LLC, 2007) form the hydrogeologic foundation used to develop the conceptualization of ground-water flow presented in this report (fig. 4). The HSUs evaluated as part of this study include 21 aquifers, 22 confining units, and 2 composite units (a combination of aquifer and confining unit).
Framework HSUs were grouped into generalized unit types on the basis of (1) whether the HSU was classified as an aquifer, composite unit, or confining unit; (2) rock type; and (3) stratigraphic position relative to other HSUs (fig. 3). The combining of HSUs reduced the number of subsurface units to seven (figs. 3 and 4). These seven units herein are referred to as subsurface hydrologic unit types, or SHUTs, and include:
1The upper carbonate aquifer SHUT, as defined for this report, is consistent with the upper carbonate aquifer defined by Laczniak and others (1996). This SHUT includes the upper carbonate aquifer HSU (Pennsylvanian-age Tippipah Limestone) and older Devonian-age to Cambrian-age carbonate rocks that structurally overlie the upper clastic confining unit HSU as a result of low-angle faulting (fig. 4).
The three-dimensional configuration and distribution of these SHUTs were developed by constructing and evaluating numerous cross sections and horizontal slices through the HFMs. Based on this evaluation, similar interconnected SHUTs were combined to form the principal aquifer- and confining-unit types of the RMSM area. Three principal aquifer types (referred to as the volcanic aquifer, upper carbonate aquifer, and lower carbonate aquifer) and one confining unit (the regional confining unit) were identified by this process (fig. 3). The volcanic aquifer includes the overlying alluvial aquifer and the volcanic composite unit. Only the saturated part of each aquifer type was mapped and contoured. For example, the volcanic aquifer is not mapped on the east side of Rainier Mesa where it is unsaturated or is known to contain only perched or semi-perched water.
Two HFMs were used to develop the aquifer distributions in the RMSM model area. The primary HFM used in this report is an alternative HFM identified by National Security Technologies, LLC (2007) as the “LCA3 at bottom of Well ER-12-1” alternative model. The only difference between this alternative HFM and the National Security Technologies, LLC (2007) base HFM is in the area of borehole ER-12-1, located just east of Rainier Mesa. Carbonate rock encountered at the bottom of borehole ER-12-1, which was modeled as lower carbonate aquifer in the base HFM, is modeled in the alternative HFM as a local, subhorizontal thrust sheet of carbonate rock (LCA3-1) that structurally is isolated from the lower carbonate aquifer. The alternative HFM is used in this report because the aquifer distribution developed from the alternative is more consistent with hydrologic conditions, as indicated by measured water levels in borehole ER-12-1. A second alternative HFM was used in this report specifically to delineate an alternative extent of the upper carbonate aquifer, which would result in a different interpretation of potential transport. This HFM, identified in National Security Technologies, LLC (2007) as the “No Redrock Valley Caldera” alternative model differs from the base HFM by the absence of the Redrock Valley caldera and its associated deposits and the presence of a more extensive section of upper carbonate aquifer. The Redrock Valley caldera, proposed by National Security Technologies, LLC (2007), is supported by an anomalous basement depression originally identified by Hildenbrand and others (2006) from gravity data but its existence has not been confirmed by borehole data.
In addition to determining the distribution of aquifers in the study area, water levels from 172 discrete open intervals in 84 boreholes (appendix 1) were analyzed. Many of these boreholes are concentrated in areas of past underground testing in the northwestern (Pahute Mesa), north-central (Rainier Mesa), and east-central (Yucca Flat) parts of the study area (fig. 2). Each unique open interval (for example, a temporary packed interval or a monitoring tube installed above a grouted section of a borehole) is referred to as a well in this report. Multi-well boreholes provide information on the changes in water-level altitude with depth. Naming conventions for wells and boreholes referred to in this report are as follows. A well that is the sole completion interval in a borehole is assigned the name of the borehole. In boreholes with multiple completions, well names typically are differentiated from each other by a parenthetical expression added after the borehole name—for example: “UE-12t 6 (1378 ft)”. A single number in the parenthetical expression refers to the depth of the well; two numbers separated by a dash refer to the depth of the top and bottom of the open interval in the well. In some cases, a well name consists of the borehole name and one of three non-parenthetical expressions (main, piezometer, or WW) that follow the borehole name. All well names in the text of this report are enclosed in quotes for clarity.
Approximately 3,400 water levels were measured in the 172 wells from 1957 to 2007. Water levels measured in each well were used to define predevelopment conditions in each aquifer. Each water-level measurement in the study area was reviewed for correctness and accuracy, assigned to the proper open interval, and remarked to document the hydrologic conditions occurring at the time of measurement. The evaluation ensures the integrity of the data and identifies the water levels that best represent predevelopment conditions. A large part of the water-level analysis was supported by on-going and completed comprehensive evaluations of water levels on the NTS (Fenelon, 2000, 2005, 2006). All water levels and well-construction information are stored in the USGS National Water Information System (NWIS) database and can be accessed from the world-wide web at http://waterdata.usgs.gov/nv/nwis/gw.
Hydrographs and locations for the 172 wells can be displayed interactively from a Microsoft® Excel workbook (appendix 1). The workbook is designed to be an easy-to-use tool to view water levels and other associated information for wells in the study area. Information for an individual well can be selected by using the AutoFilter option available in Excel. An example of the information available in the appendix is provided for well “UE-8e (2295 ft)” in figure 5. The information presented on the page includes measurement method, accuracy, and status for each water level.
Nearly all water-level altitudes computed from depth-to-water measurements provided in appendix 1 are considered accurate to within 2 ft. In most cases, actual depth-to-water and land-surface altitude measurements are accurate to 1 ft or less, depending on the method of measurement. Water-level measurement errors caused by borehole deviation generally are less than 0.5 ft. Where errors are larger, the measured water levels were corrected for borehole deviation. The only measurements requiring correction in the study area were those made in boreholes WW-2, UE-10j, and ER-16-1. Water levels in the latter borehole have a borehole-deviation correction of about 80 ft. The magnitude of this correction may be in error by as much as 10 ft because the deviation survey is incomplete.
Water-level altitudes are used to represent the hydraulic head at each well opening. However, hydraulic head is dependent on the density (temperature and salinity) of the water. Wells in the study area that have a long (several thousand feet) water column (appendix 2) in combination with a warm water-column temperature (more than 10°F greater than typical ground-water temperatures of about 80°F) could have a hydraulic head several feet lower than would be computed directly from the depth-to-water measurement. No attempt was made in this report to adjust water-level measurements for variations in water temperature because the potential error in the hydraulic head caused by these temperature differences is considered trivial given water-level contouring intervals of 50 ft and greater.
Water levels from each well were evaluated further to determine if and which water levels represent predevelopment hydrologic conditions. Hydrograph trends were analyzed and water levels that were attributed to unnatural influences such as recent well construction, pumping, or nuclear testing were filtered from the datasets. Of the 172 wells analyzed for this study, 133 of the wells (table 1; appendix 2) from 73 boreholes (fig. 2) had at least one water level identified as being representative of predevelopment conditions.
A single estimate of the water-level altitude was used to represent predevelopment conditions in each of the 133 wells identified as having at least one predevelopment water level (table 1). For wells with multiple measurements, the mean of the predevelopment measurements was used as the predevelopment estimate. A synoptic set of water-level measurements for all wells in the study area would be preferable to using mean water levels but this is not possible because many wells previously measured no longer exist and current hydrologic conditions monitored by some existing wells no longer represent predevelopment conditions. The error associated with comparing water levels that span decades is assumed to be minor because long-term, naturally occurring, water-level fluctuations generally are less than 5 ft. Water levels used to estimate the predevelopment altitude at each of the 133 wells listed in table 1 are shown as red circles on hydrographs that can be plotted interactively by using appendix 1 (fig. 5).
The predevelopment, water-level altitude estimate was determined from a single water-level measurement in 65 of the 133 wells. In about one-half of these 65 wells, the single measurement represents transient, non-equilibrium conditions and thus could only be used as an upper or lower bound for the predevelopment water level. For example, on a rising water-level hydrograph that has not yet reached equilibrium, the last water level can be used as a lower bound for the expected predevelopment, water-level altitude in the well. In this example, if the altitude of the last water-level measurement was 1,000 ft, the predevelopment, water-level altitude is expected to be greater than 1,000 ft. For measurements made in a dry well, the bottom-of-the-well altitude is assigned a “less than” qualifier and is used as an upper bound for contouring. Only mean water levels representing predevelopment conditions, or those that were assigned a “less than” or “greater than” qualifier to constrain the predevelopment level, were used to guide the contouring process.
The predevelopment water-level altitude estimate for each well was assigned to a subsurface hydrologic unit type (SHUT). The assignment is made in accordance with the SHUT encountered at the open interval (table 1). Wells with long open intervals commonly penetrate multiple SHUTs. In these cases, water levels generally were associated with the most transmissive SHUT. The top and bottom SHUT altitudes at each well location were determined from the HFMs, which, in general, are in good agreement with well logs. The value of using the HFM in assigning the contributing SHUT is that it provides a consistent method for assigning SHUTS to water levels across the entire study area, regardless of whether or not a log or other lithologic information exists.
The HSUs and corresponding SHUTs for wells having predevelopment water-level altitudes can be displayed interactively from a Microsoft® Excel workbook (appendix 3). The workbook is designed to view the HFM-interpreted, stratigraphic column, the predevelopment water-level altitude, and basic well-construction information for wells in the study area. Information for an individual well can be viewed by selecting the well from the column-header dropdown list. An example workbook page for well “UE-8e (2295 ft)” is shown in figure 6.
Each estimate of the predevelopment water-level altitude is assigned a single qualifier that describes how the estimate was used in the water-level contouring process (appendix 2). The five descriptive qualifiers describe the water level as:
In cases where the direction of the vertical hydraulic gradient is known, a water level from a well open to a confining unit was used to constrain contours in an overlying or underlying aquifer, and consequently, is assigned to the aquifer but given a “less than” or “greater than” qualifier. For example, at a location with a known downward vertical hydraulic gradient, a predevelopment water-level altitude measured in a well open to the volcanic confining unit is assigned to the underlying volcanic aquifer with a “less than” qualifier. Remarks stating the relevance of each predevelopment water level as used in the contouring process are documented in appendix 2.
The configuration and extent of continuous and isolated aquifers within each of the three aquifer types were based on the distribution and lateral and vertical extent of its component SHUTS. The magnitudes of water-level differences between wells in the same aquifer type were used to help evaluate aquifer continuity. For example, where the continuity between two areas of the same aquifer type was in question, the similarity or difference in the water level was used to support or refute a hydraulic connection.
Water levels in each of the mapped continuous aquifers were contoured manually. Only rarely were water-level contours inconsistent with local well data. Any discrepancy between contours and data typically were minor (less than 5 ft) and often the result of differences in water levels measured in closely spaced wells. In most cases, the inconsistency between measured and contoured water levels can be attributed to local vertical hydraulic gradients, unrecognized hydrologic anomalies, or small measurement errors. The manual contouring process took into consideration water-level gradients, recharge areas, discharge areas, and lateral and vertical continuity of flow systems (Blankennagel and Weir, 1973; Winograd and Thordarson, 1975; Laczniak and others, 1996; Belcher and others, 2004). Lastly, the contoured surfaces of the continuous aquifers were used to delineate tributary and regional flow systems.
U.S. Department of the Interior | U.S. Geological Survey
URL: http://pubs.usgs.gov/sir/2008/5044
Page Contact Information: Publications Team
Page Last Modified: Thursday, 10-Jan-2013 18:49:46 EST
